Ecology of Two Related Species of Caddis Fly Larvae in the Organic Substrates of a Woodland Stream

Two species of Pycnopsyche larvae inhabiting allochthonous organic materials in West Creek are contemporaneous and similar in size. Field studies and laboratory experiments show that about 90% of P. gentilis larvae live in fallen leaves, which they use as food and for case materials; the remaining 10% occur in detritus with P. luculenta. P. luculenta can utilize leaves or woody materials: 50% of the population is in detritus, and 50% is in more leafy habitats, but not far from detritus. The distribution of each species from September through March depends on the amount of preferred habitat space available. Larvae in leafy habitats are washed downstream during the spring thraw by habitat displacement. In 1971, two—thirds of the P. gentilis population and nearly half the P. luculenta population were displaced. The subsequent shortage of leafy habitat space and crowding of remaining larvae may result in competitive interaction and predation by grackles. In the laboratory, both species feed preferentially on leaves that decay quickly, especially on leaves infected with fungi. Dry weights of leaf tissue ingested per larva in 24 hours frequently exceed 50% of the dry body weight of the larva. P. luculenta ingests less leaf material if twigs are available. The niche size of P. gentilis is smaller than that of P. luculenta. There is some niche overlap, but it apparently does not normally result in harmful competition between the species.