288 



THE STUDY OF INSECTS 



the puparium. In this way the head end of the puparium is forced off, 

 making a large opening through which the adult escapes; afterwards the 

 ptilinum is withdrawn into the head. If a specimen is captured soon 

 after its emergence from the puparium, there may be seen instead of 

 the frontal suture the bladder-like ptilinum projecting from the head, 

 immediately above the antennae. 



The form of the antennae is of prime importance in determining to 

 what family a fly belongs. In the more generalized families the antenna 

 consists of many segments, which, except the basal two, are similar in 



Fig. 490. 



Fig. 491. 



Fig. 492. 



Fig. 403. 



Fig. 494, 



Fig. 495. 



form (Fig. 490). Frequently such antennae bear whorls of long hairs 

 (Fig. 491). In the more specialized families there is a reduction in the 

 number of segments of the antenna. This is brought about either by a 

 more or less complete consolidation of the segments beyond the second 

 into a single segment (Figs. 492 and 493), or by a dwindling of the terminal 

 segments, so that they form merely a slender style (Fig. 494) or bristle (Fig. 

 495). Such a bristle is termed by many writers the arista. In most cases 

 where a style or arista exists it is borne by the third segment, and this 

 segment is then usually greatly enlarged. When the enlargement of this 

 segment has taken place evenly the style or arista is terminal; but fre- 

 quently one part of the third segment is expanded so that it projects 

 beyond the insertion of the arista (Fig. 496) ; 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 de- 

 veloped they are membranous pads, one 

 beneath each tarsal claw. A third append- 

 age, the empodium, often exists between the 

 two pulvilli of each tarsus. The empodia 

 may be bristle-like, or tapering (Fig. 497), 

 or membranous, resembling the pulvilli in 

 form (Fig. 498) ; in the last case they are 

 described as pulvilliform. 



Variations in the form and venation 

 of the wings afford characters that are much used in the classification of 

 flies. In many families there is a notch in the inner margin of the wing 

 near its base (Fig. 499, 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 pos- 



FiG. 496. 



