458 



THE STUDY OF INSECTS. 



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 



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^: (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). 



FIG. ^.-Ptero- The liead is composed almost entirely of 

 dontia miseiia. eves> an( j j n some genera is minute. The 



