The structure and function of the spiracular gill of the fly Taphrophila vitripennis

Abstract

The spiracular gill is a pupal structure, but it is the chief respiratory organ of the adult of Taphrophila before the pupal cuticle is shed. At the pupa-adult moult, the epidermis and blood in the spiracular gill are completely isolated from the living insect by two cuticles between which is the moulting fluid. A few hours after the isolation of the tissue, the epidermis of most parts of the gill begins to dissociate, the cells become rounded, separate away from the cuticle and from one another, and in due course form loose clumps usually far from the gill walls. The tissue isolated in the gill repairs cuts and tears in the gill walls with a tanned cuticle. At 16 to 18 degrees C the competence of the isolated tissue to repair damage to the gill walls lasts about 14 days. The tissue is isolated 8 to 9 days before the emergence of the adult, and it repairs the gill up to 5 days after the insect has shed it and flown away. The isolated tissue tolerates complete dehydration and high temperatures. In water the isolated tissue of gills previously dried for 70 days over phosphorus pentoxide and heated when dry to 103 degrees C for 2 h, or to 130 degrees C for 30 min, successfully repaired wounds. The epidermis of the adult and larva of Taphrophila also repairs wounds after complete dehydration. The epidermis of other insects is shown to exhibit a similar tolerance to dehydration even when no such tolerance is shown by the insect: the epidermis of some insects that are killed when they lose about 20% of their moisture content will repair wounds after complete dehydration if dried rapidly. The gill of Taphrophila has a plastron that is not wetted at pressures below about 0.3 atm above normal pressure, and it is only wetted by surface active substances that reduce the surface tension to about 25 dyn/cm. Apart from its plastron, the gill is not an effective respiratory organ. The gill walls are not rigid. In water, the internal pressure maintains turgidity and maximum surface area necessary for the efficient functioning of the plastron. The internal pressure of intact and unscarred gills is 4.3 atm. When the gill is torn or cut open, blood and cells spurt out and there is an immediate equilization of internal and hydrostatic pressures. A clot is rapidly formed at the site of the injury. The increase in the mechanical strength of the clot outpaces the increase in the internal pressure brought about by water that diffuses into the gill. In a number of other Tipulidae beside Taphrophila blood and epidermis are isolated in the respiratory horn or the spiracular gill at the pupa-adult moult. In some of these, such as the species of Lipsothrix, the epidermis does not dissociate nor separate from the cuticle but becomes syncytial and remains closely attached to the cuticle.