The Significance of Case Material Selection for the Survival of Caddis Larvae

Abstract

SUMMARY (1) Young larvae of Potamophylax cingulatus construct their cases of leaf discs, but as the larvae grow, the material used to construct the cases changes to pieces of bark and then to mineral particles. The relative amount of secretion used for the construction of the leaf cases was low compared to that used for cases of older larvae. As the larvae grew and the cases changed, there was an increase in the pressure which the cases could withstand before collapsing. (2) Larvae with leaf cases were more often taken by trout when they were on a sandy bottom than when they were on a leaf bottom, while the predation on larvae with mineral cases was the same, irrespective of bottom type. Larvae inhabiting natural leaf cases were more vulnerable to trout predation when the portions of the cases extending beyond the tube inhabited by the larvae were cut off. Larvae with mineral cases suffered a much lower predation than did those with leaf cases. (3) Information from South Sweden and North America supported the hypotheses that: (a) most summer species make mineral cases whilst autumn species preferably construct organic cases; (b) the final larval length of mineral case makers is small compared to that of organic case makers, and (c) the tendency to use case enlargement is most pronounced among the organic case makers. These adaptations could all be related to the selection pressure by predation.