OBSERVATIONS ON THE EMERGENCE OF PHOROCERA HAMATA A. AND W. (TACHINIDAE) FROM SAWFLY COCOONS

Extract

The literature dealing with the methods by which the larvae and adults of tachinid parasites escape from sawfly cocoons indicates in certain cases that only circumstantial evidence is available concerning the formation of the exit hole in nature. Among these parasites a large number leave their hosts while the sawflies are in the larval stages. The larvae make their escape as mature maggots through a hole gnawed in the soft host integument. A few species, however, continue to develop within the host after the host cocoon has been formed. Some of these cut through the cocoon as mature larvae and later pupate in a suitable medium, while others form their puparial cases within the host cocoons, the adults later escaping from both the puparial cases and from the sawfly cocoons. The only escape mcchanism possessed by an adult tachinid fly consists ot the alternate expansion and contraction of the ptilinum against the surrounding wall. Since the cocoon is usually quite tough and leathery it is essential that some provision be made in advance to enable the adult to escape. This seemed to occur with adults of Phorocera hamata A. and W. which emerge from their puparia within the host cocoon and then escape from the cocoon proper, leaving a characteristic round cap which appears to have been cut (Fig. 1, C). The method of formation of this cap has been the subject of discussion by several authors.