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).

Philocasca banksi is a species within the family Limnephilidae, the largest family of Trichoptera in North America with 41 genera there. This family dominates at high latitudes and elevations. All Limnephilid larvae have a characteristic prosternal horn and their antenna are uniquely midway between their eyes and the front edge of the head capsule (Wiggins 1996).

Philocasca banksi is a species within the subfamily Stenophylacini. The larvae in this subfamily are similar to larvae within the subfamily Dicosmoecinae, but Stenophylacini usually has less setae (less hairy). Stenophylacini has fewer mesonotal setae, less setae on the dorsal sclerite of segment 9, and the ventral edges of the middle and hind femora have two major setae (Wiggins 1996). Stenophylacini larvae have single abdominal gills (gills do not branch) and usually live in cool lotic waters. Most species build their larval cases from plant material, but some build their cases from small rocks.

The genus Philocasca has seven species that all occur in western North America. Larvae measure =18mm long, are reddish brown, and have single abdominal gills that often vary in arrangement between species (Wiggins 1996). The tops of their heads are flattened with a prominent ridge, and the pronotum has a roughened texture. The larvae in the genus Philocasca have characteristically enlarged and flattened scale-hairs along the front edge of their pronotums. Philocasca larval cases are =23mm long, made from rock fragments, and are built cylindrically with a slight curve.

Unlike most yellow-brownish Limnephilid caddisfly adults, Philocasca banksi individuals are almost entirely dark brown. Adult Philocasca banksi can be distinguished from other Philocasca species by their tenth segment, as this species has a pair of short stout lobes (in dorsal view) on this segment that other Philocasca species do not have (Wiggins and Anderson 1968).

Adult female P. banksi are not known, but adult males are known to be entirely dark brown with 15mm long forewings (Wiggins and Anderson 1968). Larval P. banksi are not well-known and more research must be done to reliably distinguish this species from other Philocasca species.

Philocasca larvae occur in small mountain springs, usually in the gravel beneath larger rocks in the streambed.

Curiously, all larval instars of one Philocasca species in Oregon (P. demita) were collected in soil and leaf litter =6m from the nearest spring stream (Anderson 1967, Wiggins 1996)! This is the first North American caddisfly species that was known to be entirely terrestrial. Philocasca demita larvae have been collected in pit fall traps and Berlese funnels, but none were collected in the nearby stream.

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.

When timber harvest and forest regeneration is concerned, many studies have found that macroinvertebrate densities decrease as forest stand age increases. However, young stands only have a higher dominance of a few tolerant taxa (clams, scuds, and non-biting midges; (Cole et al. 2003). Older stands show lower dominance of tolerant taxa likely because stream conditions are more stable, thus allowing the abundance of sensitive taxa, like caddisflies, to increase.

Philocasca larvae are shredders, meaning they reduce larger pieces of plant debris to small particles, although the fungi on the dead plant material provides most of their nutrients (Wiggins 1996). Mostly vascular plant pieces were found in the gut contents of Philocasca larvae confirming they are primarily shredders (Wiggins 1996). Nevertheless, Philocasca larvae are sometimes found on the bodies of dead animals, likely attracted to the microorganisms contributing to decomposition (Wiggins 1996).

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

Little is known about the life cycle of P. banksi and more surveys should be done to gain a better understanding of the emergence timing of this species. Most caddisflies have a one-year life cycle (univoltine), but some species may need more than one year to fully develop (semivoltine). Additionally, some species that are univoltine in lower elevation temperate streams may be semivoltine at higher latitudes or elevations because the growing season is too short for larvae to fully develop (Giersch 2002).

After pupation, P. banksi transitions from the aquatic to the terrestrial environment. Most caddisflies emerge in late summer, but semivoltine species can emerge at different times of the year.

Caddisfly adults tend to remain near the emergence site where oviposition occurs (Collier and Smith 1998). 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. In other words, caddisflies tend to disperse shorter distances in dense forest compared with more open areas.

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). Forest riparian areas 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.