The grasshoppers and crickets (Orthoptera) of the Socotra Archipelago (Yemen): a comprehensive overview and a description of a new Oecanthus Tree Cricket (Oecanthidae)

Rob Felix; Jaap Bouwman; Baudewijn Odé; Robert Ketelaar; Duc Minh Pham; James Bailey

doi:10.3897/contrib.entomol.75.e144389

Research Article

The grasshoppers and crickets (Orthoptera) of the Socotra Archipelago (Yemen): a comprehensive overview and a description of a new Oecanthus Tree Cricket (Oecanthidae)

Rob Felix^‡, Jaap Bouwman^§, Baudewijn Odé^|, Robert Ketelaar^¶, Duc Minh Pham^#, James Bailey^¤

‡ Unaffiliated, Nijmegen, Netherlands

§ Unaffiliated, Arnhem, Netherlands

| Unaffiliated, Plasmolen, Netherlands

¶ Unaffiliated, Warnsveld, Netherlands

# Unaffiliated, Giessen, Germany

¤ Unaffiliated, Laguna Niguel, United States of America

Corresponding author: Rob Felix ( robfelix1@gmail.com )

Academic editor: Lara-Sophie Dey

This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Citation: Felix R, Bouwman J, Odé B, Ketelaar R, Pham DM, Bailey J (2025) The grasshoppers and crickets (Orthoptera) of the Socotra Archipelago (Yemen): a comprehensive overview and a description of a new Oecanthus Tree Cricket (Oecanthidae). Contributions to Entomology 75(1): 21-166. https://doi.org/10.3897/contrib.entomol.75.e144389

ZooBank: urn:lsid:zoobank.org:pub:57F30CBD-C51F-4D9A-A280-8EF2CE6D2E8E

Abstract

This paper presents all available information on the Orthoptera of the Socotra Archipelago, an area well-known for its endemic flora and fauna. General information is provided about the climate and geology of the Socotra Archipelago. The various habitats where grasshoppers have been found are described and illustrated, followed by a concise history of Orthoptera research on Socotra. Besides an identification key to the species, additional information about the material examined, taxonomy, diagnostic notes, distribution and occurrence, including maps, habitat, biology and bioacoustics, is provided for each species. In total, 65 Orthoptera species are reported here from Socotra, Abd el Kuri, Samha and Darsa, including Oecanthus castaneus Felix & Bouwman, sp. nov. and two unknown species assigned to Ectatoderus. Of these 65 species, 30 (46%) are endemic to the Socotra Archipelago. Re-descriptive notes on Acrotylus innotatus Uvarov, 1933 and Glomeremus capitatus Uvarov, 1957 are provided, including the description of the female of the latter species and the male of Oxytruxalis ensis (Burr, 1899). Acrotylus innotatus Uvarov, 1933, Dictyophorus griseus (Reiche & Fairmaire, 1850), Eumodicogryllus chivensis (Tarbinsky, 1930), Ochrilidia geniculata (Bolívar, 1913), Sphingonotus rubescens (Walker, 1870) and S. balteatus (Serville, 1838) are recorded for the first time from the Archipelago. Bioacoustics are presented for: Ochrilidia socotrae Massa, 2009, Stenohippus socotranus (Popov, 1957), Sphingonotus ganglbaueri Krauss, 1907, S. insularis (Popov, 1957), Acheta rufopictus Uvarov, 1957, Eumodicogryllus chivensis (Tarbinsky, 1930), Ectatoderus guichardi Gorochov, 1993 as well as two other species assigned to Ectatoderus, Oecanthus castaneus Felix & Bouwman, sp. nov., Ruspolia aff. R. basiguttata (Bolívar, 1906) and Pachysmopoda abbreviata (Taschenberg, 1883). Red List Assessments for 29 endemic species have been prepared including Oxytruxalis ensis (Burr, 1899) (Critically Endangered, CR), Cataloipus brunneri (Kirby, 1910) (Endangered, EN) and Glomeremus capitatus Uvarov, 1957, Phaneroptila insularis Uvarov, 1957, Phaulotypus granti Burr, 1899, Socotracris kleukersi Felix & Desutter-Grandcolas, 2012, Socotrella monstrosa Popov, 1957 and Xenephias socotranus Kevan, 1973 (all Vulnerable, VU).

Key Words

Bioacoustics, checklist, endemism, Indian Ocean, island, new species, Red List, taxonomy

Introduction

The Socotra Archipelago is an area well-known for its endemic flora and fauna. The grasshoppers and crickets (Orthoptera) have been studied extensively by Uvarov and Popov (1957), taking into account all the previous literature. Subsequently, Kevan (1973), Popov (1981, 1984, 1997), Gorochov (1993), Wranik (2003), Massa (2009, 2017) and Desutter-Grandcolas and Felix (2012) added information about the Orthoptera fauna of Socotra. Apart from Uvarov and Popov (1957) and Wranik (2003), no comprehensive overview of the Socotran Orthoptera fauna has been published. The current paper is a comprehensive overview of all information available on the Orthoptera of the Socotra Archipelago.

The paper is based on two collecting trips to the main island in February/March 2009 and October/November 2010 by RF, JBo and RK. In addition to the dataset resulting from these two trips, which has more than 400 records, some 350 additional field observations have been added, as well as data from collection specimens from diverse institutions, information from relevant literature and observation records from online platforms of iNaturalist and Observation.org. The current paper encompasses more than 2000 Orthoptera records from 1896 to 2024.

The Socotra Archipelago

Topography

The Socotra Archipelago is located in the Gulf of Aden, in the north-western part of the Indian Ocean and belongs administratively to Yemen (Fig. 1). The Archipelago consists of four islands and two small rocky islets. Socotra, the main island, lies 230 km from Somalia and 380 km from the Arabian Peninsula. It measures 135 km from west to east, 42 km from north to south at its widest point and has a surface area of 3625 km². Abd el Kuri lies closer to Somalia than to the main island and measures 133 km². Samha (41 km²) and Darsa (10 km²) are small islands in between Abd el Kuri and Socotra, known as “The Brothers”.

Figure 1.

The Socotra Archipelago (Yemen) with toponyms. 1. Abataro (settlement); 2. Adah (wadi); 3. Aduno (pass); 4. Aloove (settlement); 5. Amak (settlement); 6. Arher (spring); 7. Ba’a (settlement); 8. Balqathan (plain); 9. Begobig (settlement); 10. Betin (settlement); 11. Bi’r Haarso (waterhole); 12. Bijo (settlement); 13. Bojhin; 14. Dahamis (basecamp); 15. Dehamd (lagoon); 16. Delisha (settlement); 17. Di Asmo (settlement); 18. Di Hamri (cape and settlement); 19. Di Hashus (settlement); 20. Dirhor (settlement); 21. Ditwah (lagoon); 22. Firmihin (protected area); 23. Goahar (valley); 24. Gubba (lagoon); 25. Gudhm; 26. Haasan (settlement); 27. Halmi (settlement); 28. Heybaq (cape); 29. Hoq (cave); 30. Hulaf (settlement); 31. Jena-agahan; 32. Kazazahn (small area); 33. Kilisan (wadi); 34. Maabad (plain); 35. Madar (wadi); 36. Matyaf (lagoon); 37. Mokasu (wadi); 38. Mori (settlement); 39. Qadub (settlement); 40. Qarmah (cape); 41. Qeysoh (spring); 42. R.A.F. Camp (military basecamp); 43. Rookeb (hill); 44. Saqal; 45. Shibhon (settlement); 46. Sink; 47. Sirhin; 48. Taaqs (plain); 49. Zeflh; 50. Zemhom (area).

Endemism

Socotra is an ancient continental island of Gondwana origin. The timing of Socotra’s separation from the African mainland is still a matter of debate and is estimated to lie between 165 Mya and 15 Mya. An important biogeographical consideration is the assumption that the Hagher Mountains (also spelt Haggier or Hajhir) have been above sea level since the end of the Cretaceous and must, therefore, be considered one of the most isolated landmasses on Earth (Brown and Mies 2012).

Consequently, Socotra is well known for its high degree of endemism in flora and fauna. Socotra is amongst the five world insular systems richest in endemic species: fifteen plant genera, 308 (37%) of the 835 plant species, 90% of reptiles, 95% of land molluscs and 73% of isopods are endemic (Van Damme and Banfield 2011). The total number of endemic insect taxa described from the Archipelago is 662 species, representing 42% of all known insect species on the islands (Bezděk and Hájek 2017). According to Purchart et al. (2020), 80.3% of tenebrionid beetles and 51.5% of spiders are endemic to the Socotra Archipelago, while 18.5% of butterflies and only one species of dragonfly is endemic (Van Damme et al. 2020). Socotra was designated a Biosphere Reserve in 2003 and a UNESCO World Heritage Site in 2008 (Van Damme and Banfield 2011).

