Montana Field Guide

Trichoptera is the largest order of insects that is entirely aquatic, with over 12,600 species worldwide (de Moor and Ivanov 2008). Caddisflies are most closely related to Lepidoptera (butterflies and moths), and they share characteristics such as spinning silk. Rhyacophilids make up one of the largest genera of Trichoptera and are some of the most primitive caddisflies.

Caddisflies spend most of their life in the water as aquatic larvae and most species build portable, protective cases made from plant material or stones. Most caddisflies either filter small particles from the water by building nets spun from silk or shredded organic matter (e.g., leaves) into smaller pieces. The genus Rhyacophila is a unique group because they do not build portable cases and are mainly predators. Not all Rhyacophila have gills; those lacking gills absorb oxygen through their skin and thus require cold, oxygen-rich, fast-flowing water to breathe.

Caddisflies typically have five larval instars before pupation. Despite larvae being free-living, Rhyacophila construct a shelter of small stones tied together with silk for pupation, and the pupa uses its mandibles to break through the case and emerge (Anderson 1976). Adults can live anywhere from a few days to several weeks.

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). Rhyacophila usually inhabit cool mountain streams and have small distributions, often restricted to only one or two mountain ranges (Anderson 1976).

Larval Rhyacophila alexanderi are about 23mm long in their final instar (Giersch and Wisseman 2012). Adult Rhyacophila alexanderi have a unique pair of setose warts on their abdomen known in only one other species (R. ardala).

Rhyacophila alexanderi larvae closely resemble some species of the betteni group of Rhyacophila, but R. alexanderi’s head shape is approximately subequal in length and width. In contrast, larvae of the betteni group have a head that is closer to 1.5 times as long as it is wide. Rhyacophila alexanderi has a dark brown head that is lighter around the eyes. Their right mandible (dorsal view) has two apical teeth and one small mesal tooth, while their left mandible has a single apical tooth, a smooth mesal margin, and a basal tooth on the ventral cutting surface (Giersch and Wisseman 2012). Giersch and Wisseman (2012) provide a key to identify larval Rhyacophila.

Adult R. alexanderi are about 9mm long, have dark grey fore-and-hindwings with no markings, and a black head and body (Denning 1950). Their antennae and palpi are also dark, but their legs and spurs are tawny. Most unique characteristics that distinguish this species from other Rhyacophilids can be seen in the male genitalia. Males have short, flattened trapezoidal claspers with a dorsal angulation together and a unique tenth tergite. For a more detailed description of distinguishing features that occur in the male and female genitalia, see Denning 1950.

Rhyacophila alexanderi only occurs in cold-water streams and prefers to inhabit small seeps often living under moss mats (Giersch and Wisseman 2012). Little is known about exactly what water temperatures this species inhabits; however, another Rhyacophila species Rhyacophila vao was recorded in a stream with an average annual water temperature of 3.7?. Additionally, R. vao was found to not pupate until stream temperature exceeded 3? (Dixon and Wrona 1992).

More sensitive aquatic insects (mayflies, caddisflies, and stoneflies) are usually found in areas with less deposited sediment, while the abundance of tolerant aquatic invertebrates (fingernail clams, scuds, and non-biting midges) increases with the percent of fine sediment in the stream (Waters 1995, Cole et al. 2003).

Many studies have found that macroinvertebrate densities decreased as forest stand age increased; however, these young stands exhibit higher dominance by just a few taxa that are tolerant to disturbance due to increased solar radiation increasing primary production (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.

Larval rhyacophilids are predaceous, but their feeding patterns change as they grow. Larvae in early instars tend to have a diet dominated by moss, diatoms, and detritus, and eat more animal material after the third instar. Later instars of Rhyacophila mainly eat Chironomidae (non-biting midge) and other small fly larvae, Ephemeroptera (mayfly), Plecoptera (stonefly), other Trichopterans, Copepoda (crustacean), Acari (water mite), and even fish eggs (Thut 1969, Dudgeon and Richardson 1988). Rather than engulfing their prey whole, larval Rhyacophila only consume the soft parts of their prey, discarding tougher structures like legs and head capsules (Giersch 2002).

Little is known about the diet of adult Rhyacophila, but other adult caddisflies do not have developed mouthparts and only eat nectar or sap from plants during their short time as adults.

Rhyacophila species are often sympatric, with several species occurring together at one site (Giersch 2002).

The lifecycle of R. alexanderi has not been studied. Most caddisflies have a one-year life cycle, though some species reproduce more than once per year and others require two years to fully develop. Some species that are univoltine in lower elevation temperate streams may be semivoltine (more than one year) at higher latitudes or elevations where the growing season is too short for larvae to complete development in one year (Giersch 2002). After a short pupation phase, Rhyacophila transitions from the aquatic to the terrestrial environment. Adult R. alexanderi emerge in late summer or early autumn (Martin 1985, Hrovat and Urbanic 2012). The remainder of their lives are spent mating and reproducing. Adult caddisflies lay their eggs in or near water, either as strings surrounded by a cement-like matrix or as gelatinous masses (Anderson 1976). As adults, they use trees as roosting structures.

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.

R. alexanderi has been described as a rare species due to few reported collections, habitat specificity and it is never abundant when collected (Wiggins 1996). It has no USFWS status at the present time, although it is currently a USFS Species of Concern (SOC); ranked globally rare/uncommon (G2) by Natureserve (2015), and ranked S2 in Montana.

There are no known specific threats to Rhyacophila alexanderi, but threats to other populations of Rhyacophila in Montana include sediment and temperature increases resulting from road building, timber harvests, and other disturbances in forested riparian areas (Stagliano et al. 2007). Global climate change is also predicted to pose a threat to R. alexanderi and other cold-water, mountain stream insects.