$26 THE STUDY OF INSECTS. 



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 

 fall into the water, and are furnished with well-developed 

 wings, with which they fly from one body of water to an- 

 other. 



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. 634), 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 

 FIG. 634. and hind legs are short, broad, and very much flat- 

 tened. 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 antennae 

 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. They 

 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. 635) are 

 long, narrow, and much flattened. Each abdominal 

 segment is furnished with a pair of tracheal gills, 

 and there is an additional pair at the caudal end of 

 the body. The elongated form of the body and the 

 conspicuous tracheal gills cause these larvae to re- 

 semble small centipedes. 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 the species in which it has been 

 observed lasts about a month. 



The family is a small one. At present only thirty-six 



