The role of the triommatidium in the visual behavior of apterous Aphis Fabae Scop

Abstract

The triommatidium is a group of three ocular facets, mounted on a tuberole and appended to the postero-lateral part of the compound eye in most aphids. It is derived from compound eye facets, differentiates precociously in the embryo and appears to act as a functionally independent receptor in the adult. Reorientation and probing in apterous, female A, fabae was examined on the fla1 and on wax ridges. To reach a black pillar on the flat, nymphs and adults less than 90 mins. old followed sinuous paths, adults over 12 hrs. old followed simple curved paths and adults between 3 and 8 hrs. old followed paths which partially orbited the pillar, without ever reaching it. Extirpation of the triommatidium prevented the paths previously typical of nymphs and adults under I2 hrs. old and it was assumed that stimulation of the triommatidium was involved in the expression of these behaviour patterns. Probing was apparently attributable to a critical angle subtended at the compound eye by an image dead ahead and the same response was seen in aphids walking , along ridges. Aphids on ridges made abrupt turns and climbed down from the ridge when the angular acceleration of an image across the compound eye reached a critical value. The movement of an image into or out of the triommatidial field of view apparent17 elicited probing. It was inferred that the compound eyes and the triommatidia comprise functionally separate visual systems which mediate different responses in walking aphids when simplified visual situations are presented under controlled conditions. A "compromise" response apparently occurs in adults between 3 and 8 hrs. old which leads to the avoidance of visually salient features in the aphid's vicinity and may contribute towards dispersal behaviour in young adult apterae.