The Location of the Photoperiodic Receptors in the Aphid Megoura Viciae Buckton.

Although the photoperiodic reactions of insects and mites have been studied quite extensively (Lees, 1955; de Wilde, 1962), the site of the photoreceptors is still unknown. The light pathway does not seem to involve the eyes. Thus Tanaka (1950) observed that the photoperiodic control of diapause in the silkworm Antheraea pernyi was not affected by cauterizing the lateral ocelli of the 4th-instar larva—a treatment that resulted in the complete disappearance of these organs in the following instar. De Wilde, Duintjer & Mook (1959) have shown that adult Colorado beetles (Leptino-tarsa decemlineata) respond normally to day-length after the compound eyes have been covered with black paint or destroyed with a cautery. Less progress has been made in identifying the regions of light sensitivity. Geispits (1957) was able to fit an opaque hood over the head of a Dendrolimus larva which was otherwise-continuously illuminated. With this covering in place for 12 hr. daily all the insects subsequently entered diapause. But none did so when only the body was shielded from light, showing that the photoreceptors were located on the head. Since the wavelength sensitivity of the visual and photoperiodic response showed some correspondence Geispits was led to the further conclusion that the eyes were in fact the photoperiodic receptors. But in view of the preceding evidence this conclusion seems questionable. The earlier report of Belov (1951: cited in Danilyevsky, 1961) that the photo-periodic response in Antheraea pernyi is disturbed when certain 'retort-shaped' (chaetoid) bristles on the body are damaged is not confirmed by the recent work of Shakhbazov (1961). This investigator has observed that photoperiodic sensitivity of the 4th and 5th larval stages extends over into the pupa. And attention is drawn to the presence in the pupa of a clear area of transparent cuticle overlying the brain. Shakhbazov is inclined to regard this ' window' as the main light pathway, since, when it is painted over, the effect is equivalent to an exposure to darkness. Although 'negative* techniques, involving the elimination or silhouetting of different body regions, may sometimes prove useful in locating the organ of perception, complementary methods of photostimulation are also needed. Yet it is by no means easy to provide an accurately located beam of light when the insect is actively feeding and moving. Moreover, it is often impossible to surmount this difficulty by temporarily immobilizing the insect, since the effects of starvation sometimes mask the photo-periodic response …