Variation and structure of the eyes in the desert locust, Schistocerca gregaria (Forskål)

Abstract

The compound eyes of the solitaria phase individuals of the desert locust, Schistocerca gregaria (Forskal), are vertically striped with mostly 6 and 7, rarely 5 and 8, dark brown stripes, and a number of cream-coloured interstripes. In phase gregaria the interstripes, which are more or less invaded by brown pigment, are partially or wholely masked, the eye in the latter case presenting an almost uniformly dark brown appearance. The postembryonic development of the striped eyes, is described stage by stage. There is no stripe at the time of hatching. In the 6-striped eye one stripe is developed in the first-stage hopper and, subsequently a stripe is added at each of the five moults. In the 7-striped eye the one-moult-one-stripe relationship holds good in most stages, but the extra seventh stripe is produced in two ways: (i) By the addition of two stripes at the second moult (i.e. the third-stage hopper has four stripes instead of three); and (ii) by the interposition of an extra-moult, usually in the third stage and rarely in the fourth, during which a new stripe is added (stripe-positive extra-moult). But extra-moulting does not necessarily lead to the addition of a stripe; stripe-neutral extra-moults are not infrequent. The development of the dorsal spot and the subdorsal streak are described. The mechanism of growth and the homology and nomenclature of the stripes and interstripes are discussed. The structure of the compound eyes and the pigmentary basis of stripe formation are described, and their effects on vision in solitaria and gregaria individuals discussed. Vision is discussed on the basis of ommatidial structure and pigmentation. In gregaria eyes a perfect apposition image is formed, the image being sharply defined. An 'anti-halation' device, produced by the post-retinular layer of pigment, is present. The eye is suited for diurnal vision, and strong direct sunlight is not avoided. In solitaria eyes the image is of the apposition type in its mode of formation but of the superposition type in effect; it has been termed a 'pseudo-superposition' image, and is more diffuse but brighter than in gregaria eyes. The 'anti-halation' device is weak and ineffective. The eye is suited for vision in subdued light and perceives movements rather than sharp images. Solitaria individuals, especially hoppers, avoid strong, direct sunlight. The effects of these differences in vision on the behaviour of gregaria- and solitaria-phase individuals are as follows: the former, owing to mutual visual impact induced by the formation of sharp images, tend to be gregarious; and further, owing to the presence of light-absorbing mechanisms, they do not avoid strong sunlight; the latter, on the other hand, owing to the want or comparative ineffectiveness of the above-mentioned features, neither tend to congregate nor to go out boldly into the bright open.