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 



VIIj+IX 



VUi 



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 



TII 4 



FIG. 552. Wing of Xyhfhagus. 



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). 

 77/i? 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. Ptero- J 



ia miseiia. eyes, and in some genera is minute. The 



