458 



THE STUDY OF INSECTS. 



decaying wood, or dry sand ; others live in naoss or in water. 

 They have either two pairs of spiracles, one at each end of 



nif 



Vlla+IX '"^^ 



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 



^ lllll I III-2+3 



VUz+IX 

 Fig. 552. — Wing of Xylo/>hagus. 



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 ACROCERlDiE (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 —Piero- '^^^ head is composcd almost entirely of 

 doniia iniseiia. gyes, and in somc genera is minute. The 



