Insect disturbance stridulation: Its Defensive role

Abstract

1. Many insects stridulate when they are handled or attacked. It has been suggested that this disturbance stridulation acts to deter predators. This hypothesis was investigated in a series of experiments. Predators were given insects which had been silenced by disruption of their sound-producing mechanism or else had been sham operated but retained their normal ability to stridulate. 2. Three types of insects (mutillid wasps, Dasymutilla spp.; water scavenger beetles, Tropisternus spp.; and round sand beetles, Omophron labiatus) were given to wolf spiders (Lycosa ceratiola and Geolycosa ornatipes) at night in the field under natural conditions. When attacking silenced insects, spiders displayed greater persistence than when attacking phonic insects (Table 1). In addition, mortality was greater among silenced insects. 3. Spiders (L. ceratiola) were also given an artificial ‘insect’—a vibrating probe whose vibration mimicked that of the cuticle of a stridulating insect. As with real insects, spiders persisted longer in their attack on the probe when it was silent than when it was ‘stridulating”. 4. Female mutillid wasps were given to wild-caught mice (Peromyscus floridanus) in the laboratory. Unsilenced mutillids survived the encounter more often than their silenced counterparts. In another experiment, the stings of mutillid wasps were removed before testing. Mice killed nearly all these ‘unprotected’ mutillids. However, it took mice significantly longer to attack unsilenced mutillids and longer to kill them after initiating the assault (Table 2). 5. These results support the view that insect disturbance stridulation deters predators. Two modes of action by which these sounds may have their effect are discussed: they may serve to startle the attacker or they may alert it to the potential harmfulness of the insect and as such may qualify as an example of acoustic aposematism.