LIFE HISTORY OF HYDROUS ( H YDROPHILUS ) 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; 16, labium; md, mandibles; mx, maxillse. 
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 