Geology

Socotra can be divided into three main regions. The granitic Hagher Mountains, consisting of Precambrian basement rock, dominate the centre of the island’s eastern half, with their prominent peaks, sheer cliffs, deep gorges and wadis. It is the only part of Socotra where the altitude exceeds 1,000 m a.s.l. The highest peak is Mount Scand at 1,530 m a.s.l. The Hagher penetrate through vast Cretaceous and Tertiary limestone plateaus occupying by far the most significant part of the island, rising to an average elevation of 300–700 m a.s.l. Well-known areas include Dixam and Momi, where vast, undulating expanses of bare limestone form the landscape. Where plateaus do not directly reach the seashore, coastal plains comprise the rest of the island. They are covered by Pleistocene and Holocene sands and gravel, like Noged in the south and Hadiboh Plain in the north (Fig. 1) (Kossmat 1907; Popov 1957; Brown and Mies 2012). Like Socotra, Abd el Kuri is made of a granite core primarily covered by limestone. It lacks a vast mountain range like the Hagher, with the highest peak being Mount Saleh at 570 m a.s.l. Basement rocks penetrate the limestone in some lower hilly areas. Darsa and Samha are flat-topped limestone plateaus (Kossmat 1907; Brown and Mies 2012).

Climate

The climate on Socotra is highly influenced by the summer and winter monsoons, separated by autumn and spring transition periods (Scholte and De Geest 2010). Hence, Socotra experiences four distinct seasons (Table 1). In the lowlands, the temperature is comparable to that of the surrounding Arabian and African continents, with mean annual temperatures approaching 30 °C. High into the Hagher Mountains, temperatures can drop below 10 °C at night in winter. The mean annual rainfall on lower to mid-elevations is around 200 mm. The south-facing slopes of the Hagher and the higher plateaus are influenced by moisture from the SW monsoon in the form of drizzle or fog (Scholte and De Geest 2010).

Table 1.

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XLSX

Seasons and weather circumstances on Socotra (derived from Scholte and De Geest (2010)).

Spring	Summer Monsoon			Autumn	Winter Monsoon
Mid-Feb – mid-Apr	Mid Apr–Jun	Jul–mid Aug	Aug–Sep	Oct	Nov – Feb
Wind from NE towards SW.	Increasing wind speed from SW			Wind from SW towards NE	Winds from NE
Dry, hot, rather cloudless	Rain influences the southern regions of the island and only sporadically reaches the northern regions.	Dry. Harsh winds create cloud cover above the southern coastal and especially at higher altitude plateaus.	High humidity	Relative humidity decreases and temperatures rise towards the end of the period when the first rains may fall.	Highest annual rainfall in Nov, affecting the entire island

Habitats

Elevational differences, exposure, precipitation (rain, fog and dew) and soil type give rise to very diverse habitats on Socotra, spanning from arid deserts to evergreen forests within a restricted geographical area (Batelka 2012). Habitats can be assigned to one of five altitudinal zones: coastal: 0–200 m a.s.l.; low elevation: 200–400 m a.s.l.; medium elevation: 400–700 m a.s.l.; montane 700–1200 m a.s.l. and high montane 1200–1540 m a.s.l. Brown and Mies (2012) describe the major plant communities linked to these altitudinal zones. Here, we present a selection of vegetation types mentioned by Brown and Mies (2012), representing the most important habitats for Orthoptera. The accompanying photos illustrate the island’s diverse landscapes. See also Uvarov and Popov (1957) for a description of orthopteran habitats.

Salt marsh

Patches with salt-tolerant (semi-)succulent plant species locally occur along the coast (Fig. 2). Vegetation cover is very high and dominated by medium-high perennial species like Arthrocnemum macrostachyum, Aerva javanica and Limonium sokotranum. The substrate is regularly flooded (Brown and Mies 2012).

Figure 2.

Salt marsh with Arthrocnemum macrostachyum and Aerva javanica. The salt marsh, separated by a coastal dune with Urochondra setulosa, is the typical habitat of Heteracris adspersa. Neet, Socotra, 28 Oct 2010 (photograph Rob Felix).

The number of Orthoptera species in salt marsh is relatively low, but a characteristic species of this vegetation is Heteracris adspersa (Redtenbacher, 1889). On Socotra, it is confined to this community. The only record of Eumodicogryllus chivensis (Tarbinsky, 1930) comes from this habitat. The widespread species Pyrgomorpha tereticornis (Brullé, 1840) also occurs here.

Beaches and dunes

Sandy coralline beaches are very common on Socotra. Locally undulating sand sheets and higher dune complexes have developed further away from the shore, like on Noged Plain (Brown and Mies 2012). Ochrilidia socotrae Massa, 2009 is strictly associated with the tussock grass Urochondra setulosa in sand sheets and fringes of coastal dunes (Figs 2, 3). A widespread and common species, like Acrotylus incarnatus Krauss, 1907, can be found in good numbers in dunes and on sandy beaches. Sphingonotus ganglbaueri Krauss, 1907 occurs near the shoreline and prefers coarser substrates like coralline gravel.

Figure 3.

Coastal coralline sand sheet with the grass Urochondra setulosa. Habitat of Ochrilidia socotrae and Acrotylus incarnatus; on patches with coarser coralline gravel, Sphingonotus ganglbaueri occurs. Erisseyl, Socotra, 3 Nov 2010 (photograph Rob Felix).

Coastal plains

Where inland mountains and limestone plateaus reach the sea, coastal plains at elevations of 0–200 m a.s.l. dominate the landscape, varying from gently sloping landscapes to flat plains of various substrates; sandy patches are intertwined with coarse gravel and small rocks. The most common vegetation here is the Croton socotranus community (Fig. 4). Various grasses and perennial herbs develop with the onset of the first rains (Brown and Mies 2012). The most extensive plain is Noged in the south, approximately 60 kilometres long and a maximum of 5 kilometres broad. Other examples are Hadiboh Plain and Hulaf in the north (Fig. 5).

Figure 4.

Typical coastal plain vegetation, dominated by Croton socotranus. Grassy patches are the typical habitat of Stenohippus socotranus. Taaqs, east of Qalansiyah, Socotra, 28 Feb 2009 (photograph Robert Ketelaar).

Figure 5.

Coastal plain at Hulaf with Acacia edgeworthii. Geophilous species like Acorypha glaucopsis and Acrotylus incarnatus occur in good numbers. The phytophilous Diabolocatantops axillaris is a common species in this habitat (photograph Robert Ketelaar).

On the coastal plains, a wide variety of grasshoppers and crickets occurs. Many geophilous species favour this habitat: Acrotylus incarnatus, Sphingonotus insularis (Popov, 1957), and Stenohippus socotranus (Popov, 1957) are most common here. Phytophilous species such as Pyrgomorpha tereticornis, Diabolocatantops axillaris (Thunberg, 1815) and Anacridium melanorhodon arabafrum Dirsh & Uvarov, 1953 occur in vegetated parts. Glomeremus pileatus (Krauss, 1902) and Pachysmopoda abbreviata (Taschenberg, 1883) can also be found here, hiding under stones during the daytime.

Littoral margins

Rushes, sedges and grasses occur along streams, pools and springs at all elevations and form the habitat of moist-loving Orthoptera species. On the limestone plateaus, Juncus can form dense stands (Fig. 6). Here, several moist-loving Orthoptera species occur, such as Aiolopus thalassinus (Fabricius, 1781), Paratettix subpustulatus (Walker, 1871), Trigonidium cicindeloides Rambur, 1838, Gryllotalpa aff. G. africana Palisot de Beauvois, 1820 and Conocephalus maculatus (Le Guillou, 1884). Many sites like this are overgrazed by cattle, resulting in permanently low vegetation, with Plantago and Eleocharis, amongst others, where Paratettix subpustulatus can be numerous (Fig. 7).

Figure 6.

A dense stand of Juncus along a pool. Habitat of Aiolopus thalassinus, Paratettix subpustulatus, Trigonidium cicindeloides and Gryllotalpa aff. G. africana. It is also the habitat the long-lost Cataloipus brunneri. Zerig, Socotra, 27 Feb 2009 (photograph Rob Felix).

Figure 7.

Moist soil on a bank of a stream in the Hagher with Plantago amplexicaulis and Eleocharis. Prime habitat of Paratettix subpustulatus. Ayhaft, Socotra, 22 Feb 2009 (photograph Rob Felix).

Lower-altitude rocky slopes

At lower-elevation rocky slopes (200–400 m a.s.l.), Adenium- or Jatropha-dominated shrubland has developed (Fig. 8). Vegetation cover and species composition vary depending on grazing pressure (Brown and Mies 2012). Characteristic grasshopper species here are Oedaleus senegalensis (Krauss, 1877), Sphingonotus insularis and Scintharista forbesii (Burr, 1899), all occurring on the ground. At the same time, the phytophilous Phaneroptera sparsa Stål, 1857, Phaulotypus insularis (Burr, 1899), P. socotranus (Popov, 1957), Oecanthus castaneus Felix & Bouwman, sp. nov. and Dioscoridus depressus Popov, 1957 can be found inside or near shrubs and small trees.

Figure 8.

Rocky slopes below 400 m a.s.l. with Jatropha unicostata. Geophilous as well as phytophilous species occur here: Oedaleus senegalensis, Sphingonotus insularis, Scintharista forbesii, Phaneroptera sparsa, Phaulotypus insularis, P. socotranus and Oecanthus castaneus Felix & Bouwman, sp. nov. Foothills south of Hadiboh, Socotra, 5 Mar 2009 (photograph Rob Felix).

