420 
THE STUD Y OF INSECTS . 
insertion of the arista (Fig. 493) ; then the arista is said to be 
dorsal. 
The legs vary greatly in length and in stoutness. The coxae are 
usually long, and in most of the fungus-gnats (Mycetophilidae) they 
are very long. When pulvilli are developed they are membranous 
pads, one beneath each tarsal claw. A third appendage, the empo- 
dium (em-po'di-um), often exists between the two pulvilli of each tar- 
Fig. 487. Fig. 488. Fig. 489. Fig. 490. Fig. 491. 
sus. The empodia may be bristle-like, or tapering (Fig. 494), or 
membranous, resembling the pulvilli in form (Fig. 495); in the last 
case they are described as pulvilliform . 
Variations in the form and venation of the wings afford charac¬ 
ters that are much used in the classification of flies. In many fami- 
Fig. 492. Fig. 493. Fig. 494. Fig. 495. 
lies there is a notch in the inner margin of the wing near its base 
(Fig. 496, a e ); this is the axillary excision / that part of the wing 
lying between the axillary excision and the base of the wing is the 
posterior lobe (Fig. 496, /). In certain families there is a membrane 
beneath the base 0/ the wing and above the halter or rudimentary 
