774 AN INTRODUCTION TO ENTOMOLOGY 



The antenncs vary greatly in form in the different families. In 



Fig. 983. 



Fig. 984. 



Fig. 985. Fig. 



Fig. 987. 



the more generalized families the antennas consist of many segments, 

 which, except the basal two, are, similar in form (Fig. 

 983). Frequently such antennse bear whorls of long 

 hairs (Fig. 984). In the more specialized families there 

 is a reduction in the nimiber of segments of the anten- 

 nae. This is brought about either by a more or less 

 complete consolidation of the segments beyond the 

 second into a single segment (Figs. 985 and 986), or by 

 a dwindling of the terminal segments, so that they form 

 merely a slender style (Fig. 987) or bristle (Fig. 988). 

 Such a bristle is termed 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 frequently one part of the third 

 segment- is expanded so that it projects beyond the 

 insertion of the arista (Fig. 989) ; then the arista 

 is said to be dorsal. 



The month-parts of flies are formed for sucking, 

 and sometimes also for piercing. Their structure 

 differs greatly in different families; and in some 

 cases it is exceedingly difficult to determine the 

 homologies of the different parts. In the more 

 typical forms the mouth-parts consist of six 

 bristle-lilce or lance-like organs enclosed in a 

 sheath and a pair of jointed palpi. There are 

 differences of opinion as to the homologies of 

 these parts, but according to the most generally 

 accepted view they are as indicated below. The 

 mouth-parts of a mosquito will serve as an 

 example of a comparatively generalized type. 



Fig. 989. 



