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 (Mycetophilidse) they 

 are very long. When pulvilli are developed they are membranous 

 pads, one beneath each tarsal claw. A third appendage, the enipo- 

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



FIG. 487. 



FIG. 



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 puhjilliform. 



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 



