486 A N INTROD UCTION TO ENTOMOLOG Y 



leaves of aquatic plants; in others they are provided with floats 

 and let loose in the water; and in still other species the cases are 

 carried by the mother underneath her body and steadied with her 

 hind legs. Frequently some of the young larvse 

 devour their companions; in this way the size 

 of the family is decreased before it escapes 

 from the egg-case. Later they live upon insects 

 that fall into the water, and upon snails. These 

 larvse resemble somewhat those of the Dytis- 

 cidae; but the body is much more plump, 

 and the mandibles are of moderate size. 



The family Hydrophilidse is represented in 

 North America by one hundred and ninety 

 species. The largest of our common species is 

 Hydrous triangularis (Fig. 573). In the genus 

 Hydrous the metasternum is prolonged back- 

 ward into a spine between the hind legs, and 

 the sternum of the prothorax bears a deep 

 Fig- 573- furrow. 



The beetles of the genus Tropisternus agree 

 with Hydrous in the form of the prosternum and metasternum, but 

 differ in size, our species measuring less than 12 mm. in length. The 

 most common species in the East is Tropisternus glabra, and on the 

 Pacific Coast T. calif ornicus. 



Next in size to Hydrous are several species of Hydrophilus. In 

 this genus the metasternum is prolonged somewhat, but does not 

 form a long, sharp spine as in Hydrous and Tropisternus, and the 

 sternum of the prothorax bears a keel-shaped projection. Our most 

 common species is Hydrophilus obtusdtus; this measures about 1 5 mm. 

 in length. 



Some of the smaller species of this family are not aquatic, but 

 live in moist earth and in the dung of cattle, where, it is said, they 

 feed on dipterous larvas. 



Family PLATYPSYLLID^ 

 The Beaver-Parasite 



Only a single representative of this family is known; this is 

 Platypsylla cdstoris, which lives parasitically on the beaver. This 

 beetle is about 2.5 mm. in length; the body is ovate, elongate, and 

 much flattened; the wing-covers are short, about as long as the 

 prothorax, and leave five abdominal segments exposed ; the eyes and 

 wings are wanting. 



Specimens of this remarkable insect are most easily obtained by 

 beating over a sheet of paper the dried skins of beavers, which can 

 be found in fur-stores. 



The family BRATHINID^ is composed of the genus Brathinus, 

 of which three species are described, two from the East and one from 

 California. These beetles are somewhat elongate, with the outline 



