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

The effect of mucus exudation on the survival of Arion fasciatusand Deroceras reticulatum was studied by exposing mechanicallystimulated and control slugs to Carabidae beetles for 24 hours.A light stimulation, lasting three minutes, exhausted the copiousflow of mucus for one day. A generalist, Pterostichus niger,significantly preferred stressed D. reticulatum over controlones. Similarly, P. niger exclusively ate stressed individualsof A. fasciatus. Two beetles which specialize in gastropods,Cychrus caraboides and Carabus violaceus, consumed an equalnumber 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. reticulatumpreyed upon was significantly higher than that of A. fasciatus.The differences in the behaviour of A. fasciatus and D. reticulatumin defending themselves against attacks is described. The mainreason why specialist beetles are able to hunt slugs successfullyis their ability to prevent the slugs from exuding large amountsof mucus. This may succeed by different means: C. violaceustargeted their killing strokes against the posterior part ofthe mantle while C. caraboides hit the head of the slug. Inboth case the strokes seemed to paralyze the slugs. (Received 31 March 1993; accepted 1 October 1993)