Probing stimulants

Probing occurs in response to the quality and quantity of host-related stimuli (Friend and Smith, 1977). As in the other phases of host location, the response is not performed in a completely stereotyped way. Anyone who has slept under a mosquito net is likely to have had firsthand experience of this flexibility of responsiveness. If the skin becomes pressed against the net, mosquitoes are quite happy to probe and feed through it. The set of stimuli received in these circumstances must be quite different from the range of host-related stimuli that an insect landing directly on the skin would normally receive. The new set of stimuli received after landing can still influence host choice even at this very late stage, with insects choosing to leave rather than feed (Gikonyo et al., 2000). Post-landing responses can also vary with internal changes of circumstance such as the insect's degree of hunger (Brady, 1972; Brady, 1973; Friend and Smith, 1975) or water deprivation (Khan and Maibach, 1970; Khan and Maibach, 1971), feeding experience (Mitchell and Reinouts van Haga, 1976) or reproductive state (Tobe and Davey, 1972). Even if internal and external factors are carefully controlled, different individual insects still show a considerable degree of innate variation in their response to a host (Gatehouse, 1970). This flexibility of close-range responsiveness allows the insect to make the most of the differing circumstances in which it contacts hosts.