140 



THE STUDY OF INSECTS 



which vanish almost as soon as made. They are social fellows, and are 

 almost always found in large numbers, either swimming or resting motion- 

 less near together. They rarely dive, except when pursued; but are so 

 agile that it is extremely difficult to catch them without a net. Many of 

 them when caught exhale a milky fluid having a very disagreeable odor. 

 They feed upon small flies, beetles, and other insects that hill into the 

 water, and are furnished with well-developed wings, with which they 

 fly from one body of water to another. 



This is one of the most easily-recognized families of the whole 

 order Coleoptera. The members of it are oval or elliptical in form 

 (Fig. 235), more or less flattened, and usually of a very brilliant 

 bluish-black color above, with a bronze metallic lustre. The fore 

 legs are very long and rather slender; the middle and hind 

 Fig. 235- legs are short, broad, and very much flattened. These insects 

 are remarkable for having the eyes completely divided by the margin of 

 the head, so that they appear to have four eyes — a pair upon the upper 

 surface of the head with which to look into the air, and a pair upon the 

 under side for looking into the water. The antenna? are very short and 

 peculiar in form. The third segment is enlarged, so as to resemble an ear- 

 like appendage, and the following ones form a short spindle-shaped 

 mass. The}* are inserted in little cavities in front of the eyes. 

 The eggs of these insects are small, of cylindrical form, and 

 are placed end to end in parallel rows upon the leaves of aquatic 

 plants. The larvae (Fig. 236) are long, narrow, and much flat- 

 tened. Each abdominal segment is furnished with a pair of 

 tracheal gills, and there is an additional pair at the caudal end 

 of the bod}-. The elongated form of the body and the con- 

 spicuous tracheal gills cause these larva? to resemble small cen- 

 tipedes. When a larva is full-grown it leaves the water and 

 spins a gray, paper-like cocoon attached to some object near the 

 water. The pupa state of those species in which it has been ob- 

 served lasts about a month. 



The family is a small one. At present only forty-one North American 

 species are known. These represent three genera. The genus Gyretes is 

 distinguished by having the last ventral segment of the abdomen elon- 

 gated and conical. It is represented by two species. In the other two 

 genera the last ventral segment is flattened and rounded at the tip. In 

 Dineutus the scutellum is invisible; there are thirteen species of this 

 genus. In Gyrinus the scutellum is visible; of this genus we have twenty- 

 six species. 



THE HERBIVOROUS BEETLES 



Suborder Polyphaga * 



In the suborder Polyphaga the ventral 

 part of the first segment of the abdomen 

 is visible for its entire breadth (Fig. 237); 

 the first three ventral segments are im- 

 movably united (except in the Cupesida?), 

 and the notum of the prothorax is not sepa- 

 rated from the pleura by distinct sutures. 

 polyphagus, eating many kinds of food. 



Fig. 236. 



FlG. 237. — Ventral aspect of part of 

 tborax and abdomen of Enchroma giganlea: 

 1 j 1, first abdominal segment. 



Polyphaga : 



