• 



l 5 8 NORTH AMERICAN DIITERA. 



mented style or arista, altogether not more than eight 

 (19); '4) the third joint simple, with or without a ter- 

 minal or dorsal arista (7, 9). Veins of the wings distinct, 

 not crowded anteriorly; third longitudinal vein furcate; 

 basal cells large; five posterior cells usually present. 



As defined above, this family includes the Xylophagi- 

 dae, Leptidse and Coenomyidae of authors. It may be a 

 question whether this union is justifiable, but, on the 

 whole, it seems that the sole character which can be used 

 to distinguish the families — the structure of the third 

 antennal joint — divides the group unnaturally, throwing 

 with the Xylophagidae forms whose affinities are great- 

 est with the Leptidae, notwithstanding the antennal char- 

 acter. 



The Xylcphagir.oe include less than one hundred 

 known forms, and many of them are remarkable for their 

 general resemblance to certain hymenopterous insects. 

 Species of Rhachicerus form a connecting link with the 

 Nemocera, and are apt to be confounded with the Rhyph- 

 idae, but the presence of the pulvilli will distinguish 

 them. 



The larvae are found in decaying wood or under the 

 bark of trees and are carnivorous and predaceous, feed- 

 ing upon the larvae of beetles and other wood insects. 

 The skin is parchment-like, the body cylindrical. The 

 mouth-parts and antennae are very small, the maxillae 

 short and hook-like. The first cr the first three segments 

 back of the head are chitinized above: the last segment 

 above with a chitinized plate terminating posteriorly in 

 two hooks. The fourth to the ninth segments have bristly 

 pseudopods below. The pupae are free. I„arvae of Xylo- 

 myia have been observed under the bark of fallen cotton- 

 wood trees. 



The Leptinae comprise something over two hundred 

 known species. They are usually of moderate size and 



