Grain size selection in case building by the mountain cased-caddisfly species Potamophylax latipennis (Curtis, 1834): a trade-off between building time and energetic costs.

Abstract

Many caddisfly larvae build cases to facilitate breathing, provide physical protection, reduce predation, avoid becoming drift, or prolong survival during drying conditions. Case building also requires significant energetic costs related to grain searching and silk production, which may involve a trade-off with the size of grain used. Thus, building cases with large grain sizes would require less time (i.e., a trait related to survival) but higher silk production (i.e., a trait related to fecundity), whereas building with small grain sizes would show the contrary pattern. Grain size selection, time spent, and energetic costs related to case building were assessed on the Limnephilid species Potamophylax latipennis. Laboratory experiments were conducted in order to force individuals to build using seven different experimental conditions with varying grain size availability. Results showed a trade-off between time and energetic costs. P. latipennis prioritized building cases with grain sizes that provide a faster building although they used larger amounts of silk. In addition, when individuals were first forced to build a case using a unfamiliar substrate and then placed in the native (i.e. from the river) substrate, most unfamiliar grains where replaced by native ones, even though it represented an extra cost for the individuals. Despite the high energetic costs of building cases in Trichoptera and their potential implications for reproductive traits in the adult stage, larvae individual survival was prioritized.