420 



THE STUD V 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 etnpo- 

 dium (em-po'di-um), often exists between the two pulvilli of each tar- 



FlG. 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 pulvill/form. 



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 of the wing and above the halter or rudimentary 



