The water balance of tsetse pupae

Abstract

1. The pupal period of Glossina morsitans can be divided into a number of different stages on the basis of anatomical changes: (a) The 3rd instar larva which comprises the first day of pupal development. During this time water is lost at relatively high rates in part by transpiration through the puparium and in part through the supernumerary stigmata of the polypneustic lobes. (b) The 4th instar larva which comprises the next 3 days; rates of transpiration are somewhat lower due to the deposition inside the puparium of a second integument. But water is lost by excretion as well as by transpiration; this excretory loss is regulated so that the more water is lost by (c) The pupa and pharate adult. The larva-pupa moult occurs on about the 5th day and a few days later the hypodermis retracts from the pupal skin and the adult cuticle begins to be laid down. Following the formation of the very impermeable pupal skin there is a substantial decrease in the rate of transpiration. At this stage water is lost also through the pupal spiracle, but this loss is under active control, the rate being a function of the water content of the pupa. (d) A few days before emergence the pupal skin dries out and its permeability increases ; spiracular control ceases at the same time, so that rates of water loss during this period are high . 2. The regulation of excretory loss and of spiracular loss, combined with extremely low membrane permeabilities, confers a high degree of resistance to desiccation on the pupa of ; it can complete its development at humidities as low as 10 /0 r.h. 3. It is shown that in areas where a prolonged dry season occurs the humidity to which pupae are subject under natural conditions may fall to about 40 % r.h.; if the pupal sites are exposed to the action of grass fibres even lower humidities may be experienced. 4. There is a close correlation between the resistance to desiccation of the pupae of different species of tsetse fly and the habitat in which they occur, and it is suggested that the water balance of the pupa may have been an important limiting factor in the invasion of semi-arid and arid habitats. 5. The development of resistance to desiccation in tsetse pupae has involved a decrease in the permeability of puparium and pupal skin, a decrease in the size of respiratory lobes and an increase in the percentage loss of water which can be tolerated before development stops.