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 of 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).

Pupae and late-instar larvae were collected in June, so adults may be flying from late-June to July.

Rossiana is one of two genera within the family Rossianidae, and each genus contains only a single species (Rossiana montana and Goereilla baumanni) (Holzenthal et al. 2007). The Family Rossianidae is closely related to Goeridae and Uenoidae. Rossiana montana larvae are =9mm long, have long, single gills, and a uniquely shaped head and pronotum that are red-brown and coarsely pebbled (Wiggins 1996). Their antennae are midway between their eye and the back of their head, and they have an unusually wide separation between the sclerites along their mid-dorsal line. Larvae have toothed mandibles, a tuft of stout setae (hairs), and a peg-shaped sclerite beneath their head (Wiggins 1996). Rossiana montana has longer basal setae on its middle and hind tarsal claws than Goereilla baumanni.

Rossiana montana builds larval cases that are =10.5mm long from small rocks. These cases are tubular, slightly curved, and taper towards the back (Wiggins 1996). Larvae have a concave pronotum on each side and a head that acts as a flange. Little is known about the morphology of Rossiana montana adults .

Larval drift and adult movements not studied in Montana.

Rossiana montana larvae are found in small, cold, mountain streams. Their occurrence is highly restricted, but they are sometimes found in multiple microhabitats within a stream (Wiggins 1996, Holzenthal et al. 2007). This species is most often found in gravel under moss at the edge of streams or on vertical rocks that have a thin layer of water on them.

Based on gut content examination (Wiggins 1996), Rossiana montana feeds on woody materials and fungi. It is classified as a scraper and a shredder (Morse 2009). Adult caddisflies usually do not have developed mouthparts and only eat nectar, sap, or nothing during their adult lifespan.

The trophic relationship of larvae of R. montana includes scrappers and shredders (eating detritus & larger plant materials) (Merritt and Cummins 1996, Wiggins 1996).

Little is known about R. montana, but this species is likely spending most of its life feeding in a stream as a larva before emerging as a winged adult in the late summer. Most caddisflies have a univoltine (one-year) life cycle. Rossiana montana collections indicate they likely have a univoltine life cycle due to pupae and larvae of the last three instars being found in June (Wiggins 1996). This means they would spend the autumn through early summer as larvae. Late instar larvae secure their cases to rocks, seal off the case’s entrance, and undergo metamorphosis (Wiggins 1996). Adults emerge several weeks later in late summer (Beam and Wiggins 1987, Wiggins 1996).

More research should be done on the life history of this species because it is also possible for this species to be semivoltine. In intermittent streams, some Trichopteran species sustain pre-pupal diapause until after summer drought, thus delaying adult emergence and breeding (Wiggins 1973, 1996). Larvae can survive up to six months during periods of high-water temperatures or reduced flow using this method. Other species undergo the same process if the growing season (e.g., high altitude mountain streams) is too short. As adults, they use trees as roosting places and live for a few weeks.

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. As adults, they use trees as roosting places.

Rossiana montana has been described as a rare species due to habitat specificity and is never abundant when collected (Wiggins 1996, Stagliano et al. 2007). It has no USFWS status at the present time, although it is currently a USFS Species of Concern (SOC). It is ranked globally at risk/potentially at risk (G2G3) by Natureserve (2015), and at risk (S2) in Montana, although not yet reported for Idaho.

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 need to understand how these insects will respond.