Mid-elevation shrubland

Large parts of Socotra are covered by limestone plateaus with an elevation of 400–700 m a.s.l. The substrate is primarily bare rock with a varying degree of vegetation. Most often, vegetation is rather scarce; on Dixam Plateau, Dracaena trees are a distinctive feature (Brown and Mies 2012) (Fig. 9). Many geophilous grasshoppers have their prime habitat here: Acorypha bimaculata (Krauss, 1902), Scintharista forbesii, Sphingonotus insularis, Acrotylus innotatus Uvarov, 1933 and the ever-present Acrotylus incarnatus are common. Scintharista prefers larger bare rock surfaces and giant boulders. The other species are present on finer substrates.

Figure 9.

Dixam is a well-known limestone plateau on Socotra. Limestone plateaus comprise large parts of Socotra; vegetation is often scarce. It is the habitat of Acorypha bimaculata, Acrotylus innotatus, Scintharista forbesii and Sphingonotus insularis. Dixam Plateau, Socotra, 5 Nov 2010 (photograph Rob Felix).

Caves

The limestone plateaus of Socotra are interspersed with extensive karst formations, including numerous caves (Fig. 10). So far, one troglobitic cricket species has been described from Socotra, Socotracris kleukersi, Felix & Desutter-Grandcolas, 2012, occurring in a cave on Dixam Plateau.

Figure 10.

Hoq Cave, Momi. Socotracris kleukersi Felix & Desutter-Grandcolas, 2012 inhabits karst formations in Dixam. Troglobitic crickets are not yet known from Momi Plateau with this and other caves (photograph Rob Felix).

Lower montane woodland

In areas of increased moisture availability, particularly on north-facing slopes, in ravines and shaded valleys at elevations above 500 m a.s.l., conditions are favourable for denser vegetation, resulting in the presence of woodland. Trees are prominent and thickets can be very dense and species-rich. Good examples are the northern slopes of the Hagher (Fig. 11) and the higher parts of Wadi Ayhaft and Moukaradia Pass (Brown and Mies 2012).

Figure 11.

Woodland and dense thickets on the flanks of the Hagher. With increasing elevation and moisture availability, these are habitats of Phaneroptila insularis and, presumably, Oxytruxalis ensis. Depicted is the site named Hijama in Uvarov and Popov (1957) or Kishen by the Oxford Expedition (1956) and Guichard (1967). Wadi Dineghen, Socotra, 30 Oct 2010 (photograph Rob Felix).

Characteristic Orthoptera of thickets and shrubs at this elevation and higher are Phaneroptila insularis Uvarov, 1957, Oecanthus chopardi Uvarov, 1957, Pachysmopoda abbreviata (Taschenberg, 1883), Ruspolia aff. R. basiguttata (Bolívar, 1906) and Socotrella monstrosa Popov, 1957. On open patches with grassy vegetation and perennial herbs, Ermia variabilis Popov, 1957, Glomeremus capitatus Uvarov, 1957 and possibly Oxytruxalis ensis (Burr, 1899) occur.

Montane cloud zone

At higher elevations in the Hagher, above 700 m a.s.l., mean temperatures are lower, dropping sharply during the late afternoon and night, while dewfall and fog add to the moist conditions. With rising elevation, the number of goats decreases rapidly and cows are more prevalent. Dense shrubs and woodlands have developed here, with Searsia thyrsiflora, Cephalocroton socotranus and Hypericum scopulorum being common species (Brown and Mies 2012). Phaulotypus granti Burr, 1899 is associated with the latter species. High-altitude shrubs are the habitat of Phaneroptila insularis, Oecanthus chopardi, Pachysmopoda abbreviata, Ruspolia aff. R. basiguttata, Socotrella monstrosa and Xenephias socotranus Kevan, 1973. Due to grazing and erosion, the shrubs are interspaced with small open areas of herbs and grasses. Here, Ermia variabilis occurs in good numbers. Some parts of the montane shrubland have been altered to montane pastures, especially in gently sloping areas with seeping water; this is the habitat of Modicogryllus perplexus Otte & Cade, 1984.

High-montane granite peaks

The highest parts of the Hagher, above 1200 m a.s.l., are covered by a unique evergreen woodland with thickets composed of many endemic plant species. The tree layer is relatively open, while the shrub layer is very dense, dominated by Hypericum and Helichrysum (Brown and Mies 2012). The Orthoptera of the highest regions of the Hagher is a continuation of the montane and high-montane cloud zone, with the following species recorded here: Glomeremus capitatus, Dioscoridus depressus, Phaulotypus granti, Socotrella monstrosa and Xenephias socotranus.

History of Orthoptera research on Socotra

A complete overview of the history of biological exploration of the Socotra Archipelago is presented by Wranik (2003) and Bezděk and Hájek (2017). Here, we mention the orthopterological aspects of these historical expeditions and collecting voyages.

The biological exploration of Socotra began in 1834 when Lieutenant James Raymond Wellsted landed on Socotra with two colleagues for a two-month stay. He made notes on the flora and fauna of the island in his Memoir on the Island of Socotra (Wellsted 1835). He was the first to mention Orthoptera and the absence of one specific member: “locusts have been rarely seen in Socotra”.

Isaac Bayley Balfour and his team from London made the first detailed biological survey of the island in 1880. They collected mainly plants and some birds, lizards and insects, amongst which some specimens of Acorypha glaucopsis (Walker, 1870), as mentioned by Popov (in Uvarov and Popov (1957)).

Emil Riebeck from Halle, Germany, who visited the island in April and May 1881 (Neumann and Gedeon 2009), collected several of Socotra’s orthopterans. Taschenberg (1883) published Riebeck’s results, presented ten specimens of four species and described the peculiar Pachysmopoda abbreviata as new to science. Riebeck’s specimens are deposited in the Martin Luther Universität collection in Halle-Wittenberg, Germany.

Ernest Bennett, who visited the island from December 1896 till February 1897 with the explorers couple Theodore and Mabel Bent, made a small collection of orthopterans. Bennett collected six specimens belonging to five species, presented by Burr (1898), who described Physemophorus sokotranus Burr, 1898. Bennet’s specimens are deposited in the Oxford University Museum of Natural History, Oxford (OUMNH).

Figure 12.

Shrub communities dominated by Hypericum and Helichrysum around 1,000 m a.s.l. Habitat of Phaulotypus granti and Xenephias socotranus. Adho Dimello, Socotra, 1 Nov 2010 (photograph Rob Felix).

The first genuine zoological expedition, which resulted in a good collection of grasshoppers and crickets, was carried out by Henry Ogg Forbes from the Liverpool Museum and William Robert Ogilvie-Grant from the British Museum (Fig. 13). They reached the Archipelago in December 1898 and stayed for three months, also visiting Abd el Kuri at the beginning of December 1898 and at the end of February 1899. Forbes (1903) published the itinerary of this expedition.

Figure 13.

Plate from Forbes (1903) with Socotran Orthoptera collected by Forbes and Ogilvie-Grant in 1898–1899. 1. Scintharista forbesii (Burr, 1899); 2. Cataloipus brunneri (Kirby, 1910); 3. Teddia dioscoris Burr, 1899; 4. Oxytruxalis ensis (Burr, 1899); 5. Forficula smyrnensis Audinet-Serville, 1839; 6. Phaulotypus insularis (Burr, 1899); 7. Phaulotypus granti Burr, 1899; 8. Loboptera peculiaris Burr, 1899.

Burr (1899b) presented the first orthopterological results of this expedition and described two new genera and six new species; in 1903, he published the comprehensive results (Burr 1903). The collection made by Forbes and Ogilvie-Grant, consisting of around twenty species and fifty specimens, is divided amongst three depositories: the Natural History Museum in London (NHMUK), the World Museum in Liverpool (WML) and OUMNH.

In more or less the same period, January–March 1899, another important zoological expedition went to Socotra, organised by the Imperial Academy of Sciences in Vienna, Austria. One of the members was entomologist Oskar Simony. He and his team visited Abd el Kuri and Samha in the second half of January 1899. He collected forty specimens of fourteen species (Fig. 14). The collection is deposited in the Natural History Museum, Vienna.

Figure 14.

Plate from Krauss (1907) with some Orthoptera species collected by Oscar Simony in 1899. 1. Truxalis viridifasciata (Krauss, 1902); 2. Scintharista forbesii (Burr, 1899); 3. Sphingonotus albipennis Krauss, 1902; 4. Sphingonotus ganglbaueri Krauss, 1907; 5. Physemophorus sokotranus (Burr, 1898); 6. Diabolocatantops axillaris (Thunberg, 1815); 7. Acorypha glaucopsis (Walker, 1870); 8. Pachysmopoda abbreviata (Taschenberg, 1883); 9. Glomeremus pileatus (Krauss, 1902); 10. Melaneremus atrotectus (Brunner von Wattenwyl, 1888) (from India).

