THE IMPORTANCE OF MUCUS AS A DEFENCE AGAINST CARABID BEETLES BY THE SLUGS ARION FASCIATUS AND DEROCERAS RETICULATUM

Abstract

The effect of mucus exudation on the survival of Arion fasciatus and Deroceras reticulatum was studied by exposing mechanically stimulated and control slugs to Carabidae beetles for 24 hours. A light stimulation, lasting three minutes, exhausted the copious flow of mucus for one day. A generalist, Pterostichus niger, significantly preferred stressed D. reticulatum over control ones. Similarly, P. niger exclusively ate stressed individuals of A. fasciatus. Two beetles which specialize in gastropods, Cychrus caraboides and Carabus violaceus, consumed an equal number of stressed and control D. reticulatum and A. fasciatus. The susceptibility of the slug species to predation was different: for each beetle species, the proportion of available D. reticulatum preyed upon was significantly higher than that of A. fasciatus. The differences in the behaviour of A. fasciatus and D. reticulatum in defending themselves against attacks is described. The main reason why specialist beetles are able to hunt slugs successfully is their ability to prevent the slugs from exuding large amounts of mucus. This may succeed by different means: C. violaceus targeted their killing strokes against the posterior part of the mantle while C. caraboides hit the head of the slug. In both case the strokes seemed to paralyze the slugs.