THK PARASITISM <>K INSECTS. 



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 h :<l 

 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 represents an adult female of Stylop* atemmus con- 

 taining two nearly hatched e^s. and the right-hand figure, a newly boru larva of 

 Sf>//o)>n an a hair of Andrfrnn Triwvifrnnn. A, ventral surface of the thorax ; 

 B, the abdomen; a. mandible* ; b labial plate* and mouth ; c, vulva ; 1, 2, 3, the 

 three thoracic segment? united. (After Newport.) 



accompanied by extreme modification of form. In this re- 

 spect the Strepsiptera, \\hich 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- 