Krauss (1902) published the results of the Austrian expedition and described six species new to science. Rebel (1907) published an itinerary of the Austrian expedition. In 1907, Krauss published a comprehensive overview of all Orthoptera known from Socotra and Abd el Kuri at that time, referring to earlier expeditions and mentioning the specimens collected by Riebeck, Bennet, Forbes & Ogilvie-Grant, and Simony (Krauss 1907).

From January to April 1953, British entomologist George Popov stayed on Socotra for the Desert Locust Survey, Nairobi and thoroughly studied the Orthoptera of Socotra. He collected 300 specimens and published his results with Sir Boris Uvarov, the most comprehensive account of Socotran Orthoptera to date (Uvarov and Popov 1957). Later, Popov published separate papers about a selection of families based on his material (Tettigoniidae, Gryllacrididae, Thericleidae, Pamphagidae and Pyrgomorphidae) (Popov 1981, 1984, 1997). Popov’s collection is deposited in the NHMUK.

The University of Oxford organised an expedition in 1956 led by Douglas Botting. The party stayed on the island from August to October 1956. The participant responsible for the collection of Orthoptera was Michael Gwynne, who collected some 250 specimens deposited in the NHMUK (not in Oxford).

In April 1967, the Middle East Command Expedition was the most important event of this period. Kenneth M. Guichard carried out the zoological observations and collected many orthopterans, more than 300 specimens, stored in the NHMUK. Amongst the taxa, several were new to science and some were new to Socotra (Guichard 1992). Guichard kept a notebook (Guichard 1967), which is now stored in the archives of the library of the NHMUK, a copy of which was sent to RF for exploration.

In more recent times, the most important collector of Socotran Orthoptera is Wolfgang Wranik, from the University of Rostock, who visited the Archipelago many times and collected on all four islands (Socotra, Samha, Darsa and Abd el Kuri) between 1982 and 2000. His 300 specimens are all deposited in the National Museum of Prague, Czech Republic. Wranik (2003) published a comprehensive book on the flora and fauna of the Socotra Archipelago, with the first photos of live specimens of Socotran Orthoptera and many specimens in collections.

The Czech biological research in 1999–2012, thoroughly described by Bezděk and Hájek (2017), has yielded a collection of nearly 200 specimens deposited in the National Museum, Prague, Czech Republic.

Francesca Pella from the University of Pavia collected some fifty specimens of Orthoptera during her studies in 2007–2009. That material is deposited in Pavia.

Several Italian scientists explored the entomological fauna of Socotra and also collected Orthoptera, the most important of them being Bruno Massa from Palermo. He visited Socotra with Attilio Carapezza in 2008 during a botanical exploration organised by the Botanical Garden of Palermo. Pietro Lo Cascio and Flavia Grita collected some specimens in 2009 and Attilio Carapezza revisited Socotra in 2014. Massa (2009, 2017) presents a selection of the collected species. Most of the material is deposited in Bruno’s private collection in Palermo.

Methods

Dataset

We compiled and analysed a dataset of more than 2000 records of orthopterans from the Socotra Archipelago (Fig. 1) covering a survey period from 1896 to 2024 (Fig. 15). The dataset consists of specimens deposited at various institutions, literature data and field observations done by RF, JBo and RK in 2009 and 2010. Kay Van Damme and Francesca Pella provided several sight records from 2022. Additional observations were downloaded through GBIF.org (2024), containing data from Observation.org (2024) and iNaturalist contributors (2024), mainly from JBa.

Figure 15.

Localities with Orthoptera records in the Socotra Archipelago, covering a survey period from 1896 to 2024.

All data can be found on the distribution maps. The collection specimens are mentioned in Suppl. material 1. Field observations are presented in Suppl. material 2.

Material examined

RF, JBo and RK collected on Socotra from 20 Feb – 5 Mar 2009 and from 25 Oct–7 Nov 2010. RF and JBo analysed specimens from Socotra in MNHN, NHMUK and NMW during visits to the individual museums between 2011 and 2018. RF identified specimens collected by Wolfgang Wranik, Czech scientists and Francesca Pella (NMPC). RF studied the collections of WML, OUMNH and MLUH from photographs of Socotran specimens, kindly provided by Tony Hunter and Ian Wallace (WML), Amoret Spooner and Darren Mann (OUMNH) and Hendrik Müller and Joachim Händel (MLUH).

Institutional abbreviations

BMPC Bruno Massa Collection, Palermo, Italy;

HDPC Hendrik Devriese Collection, Corbion, Belgium;

MLUH Martin Luther University Halle-Wittenberg, Germany;

MNHN National Museum of Natural History, Paris, France;

MSNG Natural History Museum Giacomo Doria, Genoa, Italy;

MSNPV Pavia University History Museum, Pavia, Italy;

NBC Naturalis Biodiversity Center, Leiden, the Netherlands;

NHMUK Natural History Museum, London, UK;

NMPC National Museum, Prague, Czech Republic;

NMW Natural History Museum, Vienna, Austria;

OUMNH Oxford University Museum of Natural History, Oxford, UK;

RFPC Rob Felix Collection, Nijmegen, the Netherlands;

WML World Museum Liverpool, UK.

Taxonomy

Taxonomy follows the Orthoptera Species File (OSF) (Cigliano et al. 2024a). We identified our specimens using Uvarov and Popov (1957) and made subsequent identifications using keys in the available literature, including original species descriptions. Ochrilidia specimens were identified with Jago (1977) and Mistshenko (1937), Aiolopus specimens with the key in Hollis (1968), Heteracris specimens with Grunshaw (1991) and Stenohippus with Jago (1996). The abbreviations aff. and cf. in taxonomic names follow the syntax suggested by Lucas (1986).

Localities, coordinates and distribution maps

Most of the old museum specimens bear limited locality information, let alone coordinates of localities. Bezdĕk et al. (2012) gave an overview of all geographical names of localities on Socotra used in entomological literature in the past and provided coordinates. We used these coordinates to plot records without detailed locality information, roughly all records from before 2000, on a map (Fig. 15). Part of the localities were interpreted, based on maps and site descriptions in various publications (Forbes 1903; Gregory, 1903; Rebel 1907; Uvarov and Popov 1957; Guichard 1967; Doe 1992). In the material examined section (Suppl. material 1), interpreted and estimated coordinates are given between square brackets ([]). From 2000 onwards, records are based on accurate coordinates, present on specimen labels or, in case of field observations, taken from GPS readings.

Locality names in the main text follow Bezdĕk et al. (2012), while, in the material examined section (Suppl. material 1), we cite the original spelling of the localities, added with the spelling according to Bezdĕk et al. (2012) between square brackets. Data missing on the label, but mentioned in literature or interpreted, based on maps is also shown in square brackets. We estimated data lacking on labels of museum specimens or in original literature, based on coordinates in GIS.

We obtained elevation data through Jarvis et al. (2008). We created the base map using ArcGIS® software by Esri (Copyright © Esri) and adopted the elevation zones depicted in the maps from Brown and Mies (2012), who distinguished the following five elevation belts linked to different vegetation types: Coastal zone: 0–200 m a.s.l.; Low elevation slopes: 200–400 m a.s.l.; Medium elevation slopes: 400–700 m a.s.l.; Montane 700–1200 m a.s.l.; High montane 1200–1540 m a.s.l.

For habitat descriptions, we derived information on vegetation types from a GIS file provided by Ing. Petr Vahalík, Ph.D. of Mendel University in Brno, Czech Republic.

Bioacoustics

Sound recordings were made by RF using an M-Audio MicroTrack II with a Sennheiser microphone, module K6 and head ME40. Unfortunately, no data on temperature were noted. The song files were sampled at 96 kHz and 24 bits and analysed with Wavelab Pro 11.2 software and Wildlife Acoustics Kaleidoscope 5.6.3 software. Oscillograms were usually prepared after filtering low-frequency noise to obtain a clearer picture. Oscillograms were made with Praat 6.2.22 software and spectrograms with Wildlife Acoustics Kaleidoscope 5.6.3 software.

Bioacoustics terminology

For bioacoustics terminology, we follow Baker and Chesmore (2020).

Sound elements

Pulse: indivisible unit of sound, typically corresponding to a single tooth impact; syllable: sound produced with a single complete stridulatory movement (the opening and closing of the tegmina in Ensifera, the up and down motion of the femora against the tegmina in Acrididae); hemi-syllable: sound produced with only one of the motion directions of a syllable; two hemi-syllables may be audible within a syllable; echeme: first-order assemblage of syllables; echeme-sequence: first-order assemblage of echemes (may include individual syllables that precede or follow the echeme).

Sound spacing of pulses, syllables and echemes in time

Duration: duration of the element itself (in s or ms); interval: duration of the silence between the elements (in s or ms); period: duration of one element, including the interval with the next element; repetition rate: number of elements repeated per unit of time (per s or minute).

The sound descriptions in this paper are based on the available sound recordings, thus, sometimes only on the song of one specimen.

