l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



segment is sometimes more or less united with the eleventh in orthop- 

 teroid insects ; but in the Holometabola the eleventh segment is 

 generally suppressed, and the body ei\ds with the tenth segment, 

 though possibly in certain holometabolous larvae the terminal segment 

 contains a remnant of the eleventh somite, while in coleopterous larvae 

 the abdomen ends in a distinct anal lobe, which appears to represent 

 the eleventh segment. 



The association of the organs of copulation and oviposition with 

 the eighth and ninth segments of the abdomen is usually accompanied 

 by adaptive structural modifications in these segments that conspicu- 

 ously differentiate the latter as the genital segments, and separate the 

 rest of the abdomen into a pregenital region and a postgenital region. 

 Since the pregenital region contains most of the internal abdominal 

 organs, its segments may be termed the visceral segments. The seg- 

 ments beyond the ninth, which are usually more or less reduced and 

 united with each other, constitute the postgenital segments. It is not 

 possible, of course, in all cases to divide the abdomen consistently into 

 visceral and genital regions, since modifications adaptive to the major 

 functions of the eighth and ninth segments often affect one or several 

 of the preceding segments, but yet, for general descriptive purposes, 

 the term " genital segments " will have a specific meaning. 



THE VISCERAL SEGMENTS 



To describe here in full the visceral region of the abdomen would 

 be to repeat many well known facts without adding anything of im- 

 portance. The seven segments of this region are usually of simple 

 structure and dift'er but little from one another. In adult pterygote 

 insects they lack appendicular organs, and the definitive sterna prob- 

 ably always include the areas of the primitive limb bases. The first 

 segment is more subject to modifications than are any of the others. 

 In winged insects the antecosta of its tergal plate bears the third pair 

 of phragmatal lobes, and the precosta is usually much enlarged, form- 

 ing the so-called postnotal, or postscutellar, plate of the mctathorax 

 (fig. 16, PN3), which, together with the base of the phragma, is 

 frequently removed from the tergal region of the first abdominal seg- 

 ment and more closely associated with that of the metathorax. The 

 rest of the first segment is often reduced, or fused with the second, 

 and the sternal sclerotization is sometimes obliterated. The first pair 

 of spiracles, however, are nearly always retained, and the spiracles 

 will generally furnish a key to the basal segmentation of the abdo- 

 men where the segmental limits are obscured. In the aculeate Hy- 



