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 

 Fig. 551.— Wing of Lej>tis. 



the body, or are furnished with tracheal gills. The last 

 segment of the body has a transverse cleft, both above and 



vife+ix 



Fig. 552.— Wing- of Xylofhagus. 



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-dse). 

 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 

 dontia viiseiia. eveS) anc } [ n some genera is minute. The 



