Among Australian Trichoptera, the family Hydroptilidae is the second largest family in terms of species richness, numbering some 158 described species. Currently, these are referred to 15 genera. In a conference paper presented in 2015, Wells and Johanson (2016) discussed possible origins of the Australian hydroptilid fauna, postulating Oriental origins for many taxa, and Gondwanan affinities for others. One of the genera suggested to have Oriental origins is the genus Orthotrichia Eaton, 1873. Such taxa are thought to have arrived in Australia when the continent moved to close proximity with the Eurasian plate some 5.3 to 1.6 mya, probably before the land bridge between New Guinea and Australia was completely disrupted around 10,000 years ago.

When Marshall (1979) reviewed the genus Orthotrichia she noted that no members of the genus were known from “… Central or South America or the Australian region”. Even today only six species of Orthotrichia are known from the Neotropical Region (Harris and Davenport 1999; Harris and Rasmussen 2019; Morse 2022). In contrast, Australia’s 55 species represent about 20% of the world Orthotrichia species. Furthermore, with around 27 species described from New Guinea (three of which are also recorded from northern Australia), and one species from Vanuatu, the fauna of the Australian Region represents close to 30% of the world’s Orthotrichia fauna of just over 270 species (Morse 2022). The genus is also rich in species in sub-Saharan Africa (Wells and de Moor 2020).

Marshall (1979) described Orthotrichia as a highly specialized and very successful genus and commented that it “… is the only hydroptilid genus which has a truly tropical component …”. Although probably other similar genera are now known, the thesis of a “truly tropical component” is supported by the currently known distribution of species in the Australian Region, namely the rich New Guinea fauna and the fact that a majority of Australia’s species are described from the warmer and wetter northeast of the continent (Cartwright et al. 2013, unpublished conference report); no Orthotrichia species have been recorded from the southwest and only one species is known from Tasmania.

What has made this genus so successful, in the sense of species richness, especially in the warmer, wetter northern regions of Australia (and in New Guinea)? Is it just the richness of suitable habitats? Immature stages of Australian Orthotrichia species are generally found in streams with moderate to swift flow over rocky substrates. This contrasts with the (putatively) slow-flowing streams or lentic habitats from which Northern Hemisphere species were reported by Ross (1944—North America) and Nielsen (1948—northern Europe), habitats also attributed to the genus by Wiggins (1977, 2004) and Marshall (1979) and recently recorded by Harris and Rassmussen (2019—Florida). In comparison with the abundance of Australian species, such as some Hellyethira and Oxyethira species, that are often abundant in light collections, most collections of adults of Australian Orthotrichia species have been small—generally few specimens of any species are collected at any one time, generally taken only at lights or in small Diptera-type suction traps; rarely have they been collected using a sweep net, or Malaise trap. Most Australian species are known from few sites, some from only the type locality. Only a few northern Australian species have been collected as adults beside still-water bodies (billabongs) and sometimes these have been taken in quite large numbers. With much searching a few larvae of two species have been found in these lentic systems (Wells 1991).

Wells (1985) suggested that the toothed labrum of larvae could have pre-adapted them for a shift to feeding on eggs. Was it significant that Nielsen (1948) noted that when offered a choice of algal filaments, O. costalis selected the tougher, thicker walled filaments? Has that some significance to a shift from feeding on algal cell contents? Could a radiation of Orthotrichia species have been influenced, at least in part, by a switch in feeding habits to a readily available and rich source of larval food? This is pure speculation, but feasible with the incursion of Orthotrichia into Australia post-dating a very extensive Australian-Region radiation of the Gondwanan family Hydrobiosidae, many of which produce ‘jelly spare’ egg masses (Bovill et al. 2014). The other oddity, mentioned above, is the parasitoid habit of the known final instar larvae of the aberrans-group (Figs 15, 16; Wells 1992, 2005) also parasitising other caddisflies.

Have adaptive shifts to these specialist predatory larval-feeding niches been responsible for the quite extensive radiation of the genus in the Australian Region or at least been a major contributing factor? Is it significant that tropical Africa is also rich in Orthotrichia species, and it is from there that predatory behavior has also been reported?