Biometric terminology

The terminology that describes the genitalia of Gryllidae follows Desutter (1987), Desutter-Grandcolas (2003) and De Campos and Desutter-Grandcolas (2020). Terminology in Ragge (1955) is followed for veins of the Caelifera, terminology in Desutter-Grandcolas (2003) and Robillard and Desutter-Grandcolas (2004) for wing venation in Grylloidea. Mounted specimens were measured with a digital calliper (precision 0.01 mm). Smaller body parts were measured through a Novex zoom RZT-SF stereomicroscope in combination with an object micrometre glass slide.

For the species description of Oecanthus castaneus Felix & Bouwman, sp. nov. and other species accounts, the following measurements were made: body length (from the tip of the labrum to the apex of the subgenital plate), pronotum length (from anterior to posterior margin along the mid-line), pronotum width (at the widest part in dorsal view), tegminal length (from the thorax joining point to the distal end of tegmina along the mid-line), tegminal width of the right tegmen (measured at the widest section of the tegmina at rest or the maximum width of the dorsal field of the right tegmen excluding the lateral folds), total tegminal width of the right tegmen (maximum width of the dorsal field and the lateral fold together), hind femur length, hind tibia length and cerci length. The male stridulatory file length was measured along the ventral surface of the right tegmen’s first anal vein (file). The female ovipositor was measured from the base of the subgenital plate to the distal tip.

Biometric abbreviations

FI, FII, FIII fore, median, hind femur

iad inner, apical, dorsal

iav inner, apical, ventral

oad outer, apical, dorsal

oav outer, apical, ventral

TI, TII, TIII fore, median, hind tibia

Abbreviations genitalia

DP Distal prolongation of the Arc

EctAp Ectophallic Apodeme

EndC Endophallic Cavity

MLPs Main Lobes of the Pseudepiphallus

Ps Pseudepiphallus

PsAp Pseudepiphallic Apodemes

PsP Pseudepiphallic Parameres

R Ramus

ScEEI Lateral Sclerotisation of the Epi-Ectophallic Invagination (“Endoparameres”).

Red List assessment

Red List assessments of 29 endemic species are based on our above-mentioned dataset of orthopterans from the Socotra Archipelago. The assessments followed the criteria to evaluate if a taxon belongs to an IUCN Red List threatened category (IUCN Standards and Petitions Committee 2022). Due to data limitations, using all five IUCN criteria categories to assess the 29 Orthoptera species was impossible. No population status or trends information was available, nor was there data to enable quantitative analysis. Therefore, assessments were only feasible, based on criteria B and D2. Both criteria rely on the geographic distribution of a species and two parameters: area of occupancy (AOO) and extent of occurrence (EOO).

The AOO of the 29 Orthoptera species under investigation was calculated by summing the total area of 2 × 2 km grid squares occupied. The estimated geographic distribution was determined as polygonal spatial data using point data and habitat information. Rules outlined in the IUCN Mapping Standards were followed (IUCN SSC Red List Technical Working Group 2021). Supporting information included a digital elevation model, the outline of the landmass and a vegetation map of Socotra. The lower EOO was determined using the minimum convex hull around confirmed occurrences, while the upper EOO was calculated using the minimum convex hull encompassing the entire estimated geographical range, which may overestimate its actual distribution.

All data have been entered into the Species Information Service (SIS), IUCN’s central database, including information on geographic distribution, threats, habitat, existing conservation measures and a range of additional information that provided further insight into each species. The SIS automatically calculated the threat category based on the input data. At the time of submission of this paper, the assessments await their review by an IUCN representative. A detailed description of the assessment methodology is provided in Pahm (2023).

Results

In the Socotra Archipelago, 65 taxa of Orthoptera occur, 64 of which are resident species (Table 2). There are nine endemic genera, 30 known endemic species, one endemic subspecies and several unidentified taxa. Comprehensive species accounts are provided in the following paragraphs.

Table 2.

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List of 65 Orthoptera taxa known to occur in the Socotra Archipelago. One non-resident species is indicated with I. The endemics are indicated with an E, endemic genera with an E^G and endemic tribes with E^T.

	Endemic	Socotra	Darsa	Samha	Abd el Kuri
Suborder CAELIFERA
Superfamily Acridoidea
Family Acrididae
Subfamily Acridinae
Tribe Truxalini
Oxytruxalis ensis (Burr, 1899)	E^G	*
Truxalis viridifasciata (Krauss, 1902)	E	*
Subfamily Calliptaminae
Acorypha bimaculata (Krauss, 1902)	E	*		*
Acorypha glaucopsis (Walker, 1870)		*	*	*
Subfamily Catantopinae
Diabolocatantops axillaris (Thunberg, 1815)		*
Dioscoridus depressus Popov, 1957	E^G	*
Subfamily Cyrtacanthacridinae
Anacridium melanorhodon arabafrum Dirsh & Uvarov, 1953		*
Cyrtacanthacris tatarica (Linnaeus, 1758)		*
Schistocerca gregaria (Forskål, 1775)		*
Subfamily Eyprepocnemidinae
Cataloipus brunneri (Kirby, 1910)	E	*
Heteracris adspersa (Redtenbacher, 1889)		*
Heteracris annulosa Walker, 1870					*
Heteracris coerulescens (Stål, 1876)		*
Subfamily Gomphocerinae
Ermia variabilis Popov, 1957	E	*
Ochrilidia cf. O. geniculata (Bolívar, 1913)					*
Ochrilidia gracilis nyuki (Sjösted, 1909)		*
Ochrilidia socotrae Massa, 2009	E	*
Stenohippus socotranus (Popov, 1957)	E	*
Subfamily Oedipodinae
Acrotylus incarnatus Krauss, 1907	E	*
Acrotylus innotatus Uvarov, 1933		*
Aiolopus thalassinus (Fabricius, 1781)		*
Oedaleus senegalensis (Krauss, 1877)		*
Scintharista forbesii (Burr, 1899)	E	*
Scintharista notabilis (Walker, 1870)		*			*
Sphingonotus (Neosphingonotus) canariensis Saussure, 1884		*
Sphingonotus (Parasphingonotus) turkanae Uvarov, 1938		*
Sphingonotus (S.) albipennis Krauss, 1902	E				*
Sphingonotus (S.) balteatus (Serville, 1838)					*
Sphingonotus (S.) ganglbaueri Krauss, 1907	E	*		*
Sphingonotus (S.) insularis (Popov, 1957)	E	*		*
Sphingonotus (S.) rubescens (Walker, 1870)		*		*
Superfamily Eumastacoidea
Family Thericleidae
Subfamily Plagiotriptinae
Tribe Phaulotypini
Phaulotypus dioscoridus (Popov, 1957)	E^G	*
Phaulotypus granti Burr, 1899	E^G	*
Phaulotypus insularis (Burr, 1899)	E^G	*
Phaulotypus socotranus (Popov, 1957)	E^G	*
Tribe Socotrellini	E^T
Socotrella monstrosa Popov, 1957	E^G	*
Superfamily Pyrgomorphoidea
Family Pyrgomorphidae
Subfamily Pyrgomorphinae
Tribe Dictyophorini
Dictyophorus griseus (Reiche & Fairmaire, 1850)		I
Tribe Phymateini
Physemophorus sokotranus (Burr, 1898)	E^G	*
Tribe Pyrgomorphini
Pyrgomorpha conica kurii Hsiung & Kevan, 1975	E				*
Pyrgomorpha tereticornis (Brullé, 1840)		*	*	*
Tribe Sphenariini
Xenephias socotranus Kevan, 1973	E^G	*
Superfamily Tetrigoidea
Family Tetrigidae
Paratettix subpustulatus (Walker, 1871)		*
Suborder ENSIFERA
Superfamily Grylloidea
Family Gryllidae
Subfamily Gryllinae
Tribe Gryllini
Acheta cf. A. domesticus (Linnaeus, 1758)		*
Acheta rufopictus Uvarov, 1957	E	*
Gryllodes sigillatus (Walker, 1869)		*
Gryllus bimaculatus De Geer, 1773		*
Tribe Modicogryllini
Eumodicogryllus chivensis (Tarbinsky, 1930)		*
Modicogryllus perplexus Otte & Cade, 1984		*
Family Mogoplistidae
Subfamily Mogoplistinae
Tribe Arachnocephalini
Ectatoderus guichardi Gorochov, 1993	E	*
Ectatoderus sp. 2	?	*
Ectatoderus sp. 3	?	*
Tribe Mogoplistini
Mogoplistes aff. M. brunneus	?	*
Family Oecanthidae
Subfamily Oecanthinae
Tribe Oecanthini
Oecanthus castaneus Felix & Bouwman, sp. nov.	E	*
Oecanthus chopardi Uvarov, 1957	E	*		?
Family Phalangopsidae
Subfamily Phalangopsinae
Socotracris kleukersi Felix & Desutter-Grandcolas, 2012	E^G	*
Family Trigonidiidae
Subfamily Trigonidiinae
Trigonidium cicindeloides Rambur, 1838		*
Superfamily Gryllotalpoidea
Family Gryllotalpidae
Gryllotalpa aff. G. africana Palisot de Beauvois, 1820		*
Superfamily Stenopelmatoidea
Family Gryllacrididae
Glomeremus capitatus Uvarov, 1957	E	*
Glomeremus mediopictus Uvarov, 1957	E	*
Glomeremus pileatus (Krauss, 1902)	E	*
Superfamily Tettigonioidea
Family Tettigoniidae
Subfamily Conocephalinae
Tribe Conocephalini
Conocephalus maculatus (Le Guillou, 1884)		*
Tribe Copiphorini
Ruspolia aff. R. basiguttata (Bolívar, 1906)		*
Subfamily Mecopodinae
Pachysmopoda abbreviata (Taschenberg, 1883)	E^G	*
Subfamily Phaneropterinae
Phaneroptera sparsa Stål, 1857		*
Phaneroptila insularis Uvarov, 1957	E^G	*

Identification key to the Orthoptera in the Socotra Archipelago

The following identification key can be used to identify adults of all known species of Orthoptera in the Socotra Archipelago, except for the different species of Ectatoderus. The key is unsuitable for identifying nymphs. It is an adapted combination of the keys published in Dirsh (1965), Jago (1967), Harz (1969, 1975), Descamps (1977), Popov (1981, 1997), Grunshaw (1991), Husemann et al. (2011), Defaut and Morichon (2015), Rowell and Hemp (2017, 2018, 2021) and Hemp and Rowell (2020).

