Montana Field Guide

Trichoptera is the largest and most diverse order of insects that is primarily aquatic, with about 13,000 species worldwide (Holzenthal et al. 2007, de Moor and Ivanov 2008). The roots Trichopteran lineages date back to at least the middle of the Jurassic period (Holzenthal et al. 2007). Caddisfly larvae are vital contributors to aquatic food webs and their presence is often used when assessing water quality. Caddisflies are most closely related to Lepidoptera (butterflies and moths), and they share characteristics such as spinning silk. Adult caddisflies are medium-sized insects with tent-shaped wings. They resemble moths, but caddisflies do not have a coiled proboscis and their wings are covered in hairs rather than scales. They tend to be secretive and slow-flying riparian insects (Anderson 1976).

Caddisflies spend much of their life in the water as aquatic larvae and most species build portable, protective cases made from plant material or stones. These cases are incredibly intricate and complex structures, especially for a non-social insect (Holzenthal et al. 2007). Trichopteran larvae have well-developed mouthparts; mandibles of shredders are broad with cutting teeth, while the mandibles of scrapers are more elongated with entire edges. Abdominal gills are present in most species.

Caddisflies typically have five larval instars before pupation (Wiggins 1996). During pupation, the insect’s antennae, legs, and developing wings are free from the body, and characteristics can still be used for identification (Holzenthal et al. 2007). Adult caddisflies are terrestrial and usually dully colored. This order of insects is very diverse and adult body length can range from 1.5mm to 45mm. Unlike Trichopteran larvae, adult caddisflies have reduced mouthparts because they only live from a few days to a couple weeks (Wiggins 1996).

The genus Allomyia is one of 18 genera within the Apataniidae family. This family is found in the northern montane regions of North America, Europe, and Asia (Holzenthal et al. 2007). Apataniid larvae live in cool running waters at high elevations and construct their cases from small rocks. All genera within the Apataniidae family have antennae located between the eye and the anterior margin of the head, and have either single or absent abdominal gills (Wiggins 1996). Allomyia larvae have a flattened head and scraper mandibles with apical edges entire and not subdivided into teeth. The head and pronotum of this genus have a pebbled texture, and the pronotum is convex. Allomyia larvae build a 13mm long tapered, cylindrical case from small rocks. The length of mature Allomyia larvae is about 11.5mm.

Allomyia picoides is 7-13mm long as a mature larvae (Ruiter et al. 2021) and the length of A. picoides pupae measures 7-9mm. Their heads and thoracic notum are blackish-brown, and their mandibles are reddish-brown. Adults are 7-9mm long with 7mm long forewings. Their forewings are covered with short dark brown hairs, while their hindwings are covered with short black hairs. Their legs are brown and have paired short spines on them.

Allomyia picoides is very closely related to A. cidoipes (BC, WA, OR, CA) and A. renoa (NV). They can be differentiated by the three-lobed apical segments of the adult male inferior appendage (Ruiter and Nishimoto 2019, Ruiter et al. 2021). The shape of the medial style of the inferior appendage is massive and contorted apically on A. cidoipes, and blade-like and acute apically ventrally on A. picoides (Ruiter and Nishimoto 2019). Allomyia renoa has a broadly separated narrow curved ventral rami of the apical inferior appendage (most apparent in ventral view) that separates this species from both A. picoides and A. cidoipes.

Allomyia picoides larvae are usually found on vertical rock faces in small, cold mountain streams (Wiggins 1996). Caddisfly adults tend to remain near the emergence site where oviposition occurs. Although dispersal flights are common, they are relatively short and only occur immediately following emergence. Dispersal from emergence sites tends to be negatively correlated with vegetation density (Collier and Smith 1998). In other words, caddisflies tend to disperse shorter distances in dense forest compared with more open areas.

Allomyia larvae graze on periphyton growing on rocks and other substrate in streams (Holzenthal et al. 2007), while clinging and crawling amongst rocks (Sueyoshi et al. 2014). When gut contents of Allomyia larvae were examined, they were largely fine organic and mineral particles, confirming that they graze on the algae on rocks (Wiggins 1996).

Adult caddisflies do not have developed mouthparts and only eat nectar, sap, or nothing during their adult lifespan.

Another Allomyia species in western Colorado (A. gnathos) also builds an armored case and its behavior does not seem to be affected by the presence of trout or predatory stoneflies (Alvarez and Peckarsky 2013). Alvarez (2013) predicted this caddisfly selects for mossy habitats for its protection against hydrological perturbation events instead of predators. They suggest that the caddisfly has a trade-off between protection in the moss and constrained mobility while searching for food.

Both A. picoides larvae and adults have been collected at the same time of year, so it is thought that this species needs more than one year to complete its life cycle (Wiggins 1996). This semivoltine life cycle (more than one year to develop to an adult) is unique because most caddisflies have a univoltine (one-year) life cycle. There have been few collections of A. picoides, but they suggest adult emergence occurs from April 20 to July 12 (Ruiter and Nishimoto 2019). As adults, they use trees as roosting places. The remainder of their lives are spent underwater feeding and growing.

Freshwater aquatic habitats are one of the most imperiled ecosystems globally because its water collects all the abuses in the entire watershed area (Holzenthal et al. 2007). Cold-water invertebrates are thought to be specialists and are sensitive to changes in aquatic habitat (e.g. flow patterns, streambed substrate, thermal characteristics, and water quality) (Green et al. 2022). Forest riparian area are prone to increases in sediment and temperature when the landscape is disturbed, such as road building and timber harvests, and may make these streams less suitable for cold-water invertebrates (Stagliano et al. 2007). Additionally, researchers have begun studying the effects of climate change on Trichopterans in alpine headwater streams (Brown et al. 2007, Holzenthal et al. 2007), but much more research is needed to understand how these insects will respond.