THE PARASITISM OF INSECTS. 



387 



the thorax, while the tergal disks develop into the tergal 

 halves of the corresponding somites, with their appendages, 

 the wings and the halteres. The anterior pair of disks origi- 

 nate the head and proboscis of the fly. As the imaginal 

 disks develop, the preexisting organs contained in the head 

 and thorax of the larva undergo complete or partial resolu- 

 tion. On the other hand the abdomen of the fly is produced 

 by the continuous modification of the constituents of the lar- 

 val abdomen. 



As in the Crustacea^ so in Insecta, the parasitic habit is 



Fig. 111.— The left-hand figure repreeeiats an adult female of Siylops aterrim'us con- 

 taining two nearly hatched eg^e. and the right-hand fiL'Ure, a newly born larva of 

 Sfyhps on a hair of Andrfvna Trimmerana. A. ventral surface of the thorax ; 

 B. the abdomen: a. mandibles ; 6 labial plates and mouth ; c, vulva ; 1, 2, 3, the 

 three thoracic segments united. (Alter Newport.) 



accompanied by extreme modification of form. In this re- 

 spect the Stre2Jsiptera^vf\\\Q\\ are parasitic upon Bees, present 

 a remarkable history. The female (Fig. Ill) has the form of 

 a sac with a short neck, and never leaves the body of the 

 Hymenopteran in which she is parasitic. The males, on the 

 contrary, are exceedingly active insects provided with a sin- 



