28 WATER-STRIDERS. 
part of the vertex is depressed. No ocelli are conspicuous, if 
indeed they are present. The thorax usually widens backwards, 
and its thickness is increased by the prominent middle and pos- 
terior coxe, which project beyond the sides. No scutellum is 
apparent, but in its place the end of the dorsal plate of the 
mesothorax is scale-like, narrowed, rounded, and depressed 
around the tip. Behind this the abdomen tapers more or less 
towards the last segment, which is usually armed on each side 
with a tooth-like process. The under side of the body is gen- 
erally minutely pubescent and sericeous like satin, and this is 
sometimes continued along the sides of the thorax. There are 
Fic. 20.—Stephania picta H. Schf. After Uhler. 
commonly two forms of the adult belonging to the same species, 
the winged and the unwinged. These do not necessarily co- 
exist. During some years only the winged forms appear, while 
in others, and especially if the spring and summer are cool, the 
individuals will all be unwinged, with perchance a single, more 
vigorous specimen, winged. In some parts of the Southern 
States three forms occur, those before cited, and another which 
has the wing-covers of scarcely half length, but with these organs 
as nicely differentiated into corium and membrane as the most 
completely developed.” (Uhler.) 
As these insects are of no economic value it is not necessary 
to give the different genera that compose the family. Fig. 20 
and Fig. 21 show two extreme forms. Some related forms are 
true pelagic insects, being found on the surface of the warmer 
