THE EXTERNAL ANATOMY OF INSECTS 75 



The anal area and the prea)ial area of the wing. — In descriptions of 

 wings it is frequently necessary to refer to that part of the wing 

 supported by the anal veins; this is designated as the anal area of the 

 wing; and that part lying in front of the anal area, including all of 

 the wing except the anal area, is termed the preanal area. 



IV. THE ABDOMEN 



a. THE SEGMENTS OF THE ABDOMEN 



The third and terminal region of the body, the abdomen, consists 

 of a series of approximately similar segments, which as a rule are 

 without appendages excepting certain segments near the caudal end 

 of the body. 



The body-wall of an abdominal segment is usually comparatively 

 simple, consisting in adults of a tergum and a sternum, united by 

 lateral conjunctivas. Sometimes there are one or two small sclerites 

 on each lateral aspect of a segment; these are probably reduced 

 pleura. 



The number of segments of which the abdomen appears to be 

 composed varies greatly in different insects. In the cuckoo-flies 

 (Chrysididffi) there are usually only three or fotir visible; while in 

 many insects ten or eleven can be distinguished. All intergrades 

 between these extremes occur. 



The apparent variation in the number of abdominal segments is 

 due to two causes: in some cases, some of the segments are tele- 

 scoped ; and in others, adjacent segments coalesce, so that two or more 

 segments appear as one. 



A study of embryos of insects has shown that the abdomen con- 

 sists typically of eleven segments; although this number may be 

 reduced during the development of the insect by the coalescence of 

 adjacent segments. 



In some insects there is what appears to be a segment caudad of 

 the eleventh segment; this is termed the telson. The telson differs 

 from the segments preceding it in that it never bears appendages. 



Special terms have been applied, especially by writers on the 

 Coleoptera, to the caudal segments of the abdomen. Thus the 

 terminal segment of a beetle's abdomen when exposed beyond the 

 elytra is termed the pygldium; the tergite cephalad of the pygidium, 

 especially in beetles with short elytra, the propygidium; and the last 

 abdominal sternite, the hypopygium. The term hypopygium is also 

 applied to the genitalia of male Diptera by writers on that oi"der of 

 insects. 



