COMMITMENT TO METAMORPHOSIS IN TSETSE (GLOSSINA-MORSITANS-CENTRALIS) - TEMPORAL, NUTRITIONAL AND HORMONAL ASPECTS OF THE DECISION

Larvae of Glossina morsitans centralis were manually extruded from the uterus of the female at different times during the third (final) larval instar to evaluate their competence to initiate metamorphosis. Parturition is a gated response in tsetse, and in this species the peak of activity occurred 8.7 h after the onset of the daily 12 h photophase. Larvae aborted a few hours prematurely were completely competent to crawl and pupariated within 1-2 h, but a portion of the puparia blackened before tanning. Younger larvae (22.9 mg vs full grown weight of 33.6 mg) were also competent to crawl, but after 1-2 h they became immobile and they performed retractory movements with their anterior segments for up to several days before eventually pupariating. The youngest category of larvae (17.5 mg) were incapable of crawling but continuously performed retractory movements with their anterior segments. Such larvae never succeeded in pupariating. The commitment to various components of metamorphosis is thus made at different stages of ontogenetic development. Some behavioural components such as retraction of the anterior segments are established relatively early, but the integument is fully competent for normal phenolic tanning only near the end of the instar. Commitment to metamorphosis is thus made much later in tsetse larvae than in free-living larvae of other species of Diptera, presumably as an adaptation to the vagaries imposed by adenotrophic viviparity. Pupariation could be accelerated in aborted larvae by injection of 20-hydroxyecdysone, thus implying that the delay in pupariation of aborted larvae may be, at least partially, the consequence of moulting hormone deficiency.