Beetles 265 



have their legs fitted for swimming, nor in any other way the body partic- 

 ularly modified for an aquatic life. They crawl around on submerged stones, 

 sticks, and water-plants, carr3'ing a supply of air with them, held by the 

 fine pubescence of the body. The larvae are curiously flattened, broadly 

 oval to nearly circular small creatures (Fig. 364), which cling to stones and 

 give the family its popular name of "water-pennies." As the legs, mouth- 

 parts, eyes, etc., are all on the under side and concealed, the flat, brownish, 

 leathery little "penny" is usually not recognized as an insect by the observer 

 of brook life. 



The family Platypsyllidae has been established to include a single 

 species of strangely shaped beetle which lives as a parasite on the bodies 

 of beavers. Its name is Platypsylla castoris; it is about ^-^ inch long, blind 

 and wingless, and with the elytra rudimentary. This degenerate condition 

 of the body is due of course to the parasitic habit. Other obscure little 

 . beetles of curious habits are the Pselaphids and Scydmicnids, many of 

 which live commensally with ants in their nests. These beetles are rarely 

 over an eighth of an inch long, and some of them have bodies strangely 

 modified to look like ants. (For a further account of these insects see 

 the discussion of myrmecophily in Cha])ter XV.) 



TRIBE SERRICORNIA. 



In this tribe of beetles, characterized by having the antennas slender, 

 with each segment projecting more or less inward so as to give the whole 

 antennae a saw-toothed or serrate character (Fig. 340, "=), are included sev- 

 eral famihes certainly not closely related and having widely different habits 

 and appearance. The serrate character of the antenns, too, is sometimes 

 so slight that it can hardly be distinguished with certainty. The more 

 important families of the tribe can bj separated by the following key: 



Head inserted in thorax as far as the eyes; body elongate or elliptical, and with unusually 

 hard cuticle. 

 Antennae finely serrate, the first two abdominal segments grown together on the ven- 

 tral side (Metallic wood-borers.) Bupeestid^. 



Antenn;E often filiform; first two abdominal segments free. 



(Click-beetles.) Elateeid.e. 

 Head free, but bent under the thorax. 



Small insects usually less than J inch long (Death-watch beetles.) Ptinid.e. 



Head free, but often partly or wholly covered by the thin anterior margin of the thorax. 

 Wing-covers flexible; body elongate and flattened; antennae not enlarged at tip. 



(Fireflies.) Lampvrid.e. 



Wing-covers firm, thorax convex, body not much flattened; antennie often enlarged 



at tip (Checkered beetles.) Ci.erid.e. 



The metallic wood-borers, or flat-headed borers, a name suggested by 

 the flat broad head of the larva, constitute the large and important family 



