504 



AN INTRODUCTION TO ENTOMOLOGY 



Fig. 602. 



California. This genus was formerly included in the following family ; 

 but it differs from the Dryopidas in having more than five ventral ab- 

 dominal segments. Our eastern species is Psephenus lecontei. These 

 beetles are found in the vicinity of running water, and often, in the 

 heat of the day, collect on stones that project from the water; they 

 fly swiftly when disturbed. The body is oval, subdepressed, nar- 

 rowed in front, and clothed with fine, sill<:en hairs, which retain a film 

 of air when the insect goes beneath water. The females deposit their 

 eggs in a layer on the under side of submerged stones in shallow 

 brooks. The beetles measure from 4.5 mm. to 6 mm. in length. 

 The larva is found clinging to the lower surface 

 of stones in rapid streams, and I have found it in 

 muck near a spring. It is very flat, circular in out- 

 line (Fig. 602), and measures about 7 mm. in 

 length. It breathes by five pairs of branched tracheal 

 gills on the ventral side of the abdomen. It is rarely 

 recognized as an insect by the young collector; in 

 fact it was originally described as a crustacean under 

 the generic name Fluvicola. I have suggested the 

 common name water-penny for the larva. When 

 mature the larva leaves the water, and pupates 

 under the last larval skin, beneath a stone or other 

 object in a damp situation. 



The family DRYOPID^E as now restricted includes only the sub- 

 family Parninae of the old family Parnidas, in which were included 

 the preceding family and the following family. The 

 Dryopidse differ from the Psephenidae in that the 

 members of it have only five ventral abdominal seg- 

 ments, and from the Elmidje in that in the Dryopidae 

 the anterior coxae are transverse, with a distinct tro- 

 chantin. 



This family includes small water-beetles in which 

 the legs are not fitted for swimming. They are 

 found most often in swift-running water, where they 

 cling to stones, logs, or aquatic plants. The body is 

 clothed with fine, silken hairs, which retain a film of 

 air when the insect is beneath the water. They feed 

 on decaying matter in the water. 



The larvae are also aquatic. The larva of 

 Helichus lithophiliis (Fig. 603) resembles somewhat 

 the water-penny {Psephenus), except that the body 

 is more elongate and is deeply notched between the 

 segments. 



Seventeen species of this family have been found in our fauna. 



The family ELMID.^ includes beetles that are closely allied to 

 the preceding family in structure and in habits; but in this family 

 the anterior coxae are rounded and without a trochantin, and the body 

 is less densely pubescent than in the Dryopidae. 



Fig. 603. 