It is beyond the scope of this paper to illustrate all body parts used as characters in the key. For this purpose, we kindly refer to additional resources available elsewhere.

1a	Antennae short, not much longer than head and pronotum together, always with less than 30 segments; hearing organs (if present) on each side of the base of the abdomen	CAELIFERA (2)
1b	Antennae longer than the body, composed of more than 30 segments; hearing organs (if present) near the base of the fore tibiae	ENSIFERA (44)
Caelifera
2a	Pronotum strongly elongated, covering the whole or a significant part of the abdomen, (almost) reaching its tip (Fig. 130); tarsi of the fore and middle legs with two segments, no arolium between the claws	Tetrigoidea – Tetrigidae – Paratettix subpustulatus
2b	Pronotum not produced backwards along the abdomen; tarsi with three segments and with an arolium between the claws	3
3a	Outer side of the basal segment of the hind tarsi proximally with a tubercle and distally with a tooth; dorso-median carina of the hind tibiae serrated, armed with spines (Fig. 105); body strongly laterally compressed, if not, then strongly rugose	Eumastacoidea – Thericleidae (4)
3b	Basal segment of the hind tarsi without a tubercle and a tooth; dorsal carina of the hind tibiae not serrated or armed with spines	8
4a	Body strongly rugose, not compressed laterally (Fig. 115); the vertex of the fastigium laterally compressed and strongly projecting forward, in front of the eyes; ten antennal segments; antennal organ on the ninth segment	*Socotrella monstrosa*
4b	Body never strongly rugose, compressed bilaterally (Fig. 113); vertex of the fastigium projecting above the eyes, not in front of the eyes; nine antennal segments; antennal organ on the eighth segment	Phaulotypus (5)
5a	Female pronotum strongly produced posteriorly in a sharp angle, covering the meso- and metanotum (Fig. 105); dorso-median carina of the hind femora in males armed with small spines (Fig. 104)	6
5b	Female pronotum not projecting posteriorly, not covering the meso- and metanotum (Fig. 108); armature of the hind femora in males stronger (Fig. 107)	7
6a	Apex of the head in males strongly projecting above the upper level of the eyes; length of the female pronotum along the median carina more than the length of the hind margin of the lateral pronotal lobe (Figs 101, 104)	*Phaulotypus granti*
6b	Apex of the head in males less prominent, length of the median pronotal carina in females less than the hind margin of the pronotal lobe (Fig. 102)	*Phaulotypus dioscoridus*
7a	Lower carina of the hind femora strongly rugulose in both sexes; pronotum in profile evenly convex; abdomen with a dark longitudinal dorso-median stripe	*Phaulotypus socotranus*
7b	Lower carina of the hind femora smooth; male pronotum in profile strongly arcuate and gibbose, without a dark dorso-median stripe	*Phaulotypus insularis*
8a	Fastigial furrow at the tip of the frons present, visible from above; ventral lobes of the hind knees longer than the dorsal lobes	Pyrgomorphoidea – Pyrgomorphidae (9)
8b	Fastigial furrow absent; ventral lobes of the hind knees shorter than or equal to the dorsal lobes	Acridoidea – Acrididae (13)
9a	Apterous species (Fig. 128)	*Xenephias socotranus*
9b	Fully-winged species	10
10a	First tergite dorsally with a knob-like tubercle, visible with closed tegmina, because of a bend in the hind margin of both tegmina (Fig. 122)	*Physemophorus sokotranus*
10b	No tubercle on the first tergite	11
11a	Head not conical. Large, robust black species with reddish tones. Tegmina broad, with a rounded apex. Pronotum heavily sculptured, in profile with a raised, rounded frontal hump (Fig. 117)	*Dictyophorus griseus*
11b	Head conical. Smaller species, slender, with narrow, pointed tegmina; pronotum different	Pyrgomorpha (12)
12a	Occurring on Socotra, Darsa and Samha; the sides of the frontal half of the female pronotum parallel or almost so, only subtly diverging posteriorly	*Pyrgomorpha tereticornis*
12b	Occurring on Abd el Kuri; with a more robust appearance; the sides of the whole female pronotum diverging posteriorly over its entire length	*Pyrgomorpha conica kurii*
13a	Prosternal process present	14
13b	Prosternal process absent; if present, then the body is elongated and the antennae are blade-like and flattened	25
14a	Mesosternal lobes rectangular	Cyrtacanthacridinae (15)
14b	Mesosternal lobes rounded, obtuse-angular or acute-angular, but never rectangular	17
15a	Prosternal process strongly curved backwards, touching or almost touching the mesosternum, inﬂated in the middle, with an acutely or subacutely conical apex; male subgenital plate acutely conical; pronotum moderately tectiform, slightly constricted; integument ﬁnely rugose or dotted, almost smooth; male cercus with a subacute apex; hind wings lemon-yellowish	*Cyrtacanthacris tatarica*
15b	Prosternal process straight or slightly inclined backwards, conical, cylindrical or compressed; male subgenital plate bilobate or trilobate; pronotum in the middle constricted.	16
16a	Male subgenital plate deeply bilobate; cerci wide, lamelliformly compressed, with an almost truncate, slightly excised apical margin.	*Schistocerca gregaria*
16b	Male subgenital plate deeply trilobate; cerci not compressed, subconical, narrow, incurved and upcurved	*Anacridium melanorhodon arabafrum*
17a	Male cerci incurved, clasping, pincers-like	Calliptaminae (18)
17b	Male cerci variable, but not incurved nor pincers-like	19
18a	Inner side of the hind femora solid black; ratio of the length to the height of the hind femora greater than 2.8; ventral femoral carina lower than the dorsal one	*Acorypha bimaculata*
18b	Inner side of the hind femora yellow (with or without separate spots); ratio of the length to the height of the hind femora less than 2.8; femoral carinas high, with the dorsal and ventral of more or less the same height; ventral carina whitish.	*Acorypha glaucopsis*
19a	Pronotum flat or weakly roof-like dorsally, with linear lateral and medial carinae (lateral carinae sometimes weak or absent); male cerci often bilaterally compressed, apically lobiform or subacute, downcurved apically	Eyprepocnemidinae (20)
19b	Pronotum of variable shape; lateral carinae, if present, not linear. Male cerci variable, but never strongly bilaterally compressed and rounded lobiform at its tip, but usually straight or upcurved apically	Catantopinae (24)
20a	Apex of the subgenital plate excised; cerci moderately wide, gradually narrowing towards the apex, the distal part being gently, in- and downwardly curved, with an acute apex, often oblique on the upper edge (Fig. 34)	*Cataloipus brunneri*
20b	Apex of the subgenital plate without a notch; cerci bilaterally flattened, downcurved in the apical part, adapted as flat clasping structures	Heteracris (21)
21a	Apex of the subgenital plate with two tubercles; the external median surface of the hind femora with extensive dark transverse bands reaching the middle line; lateral surface of the head yellow-brown or light green; a distinctive yellowish stripe behind the eyes (Figs 36, 38)	*Heteracris adspersa*
22b	Apex of the subgenital plate without a pair of tubercles	23
23a	General colouration of the male brown with two yellow or yellow-green dorsolateral stripes; tegmina unicolorous infuscate brown, with dorsal longitudinal yellow-green stripe; external surface of posterior femora dull brown without boldly marked spots or transverse bands, sometimes with an indistinct distal band before the dull cream-yellow pre-genicular annulae (Fig. 41); hind wings blue at their base	*Heteracris coerulescens*
23b	General colouration variable; tegmina with large brown spots merging to form transverse bands; external femoral markings variable, generally with median and distal spots, but sometimes absent; if present, median spots never extending to the median line; hind wings colourless	*Heteracris annulosa*
24a	Completely apterous; body cylindrical, somewhat depressed with a light longitudinal line along the mid-dorsal axis from the fastigium to the abdomen’s tip; two irregularly swollen ridges on the pronotum’s sides represent the lateral carinae (Fig. 27)	*Dioscoridus depressus*
24b	Fully winged; body shape different than above; characteristic vertical whitish line on the posterior margins of the metathorax (Fig. 25)	*Diabolocatantops axillaris*
25a	Large, slender species with strongly elongated, almost stick-like bodies; head elongated and conical with a long fastigium and an incurved, sloping frons; antennae ensiform; hind wings with a tessellated pattern formed by dark transverse veinlets (Figs 16, 17, 19)	Acridinae (26)
25b	Other combinations of characters, not like Figs 16, 17, 19	27
26a	Antennae longer than the head and pronotum together; pronotum strongly elongated, almost flat; tegmina very long, gradually narrowed and pointed in the apical half; hind wings much shorter than the tegmina; internal upper lateral lobes of the hind knees elongated and much longer than the external upper lobes (Figs 16, 17)	*Oxytruxalis ensis*
26b	Antennae shorter than the head and pronotum combined, pronotum saddle-shaped, tegmina more abruptly narrowed and obtusely pointed apically, hind wings somewhat shorter than the tegmina; internal and external upper lateral lobes of the hind knees equal in length (Fig. 19)	*Truxalis viridifasciata*
27a	Head subconical with a sloping frons relative to the vertex; temporal foveolae always present; stridulatory mechanism consisting of a serrated vein in the tegmen (primarily the radius) and a file of pegs on the lower edge of the inner area of the male hind femur; hind wings always hyaline without dark markings.	Gomphocerinae (28)
27b	Head subglobular with vertical frons relative to the vertex; temporal foveolae absent or irregularly indistinct, except in Aiolopus; medial area of the tegmen with a raised and often serrated intercalary vein serving as stridulatory mechanism; hind wings often brightly coloured with or without dark wing bands, except in Aiolopus and some Sphingonotus	Oedipodinae (32)
28a	Short vestigial wings on the sides of the body; small species with a relatively large head, strongly sloping frons, ensiform antennae and large elongated eyes	*Ermia variabilis*
28b	Fully winged; tegmina posteriorly passing the hind knees	29
29a	Small; antennae filiform; pronotum slightly constricted in the prozona with angularly incurved lateral carinae	*Stenohippus socotranus*
29b	Small to medium-sized; antennae ensiform; lateral carinae of the pronotum straight and parallel or slightly diverging posteriorly, not constricted	Ochrilidia (30)
30a	Length of the fastigium of the vertex in front of the eyes, in dorsal view, more than the maximum width of the vertex at the frontal edge of the eyes; temporal foveolae invisible when seen from above (Fig. 47); lower inner lobes of the hind knees without a black dot	*Ochrilidia gracilis nyuki*
30b	Length of the fastigium of the vertex in front of the eyes equal to the width of the vertex at the frontal edge of the eyes, in dorsal view; lower edges of the temporal foveolae partly or well visible from above; lower inner lobes of the hind knees with a block dot	31
31a	Larger size; lower edges of the temporal foveolae completely visible from above; within the Archipelago only known from Abd el Kuri	*Ochrilidia geniculata*
31b	Smaller size; lower edges of the temporal foveolae partly visible from above; within the Archipelago only known from Socotra	*Ochrilidia socotrae*