Larvae of the two Australian species collected from lentic waters have curiously specialized cases, suggesting that their adoption of the lentic niche could be secondary. These species are found in billabongs in the seasonal monsoon area of the north of Australia and their cases are equipped with a pair of small dorsal vents. Billabongs are anabranchs of rivers and, when temperatures are high towards the end of the wet season and later in the ‘build up’ season, the waters in these shallow macrophyte-rich lakes are often low in oxygen (Walker et al. 1984). The unusual vents in these cases (Figs 5, 6) presumably aid circulation of water in the case (Wells 1985), and hence obviate the total reliance on diffusion of oxygen across the case wall, probably being especially important in the pupal stage. The case adaptations have probably developed independently in the two species as, based on male genitalia, they are assigned to two separate species groups. (Similar vents are seen in cases of a New Caledonian hydroptilid species, Caledonotrichia illiesi Sykora).

However, in the several aberrans-group species for which immatures have been associated, the cases and the pupae are distinctive (Figs 15–17): the cases are very flimsy and transparent and the pupae bear varying-sized ‘beaks’ on the head (Fig. 17). The final cases are protected within the pupal cases of the host and putting less energy into case construction could be adaptive; Wells (1992) suggested that the pupal ‘beak’ probably assists the pharate adult to break out of the coarse sand-grain pupal case of the host.

While adaptive shifts by larvae could have led at least in part to the radiation of Orthotrichia in the Australian Region, as with many hydroptilids, it is the distinctive male genitalia that attract one’s notice. Our species discrimination of hydroptilids is based for the main part on features of male genitalia. In the genus Orthotrichia, these male structures are often highly complex, and, as noted above, distinguishing features can be recognized among the few associated females.

In contrast to some of the diurnally active Trichoptera species, e.g., some leptocerids (Gullefors and Petersson 1993), the hydropsychid genus Macrostemon (Silva and Paprocki 2020) and some hydroptilids (e.g., Wells 2002), no evidence of any pre-mating courtship behavior has been reported or observed (by the author) in Orthotrichia species. The complexity of male genitalic structures in Orthotrichia species immediately suggests the operation of sexual selection involving a ‘lock and key’ mechanism. Evidence is accumulating in some non-Trichoptera groups (Ah-King 2014) on the matching complexity of female structures to male. As yet only few Trichoptera studies have associated females and males and certainly, as noted above, very few for Orthotrichia species.

Hypotheses of sexual selection involving lock and key mechanisms are not particularly popular (see Simmons 2014). Most recent hypotheses invoke sensory lock and key systems (Masly 2012), rather than purely mechanical systems, since the non-intromittent structures (i.e. the ‘secondary genitalic structures’ sensu Sloan and Simmons 2019) are the more elaborate parts, and are presumed to require a precise fit.

In males of most species of Orthotrichia the inferior appendages are generally so greatly reduced that they appear to be unsuited to perform the more usual clasping function. Perhaps the often berry-like complex formed by the inferior appendages and their dorsal bilobed processes is involved in sensory stimulation of the female, while other structures could hold the male abdomen in place during copulation.

The simple almost invariant form of the intromittent organ (Figs 18, 20, 25, 28, 30, 32) contrasts with the phallus of many other hydroptilid genera, in which the phallus is often equipped with hooks or spines. Such structures have been demonstrated in some non-Trichoptera groups to play a role in displacement of rival sperm from the female tract (Córdoba‐Aguilar and Cordera Riviera 2006). However, in Orthotrichia species, the ‘paramere’ could play a similar role and the cup-shaped apex could also function in spermatophore removal as postulated in an anisolabidid earwig by Kamimura (2000).

Instability among genes controlling development of male genitalic structures, leading to sensory or mechanical modifications and consequent incompatibility, thence reproductive isolation, could explain, again at least in part, the radiation of the genus (Wojcieszek and Simmons 2013).

So much of this is speculative. I hope it provides a stimulus for others to explore the life histories, distributions, morphological adaptations, and behavior of Trichoptera.