Observations on cocoon formation, the pupal stage, and emergence of the adult ofSimulium damnosumTheobald in Ghana

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Abstract
Observations on the life-cycle of Simulium damnosum in Ghana, from the spinning of the cocoon to the emergence of the adult, were made in the laboratory under a horizontal stereoscopic microscope set up against a museum-jar breeding-apparatus at a water temperature of 78[degree]F. and a pH of 6.0. The behavior of the larvae, pupae and emerging adults was also photographed with a 16-mm. cine camera for subsequent study on a screen. The construction of the entire cocoon took from 40 to 70 min., the threads being laid down in an irregular pattern. The time from the initial doubling up of the last larval form to the completion of ecdysis in the anterior half of the comoon was about 25 min., and the time for the pupa to stretch out into normal position was also about 25 min., i.e., a total of approximately 50 min. The complete emergence process, beginning with the first appearance of the peri-abdominal silvery film of air within the pupal skin, took from 21 to 27 min., the adult being slowly forced out of the cocoon by pressure of an enlarging air bubble posteriorly. Thus the time from the initiation of cocoon spinning to the exit of the adult was approximately 2-21/2 hrs. The adult shot to the surface completely enveloped in a form-fitting layer of air (rather than in a bubble), breaking through the surface film and taking to flight instantly. Mortality of the flies occurred whenever the legs became entangled in the retained last larval skin, pupal skin or respiratory filaments, which resulted in drowning. Mortality also occurred when masses of pupae were heavily layered upon the substratum, with the result that the bottommost flies were impeded in their escape by superimposed layers of cocoons. The observations descrived are illustrated by 13 photomicrographs.