Visual recognition of the host in the parasitoid fly Exorista japonica.

We presented the tachinid fly Exorista japonica with moving host models: a freeze-dried larva of the common armyworm Mythimna separata, a black rubber tube, and a black rubber sheet, to examine the effects of size, curvature, and velocity on visual recognition of the host. The host models were moved around the fly on a metal arm driven by motor. The size of the larva, the velocity of movement, and the length and diameter of the rubber tube were varied. During the presentation of the host model, fixation, approach, and examination behaviours of the flies were recorded. The fly fixated on, approached, and examined the black rubber tube as well as the freeze-dried larva. Furthermore, the fly detected the black rubber tube at a greater distance than the larva. The rubber tube elicited higher rates of approach and examination responses than the rubber sheet, suggesting that curvature affects the responses of the flies. The length, diameter, and velocity of host models had little effect on response rates of the flies. During host pursuit, the fly appeared to walk towards the ends of the tube. These results suggest that the flies respond to the leading or trailing edges of a moving object and ignore the length and diameter of the object.