LIFE HISTORY OF HYDROUS ( HYDROPHILUS) TRIANGULARIS. 



15 



center of each cercus on the ventral surface of the last abdominal segment pro- 

 jects a short rounded knob, carrying on its tip a tiny papilla armed with a single 

 seta (fig. 6). 



Fig. 7.— Side view of head, showing its in- 

 clination to thorax. 



Fig. 8. — Dorsal view of head, an, antenna; e, 

 eyes; labium; ind, mandibles; mi, maxillae. 



Fig. 9.— Ventral view of head, an, antenna; 

 lb, labium; md, mandibles; mx, maxillae. 



Fig. 10.— Dorsal view of 

 left antenna. 



HEAD. 



The head has the form of an ovoid, flattened dorsoventrally, evenly rounded 

 posteriorly, narrowed and squarely truncated anteriorly, and a little more convex 

 on the dorsal than on the ventral surface, the latter being divided by a central 

 longitudinal groove. The head is inclined upward at an angle of 30 to 45° with 

 the body axis, and its outer surface is very hard chitin (fig. 7). It is brownish 

 39557°— 23 2 



