Haemocyte activity in relation to epidermal cell growth, cuticle secretion, and cell death in a metamorphosing cyclorrhaphan pupa

Abstract

The fate of a group of specific blood cells is traced from the time of the cryptocephalic pupal stage to 3 weeks following emergence of the adult fly. The specific cells studied are located in the terminal ‘blind’ end of the developing pre-tarsus of the leg. They are phagocytic blood cells, are derived from larval granular haemocytes, and represent some 95 per cent of the total haemocyte population during the pupal stage. From cryptocephalic pupa to emerging adult takes 11 days at 25°C. The blood cells are seen during the first day to be gorged with larval fragments. At day 2 digestion of fragments progresses; at day 3 there are a large number of osmiophilic globules present in the blood cells which become reduced in number while immediately adjacent epidermal cells accumulate similar globules; at this time the blood cells are closely interdigitated with ventrally located tenent cells of the pulvillus. At day 4 connexions with epicermal cells become less intimate, digestion of larval fragments continues, osmiophilic globules are seen being extruded from larger masses, and large numbers of lysosomes are identifiable in the vicinity of the blood cell nucleus. At day 5 close association of the epidermal and blood cells is again evident: a large area of the blood cell surface interdigitates with that of the dorsal giant cells of the pulvillus. Again enormous numbers of osmiophilic globules accumulate in the dorsal giant and ventral tenent cell cytoplasm, with corresponding depletion of similar bodies in the blood cells. The remainder of the giant cell plasma membrane is thrown into microvilli exhibiting pinocytosis, suggesting the uptake of soluble materials from the haemolymph. Through day 7 continued breakdown of fragments occurs. At day 8 the blood cells are relatively devoid of contents and are correspondingly smaller. Tenent cells begin to show signs of autocytolysis at late day 8, before any association with the blood cells is seen. Through days 9 to 10 phagocytosis of the pycnotic tenent cell nuclei occurs. Giant cells and tracheoles are untouched but the contents of the former are also phagocytosed later, at the time of emergence. The blood cells are also involved in some complex way in the intracellular (giant and tenent) deposition of endocuticle, and in the emerged adult would appear to contribute to the formation of connective tissue. Since these haemocytes appear to be the only cells remaining at the endocuticle-haemolymph junction in the adult pre-tarsus, it may be these blood cells, rather than epidermal cells, that are involved in the attack by, and be the source of resistance to, modern contact insecticides.