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, carrying 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 -jf-g- 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 Pselaphidae and Scydmaenidae, many of 
which live commensally with ants in their nests. These beetles are rarely 
pver 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 Chapter XV.) 
TRIBE SERRICORNIA. 
In this tribe of beetles, characterized by having the antennae slender, 
with each segment projecting more x>r less inward so as to give the whole 
antennae a saw-toothed or serrate character (Fig. 340, 10), are included sev¬ 
eral families certainly not closely related and having widely different habits 
and appearance. The serrate character of the antennae, too, is sometimes 
so slight that it can hardly be distinguished with certainty. The more 
important families of the tribe can b^ 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.) Buprestid^e. 
Antennae often filiform; first two abdominal segments free. 
(Click-beetles.) Elaterid^e. 
Head free, but bent under the thorax. 
Small insects usually less than \ 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.) Lampyrid^e. 
Wing-covers firm, thorax convex, body not much flattened; antennae often enlarged 
at tip..(Checkered beetles.) Clerid^:. 
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 
