THE STUDY OF INSECTS. 
45 * 
decaying wood, or dry sand ; others live in moss or in water. 
They have either two pairs of spiracles, one at each end of 
Fig. 551.—Wing of Leptis. 
the body, or are furnished with tracheal gills. The last 
segment of the body has a transverse cleft, both above and 
m. 
vin+ix 
Fig. 552.—Wing of Xylophagus. 
below, which is furnished above with two processes. The 
pupae are free. 
The family is of moderate size; about seventy North 
American species have been described. 
Family ACROCERID^E (Ac-ro-cer'i-dae). 
The Small-headed Flies . 
These flies are easily recognized by the unusually small 
head, the large humpbacked thorax, the in¬ 
flated abdomen, and the very large alulets 
(Fig- 553)- 
_ The head is composed almost entirely of 
Fig. 553. — Ptcro- % r ' 
dontia viiseiia. e yes, and in some genera is minute. Ihe 
