Acroceridae have rarely been recorded from Taiwan in the past. To our knowledge, only four named species have been cited so far, all of them being considered endemic to the island (Chung and Shao 2025). Breeding records of Acroceridae are largely based on incidental findings by arachnologists, collecting subadult spiders and taking them into the lab in order to breed them to adulthood for unambiguous species identification. Most Acroceridae larvae develop as endoparasitoids in the opisthosoma of true spiders (Araneae), leaving them shortly before or after the spider undergoes its final moult. Known exceptions comprise the Neotropical Sphaerops appendiculata Philippi, 1865, developing in an ectoparasitic mode on Ariadna maxima (Nicolet, 1849) (Schlinger 1987). Mature larvae often seem to induce an aberrant web-building behaviour in the hosts previous to their emergence, the resulting web serving the acrocerid as a “cradle” or shelter during pupation (Winterton and Barraclough 2017). A synopsis of the life history of Acroceridae is provided by Schlinger (2003).

On 10 May 2024, several subadult specimens of the huntsman spider Gnathopalystes taiwanensis Zhu & Tso, 2006 were collected in Yangmingshan, northern Taiwan (25°7.976658'N, 121°32.141052'E) by the senior author. The spiders were kept in the lab for rearing on moistened wet wipes and leaves and at 28 °C with a light cycle of approximately 10 hours of light and 14 hours of darkness. All except one parasitised specimen developed to adulthood and were identified via the morphology of the genitalia using Zhu and Tso (2006). The identification of the acrocerid fly was achieved with Winterton and Gillung (2012).

After emergence of the acrocerid larva from the spider, the host and each larval development stage were documented with a Panasonic Lumix DMC-LX5 digital camera, with subsequent processing of the images using ImageJ. Micrographs of the reared Ogcodes sp. were taken with a Zeiss Axiocam 712 colour, equipped on a Zeiss AXIO Zoom.V16.

The adult Ogcodes sp. is preserved in ethanol at Senckenberg Natural History Collections Dresden, whereas the spider remains are preserved dry and are deposited at the Systems and Evolutionary Biology Laboratory, Department of Life Science, National Taiwan Normal University.

The type specimen of Ogcodes taiwanensis Schlinger, 1971 is currently placed in the collection of the late E. I. Schlinger housed at the California Academy of Sciences Entomological Collection in San Francisco (courtesy of Christopher C. Grinter), but is in the process of being returned to the Institute for Agro-Environmental Sciences, NARO, Tsukuba (formerly National Institute of Agricultural Sciences, Tokyo).

The systematics of the almost cosmopolitan genus Ogcodes Latreille, 1796 is poorly understood. Currently, 115 species are being considered (Winterton and Barraclough 2017), making Ogcodes the species-richest genus of Acroceridae (Nearctic: 17 (Schlinger 1981); Neotropic: 9 (Schlinger 1960); Afrotropic: 10 (Winterton and Barraclough 2017); Palaearctic: 27 (Nartshuk 1988); Oriental: 16 (Schlinger 1960, 1971); Australasia: 23 (Schlinger 1960); Polynesia: 3 (Schlinger 1960)). The latest world revision was presented by Schlinger (1960) who later described six additional Oriental species, including O. taiwanensis (Schlinger 1971). Differentiation of the individual species is largely based on external morphological features which might show some degree of variability. So far, phenotypic plasticity regarding body size and colouration within a single sex of a species of Ogcodes has never been investigated in detail, but a study on Acrocera orbiculus (Fabricius, 1787) showed that the observed variability can be very large, potentially leading to an oversplitting of species (Kehlmaier and Almeida 2014). For these reasons, we currently refrain from officially describing and naming the reared male of Ogcodes. Instead, we hope our finding will encourage additional collecting efforts in Taiwan in order to increase the available material at hand. This should best be achieved by a mixture of collecting techniques, such as Malaise trapping, hand netting and by rearing of possible host spiders.

So far, host data for more than 60 Acroceridae taxa (about 10% of the world fauna), recorded from 25 spider families, have been gathered (Gillung and Borkent 2017). Including the current observation, members of Ogcodes have been reared from 16 families of Araneomorphae: Agelenidae, Amaurobiidae, Anyphaenidae, Araneidae, Cheiracanthiidae (= Eutichuridae), Clubionidae, Desidae, Gnaphosidae, Lycosidae, Oxyopidae, Philodromidae, Salticidae, Sparassidae, Theridiidae, Thomisidae and Trachelidae (Gillung and Borkent 2017). This listing results from 95 acrocerid-spider interactions summarised or observed by Gillung and Borkent (2017), Yuan et al. (2019), Abhijith et al. (2022), Hagopián and Laborda (2023), Kehlmaier et al. (2024) and Shih and Kehlmaier (present study).

Acroceridae have rarely been recorded from Taiwan in the past. Only four named species from three subfamilies have been cited, all of them being considered endemic to the island: Ogcodinae—Ogcodes taiwanensis Schlinger, 1971; Cyrtinae—Hadrogaster formosana (Shiraki, 1932); Nipponcyrtus taiwanensis (Ôuchi, 1938); Philopotinae—Oligoneura takasagoensis (Ôuchi, 1942). In addition, Schacht (2010) recorded eight unidentified acrocerid flies from various localities. Six of these flies are currently deposited in the Bavarian State Collection of Zoology (Zoologische Staatssammlung München) and belong to Hadrogaster formosana (one specimen, Meifeng, N-Nantou County, Road No. 14, NE Puli, Meifeng Experimental Farm, ca. 24°06'N, 121°10'E, 2200 m a.s.l., 9–11 Sep. 2002) and Oligoneura sp. (one specimen, Fushan Botanical Garden, ca. 24°46'N, 121°35'E, 650 m a.s.l., 7–9 May 2001; four specimens, Tengir, Kaohsiung County, Jungyang Mountains, Tengir Endemic Species Research Institute, ca. 23°07'N, 120°47'E, 1700 m a.s.l., 14–19 Sep. 2002). It can be assumed that the true diversity of this Diptera family in Taiwan is considerably higher than currently known.