32a	Pronotum as long as, or shorter than its width, strongly saddle-shaped; lateral lobes of the pronotum with a dark spot with an off-central white dot; hind wings basally orange or pinkish-red, without a dark fascia, but sometimes with some dark spots in the apex	Acrotylus (33)
32b	Pronotum of variable shape, but always longer than its width	34
33a	Pronotum with a rounded posterior margin; first half of the prozona of the pronotum gently sloping (Figs 59, 64C, F); hind wings basally orange-red (Fig. 65)	*Acrotylus incarnatus*
33b	Pronotum rather rugose with a subrounded to slightly angular posterior margin; frontal half of the prozona in lateral view step-like raised (Fig. 64A, B, D, E); hind wings basally pinkish-red and apically slightly infumated, often with prominent black apical spots (Figs 63, 65B)	*Acrotylus innotatus*
34a	Frons sloping (Fig. 67); temporal foveolae well developed, elongate, trapezoidal; pronotum weakly saddle-shaped, dorsally flat, with a clear but low median carina, cut by one or two transverse sulci; pronotum without lateral carinae (be aware of the contrasting pattern of two white incurved lines bordered by dark markings, suggesting the presence of two lateral carinae); hind wings hyaline or with a greenish hue, slightly infumated in the apex and posterior margin; fastigium of the vertex concave; species of relatively moist habitats	*Aiolopus thalassinus*
34b	Frons slightly sloping to straight, relatively to the vertex (Fig. 70); hind wing basally often colourful with or without a dark fascia, sometimes hyaline with or without a blue hue	35
35a	Median carina of the pronotum straight, not interrupted by one or more transverse sulci; hind wings basally yellowish, apex hyaline, with a dark fascia in between; pronotum marked with an X-shaped pattern	*Oedaleus senegalensis*
35b	Median carina of the pronotum incomplete or complete, but always interrupted by one or more transverse sulci	36
36a	Viewed in profile, the head positioned on the same level as the pronotum; pronotal median carina well marked along the entire length of the pronotum, clearly raised and only interrupted by the principal sulcus; robust species	Scintharista (37)
36b	Viewed in profile, the head clearly rises above the pronotum; pronotal median carina weak or only developed in the first half of the prozona and metazona, interrupted by more than one transverse sulci; slender species, except for the rather stout Sphingonotus insularis	Sphingonotus (38)
37a	Hind wings pure black, except for a hyaline band bordering the infumated apex (Fig. 73)	*Scintharista forbesii*
37b	Hind wings basally yellow to red with a dark fascia and a black apex (Fig. 76)	*Scintharista notabilis*
38a	Male intercalary vein in the medial area of tegmina serrate, projecting above the radial and medial veins, in females sometimes less serrate or smooth; radial area at the same level as the medial area, without cross veinlets between the radial and medial veins;	Sphingonotus s.s (40)
38b	Male intercalary vein in the medial area of the tegmina smooth (very rarely with serration), not projecting above the radial and medial veins	39
39a	Hind wings hyaline with a narrow complete dark fascia continuing on the anal fan; thickened cross veinlets between the radial and medial veins (less developed in females), which project above the intercalary vein; radial vein without serration	Sphingonotus (Neosphingonotus) canariensis
39b	Hind wings hyaline; radial vein serrated and stronger raised than the subcostal vein; without thickened cross veinlets between the radial and medial veins; smaller species; supra-anal plate trilobite	Sphingonotus (Parasphingonotus) turkanae
40a	Large species; hind wings basally violet or purple with an extensive black fascia and a hyaline apex (Fig. 88); inner side of the hind tibiae bluish or bluish-grey; only recorded on Abd el Kuri	Sphingonotus (S.) balteatus
40b	Smaller species with hyaline or basally light blue hind wings, without a dark fascia or with an incomplete fascia formed by a row of dark spots	41
41a	Robust species, with strongly undulated margins of the pronotum; sudden notch in the dorsal carina of the hind femora, close to the knee; hind wings basally light blue with a short, often incomplete fascia not reaching the hind margin and covering only the first anal veins; male supra-anal plate trilobate and deep blue in the male	Sphingonotus (S.) insularis
41b	Slender species without the above characters	42
42a	Hind wings hyaline; transverse bands of the tegmina weakly defined with discontinuous margins (particularly the second and the third band), usually built by a variable number of speckles; pronotum dorsally flat, prozona at the same level as the metazona, the latter with weak “shoulders”, formed by the lateral carinae; intercalary vein variable, but never straight, often clearly sinuous with apical part almost touching the medial vein; apical part of the second branch of the radial vein with three branches; male supra-anal plate triangular, gradually narrowing towards the apex (sometimes with a slight hint of two lateral lobes, hence irregular triangular)	Sphingonotus (S.) rubescens
42b	Hind wings hyaline, sometimes with a bluish hue and a hint of a dark fascia; transverse bands of the tegmina distinct and continuous (particularly the first and second band, mostly also the third); pronotum dorsally unevenly levelled, with a depressed prozona and an elevated metazona, separated from each other by a clear and deep principle sulcus; metazona with strong “shoulders”, formed by the lateral carinae; intercalary vein variable; apical part of the second branch of the radial vein with two branches; male supra-anal plate pentagonal with two lateral lobes and a median lobe	43
43a	Socotra, Samha; posterior margin of the pronotum acutely angled, frons vertical relative to the vertex; hind femora shorter; hind wings hyaline or basally with a hint of a bluish hue, often with a trace of a smoky dark fascia	Sphingonotus (S.) ganglbaueri
43b	Abd el Kuri; posterior margin of the pronotum obtusely angled; frons sloping relative to the vertex; hind femora longer; hind wings whitish/hyaline, never with a bluish hue, sometimes with a trace of a dark fascia	Sphingonotus (S.) albipennis
Ensifera
44a	Tarsi with three segments	45
44b	Tarsi with four segments	57
45a	Forelegs modified for digging habits	Gryllotalpoidea – **Gryllotalpa aff. G. africana**
45b	Forelegs similar to the mid-legs	46
46a	Cerci very long and thin, much longer than the body; habitus as in Fig. 178; cave-dwelling species	Phalangopsidae – Socotracris kleukersi
46b	Cerci not longer than the body, not cave-dwelling	47
47a	Small, shiny black cricket, except for light brown to orange-red hind legs and cerci (Fig. 180); body length smaller than 5 mm; marshy habitats	*Trigonidium cicindeloides*
47b	Habitus different	48
48a	Small, flat, dull crickets, covered with small, flaky scales; posterior tibiae without spines, only finely serrulated on the superior margins (but with apical spurs)	Mogoplistidae (49)
48b	Habitus different	50
49a	Male pronotum strongly prolonged backwards, broadly rounded, smooth and completely covering the wings; female apterous; habitus as Fig. 153	Ectatoderus spp.
49b	Male pronotum not strongly prolonged backwards; both male and female apterous; habitus as Fig. 161	**Mogoplistes aff. M. brunneus**
50a	Head more or less elongated; body depressed and slender; delicate creatures with a habitus as in Fig. 162; tegmina glassy, long, extending beyond the tip of the abdomen	Oecanthus (51)
50b	Head roundish, broader than long; body cylindrical (Fig. 132)	Gryllidae – Gryllinae (52)
51a	Colour pale straw to whitish, with sometimes greenish tones; two small dark spots on the tegmina Figs 173, 174); apex of the lobe of the hind knee tipped black (Fig. 165B); female cerci > 4.5 mm; ovipositor > 5.0 mm	*Oecanthus chopardi*
51b	Colour warm brown with extensive dark markings on the wing (Fig. 162); lobe of the hind knee dorsally lined black (Fig. 165A); female cerci < 4.0 mm; ovipositor < 4.5 mm	Oecanthus castaneus Felix & Bouwman, sp. nov.
52a	Length > 1.9 cm; large all-black cricket, including the head; only two yellow markings at the base of the tegmina	*Gryllus bimaculatus*
52b	Length < 1.9 cm; colour different; head never wholly black, always with some light, yellowish spots or stripes on the occiput, behind the eyes or around the ocelli; legs light	53
53a	Male tegmina truncated, posterior margin rounded square, reaching halfway through the abdomen; females with tiny, reduced scaly wings; pronotum seen from aside longer than high, with a dark hind margin; head light with a broad dark line between the eyes	*Gryllodes sigillatus*
53b	Male tegmina not truncated, if somewhat shortened, then posterior margin rounded, not rounded square; female tegmina developed; pronotum in lateral view as long as high	54
54a	Larger species, body length more than 14 cm; male tegmina with four harp veins	Gryllinae – Gryllini (55)
54b	Smaller species, body length less than 14 cm; male tegmina with two harp veins	Gryllinae – Modicogryllini (56)
55a	Head yellowishbrown, with broad dark bands on the occiput and between the eyes, with a darkly marked frons; pronotum with a characteristic pattern of dark spots	Acheta cf. domesticus
55b	Head dark reddish-brown to black, with only light median and lateral ocelli and a light spot behind the eye (Fig. 134); pronotum uniformly blackish-brown on the disc, sometimes with a reddish hue and with a light hind margin and lateral lobes with a broadly yellowish margin	*Acheta rufopictus*
56a	Epistomal suture (between frons and clypeus) slightly curved, with an obtuse angle and a rounded apex	*Modicogryllus perplexus*
56b	Epistomal suture strongly curved, forming an almost right angle with a sharp apex (Fig. 144)	*Eumodicogryllus chivensis*
57a	Typical bush-crickets; fully winged, with fore- and hind wings; tympanum present on the fore tibiae	Tettigonioidea (58)
57b	Cylindrical, sandy-coloured, non-jumping crickets with soft, fleshy bodies, covered by black markings; wing-less or with very small vestigial wings as scales on the side of the body; no tympanum on the fore tibiae	Stenopelmatoidea – Gryllacrididae (62)
58a	Huge, stout, unmistakable reddish-brown or green bush-cricket, with broad, rounded and heavily veined tegmina, dotted with little creamy spots and some larger black ones (Fig. 208)	*Pachysmopoda abbreviata*
58b	Habitus different, body more slender and delicate, without black markings	59
59a	Head more or less pointed because of a strongly sloping frons relative to the vertex	Conocephalinae (60)
59b	Head globose, not markedly inclined; frons vertical relative to vertex	Phaneropterinae (61)
60a	Smaller species; body size including wings in males less than 27 mm, in females less than 30 mm; greyish band extending from the frons to the posterior margin of the pronotum	*Conocephalus maculatus*
60b	Larger species; body size including wings always more than 35 mm	**Ruspolia aff. R. basiguttata**
61a	Hind wings shorter than or of the same length as the tegmina; habitus as in Fig. 216	*Phaneroptila insularis*
61b	Hind wings much longer than the tegmina, extending a quarter of their length beyond the tegmina; habitus as in Fig. 213	*Phaneroptera sparsa*
62a	Smaller species; tiny scale-like wings present; head yellow; habitus as in Fig. 198	*Glomeremus mediopictus*
62b	Larger species; wing-less; head with a dorsal black spot	63
63a	Two black markings on the pronotum: one on the anterior, one on the posterior margin, as in Fig. 185	*Glomeremus capitatus*
63b	Two black or reddish markings in the centre of the pronotum, as in Figs 191, 194	*Glomeremus pileatus*

