THR WTTIRLTCin REETLER. 237 



ously at any small insect they meet upon the surface of the water. 

 Sometimes the outsider becomes alarmed and skates back at full 

 speed into the assembly, putting its members into wild stampede. 



Although easily recognized by their form and habits, the Gyri- 

 nidcB are distinguished from allied families by having the eyes 

 completely divided by the sides of the head into 

 upper and lower parts, both rounded, so that 

 they appear to have four eyes, two for looking 

 up into the air and two for gazing down into the 

 water. (Fig. 120.) The antennae are very 

 short, thick, inserted behind the mandibles, the 

 third joint enlarged so as to resemble an ex- 

 ternal ear, the following joints rather broad and bee^e^neSwfto 

 united so as to form a spindle-shaped append- vided ' eye ' (Aft ' er Folsora ' ) 

 age (Fig. 4, No. 16) ; front legs very long, rather slender, and when 

 at rest placed in oblique grooves between the pro- and mesosterna ; 

 middle and hind legs short, broad, very much flattened; tibiae with- 

 out spurs; tarsi 5-jointed, the joints in part flattened and trian- 

 gular. 



The eggs of the Gyrinidas are laid in parallel rows upon the 

 leaves of aquatic plants. The grubs are long, narrow and much 

 flattened, and breathe by means of tracheal gills attached to the 

 abdominal segments. (Fig. 121.) They are chiefly carnivorous, 

 and when full grown leave the water and spin a gray, paper-like co- 

 coon on some nearby object, in which they undergo the pupal stage. 



Only about 40 species of Gyrinida? are known from the United 

 States. These are divided among three genera, two of which are 

 known to occur in the State, while the single species of the third 

 is recorded from Quincy, Illinois, and may yet be found in the 

 western part of Indiana. 



The only papers treating of the North American species of the 

 family are as follows: 



LeConte. "The Gyrinida? of America North of Mexico," in 

 Proc, Phil. Acad. Nat. Sci., 1868, 365. 



Roberts, C. H. "The Species of Dineutes of America North of 

 Mexico," in Trans. Amer. Ent. Soc., XXII, 1895, 279. 



KEY TO GENERA OF OYRINID/E. 



". Last ventral segment depressed, rounded at tip. 



It. Scutellum distinct; length less than 8 mm. I. GYRINUS. 



/>/>. Scutellum invisible; labrum transverse; length or more mm. 



II. DINEUTES. 



