NORTH AMERICAN DIPTERA. 



ventral side, where they serve as prolegs. The head is 

 small, but distinct, and the mouth-parts are peculiar; 

 the mandibles are two strongly chitinized pieces which 

 work antero-posteriorly ; when they are retracted the an- 

 terior ends point directly forward, but when protruded, 

 they point downward and backward, thus forming a pair 

 of hooks by which the prey is held. 



TABLE OF GENERA. 



1. Hind tibiae with spurs at tip. ....... 2 



Hind tibiae without spurs at tip. ...... 8 



2. First six segments of the third joint of antennae each produced 



into a pair of long processes (16) .... Pityocera. 

 None of the segments of third joint of antennae produced into lat- 

 eral processes. ......... 3 



3. Third joint of antennae composed of five segments, the first of 



which is much longer than the following ones; ocelli present. 4 



Third joint of antennae composed of eight segments, the first of 



which is only slightly longer than the following ones. . 5 



4. Second joint of antennae only half as long as the first (14) . 



Silvius. 



Second joint of antennae distinctly more than half as long as the 

 first (11,12) Chrysops. 



5. Fourth posterior cell of wing closed (Dielisa}. . . Scione. 

 Fourth posterior cell open. ....... 6 



6. Eyes of female acutely angulated above; wings in both sexes dark 



on anterior part, hyaline behind (13). . . Goniops. 



Eyes of female not acutely angulated above; wings nearly uniform 



in color or hyaline. ........ 7 



7. Front of female wide; much wider below than above; proboscis 



only a little longer than the palpi (15). . Apatolestes. 



Front of female of normal width or narrow; its sides usually par- 

 allel; ocelli present or absent; proboscis often long or very long. 



Pangonia. 



8. Third joint of antennae composed of four segments; front of female 



very wide (6) Hamatopota. 



Third joint of antennae composed of five rings or segments; front 

 of female not unusually wide. ...... 9 