Species accounts

Suborder Caelifera

Acridoidea

Acrididae

Acridinae

Truxalini

Oxytruxalis ensis (Burr, 1899)

Figs 16, 17, 18

References for Socotra.

Burr 1899b: 43–44 [as Truxalis ensis]; Burr 1902: 161–162 [as Acrida ensis]; Burr 1903: 412, 413, 416, plate XXV: figs 4, 4a [as Truxalis ensis]; Krauss 1907: 29 [as Acrida ensis]; Dirsh 1950: 149–151, figs 50, 51; Popov (in Uvarov and Popov (1957)): 384–385; Wranik 2003: 325, plates 153, 158.

Diagnostic notes.

Oxytruxalis ensis is a very slender and elongated species (Figs 16, 17). It differs from Truxalis viridifasciata (Fig. 19) in very long antennae that exceed the combined length of the head and pronotum, an almost flat and strongly elongated pronotum parallel in the prozona, very long tegmina that are gradually narrowed and pointed in the apical half and hind wings that are much shorter than the tegmina (Figs 16, 17) (Burr (1899b, 1902, 1903). The inner upper lateral lobe of the hind knee is elongated and longer than the outer upper lobe (Fig. 17B) (Dirsh 1950; Popov in Uvarov and Popov (1957)).

Figure 16.

Oxytruxalis ensis (Burr, 1899), male. The only known male specimen collected by Kenneth Guichard at Wadi Dineghen in April 1967. Scale bar: 1 cm (photograph Rob Felix).

Figure 17.

Oxytruxalis ensis (Burr, 1899), female, holotype. A, D. Habitus; B. Left hind knee with elongated inner upper lobe; C. Label. Collected by Forbes & Ogilvie-Grant at Moukaradia Pass, Rooget Hill, Socotra in 1898. Scale bar: 1 cm (photograph T. Hunter, WML, Liverpool).

Taxonomic notes.

Burr (1899b, 1903) described the species, based on two female syntypes (Fig. 17). Dirsh (1950) gave a re-description of the genus and species. The male of O. ensis has never been described. A short description of the male is provided here, together with a photo of the only known adult male specimen (Fig. 16).

Description of the male.

Smaller than the female; body length: 39 mm; frons 10 mm; antenna length: 18 mm; tegmen length: 26 mm; hind wing length: 23 mm. The apex of the fastigium tapers more sharply and is less rounded than in the female. The hind wing is relatively longer than in the female: it is only 12% shorter than the tegmen; in females, this is 20% (Figs 16, 17A) (Dirsh 1950). The pronotum is malformed, making it indescribable in detail. The subgenital plate is short and conical with a sharp apex. The general colouration is yellowish-green. The tegmina have longitudinal reddish, brownish and white lines. The basal disc of the hind wing is reddish with a tessellate pattern (Fig. 16). Other characteristics are the same as in the female (see Dirsh (1950)).

Distribution and occurrence.

Oxytruxalis ensis is endemic to Socotra and a very scarce species. It is only known from one adult male, two adult females and a nymph found in the mid-elevations in the Hagher massif and on the surrounding limestone plateau (Fig. 18). There are no records after 1967.

Figure 18.

Distribution of Oxytruxalis ensis (Burr, 1899) in the Socotra Archipelago.

Habitat and biology.

The habitat is unknown. Based on its distribution, the species probably inhabits herbaceous or grassy sites in wood- and shrubland at 250–950 m a.s.l. All known localities, Moukaradia Pass, Wadi Dineghen and Jena-agahan, are in Frankincense woodland. Records are from January, March (a nymph), April and December.

Bioacoustics.

It is unknown if this species emits a calling song. Members of Truxalini are known to possess a stridulatory apparatus and can produce sounds through crepitation by snapping their hind wings during flight (Harz 1975; Haggag and Badawy 2017).