386 MORPHOLOGY OF THE ABDOMEN IN THE ODONATA 



central portion, but with a tendency to lateral compression. The 

 swollen region of the first three segments is about equal in the 

 two sexes, though the male organs project more beneath. A 

 thickening of the i-egments from seven to ten in most of the 

 Gomphinae and in Macromia produces a clubbed effect. This 

 appe ranee is enhanced in the former by the oat-turning of the 

 usually ventral, latera^ margins of the terga, forming narrow 

 winglike expansions. Females show less of this and are stouter. 

 In Macromia the constriction beyond the second segment occurs 

 rather suddenly and is soon followed by a gradual thickening 

 until the greatest diameter is reached in the eighth, giving a 

 very graceful and mobile form. In these and most of the follow- 

 ing groups the swollen base of the abdomen so approaches the 

 massive thorax in size and contour as to cause a gradual transition 

 from one to the other of these regions of the body. 



In the Aeshninae various modifications exist from the tapering, 

 tubular form of the great Epiaeshna heros to the conditions in 

 Aeshna where the venter becomes flatter, the longitudinal carinae 

 distinct and the great enlargement of the anterior segments is 

 immediately succeeded by the narrowest part of the abdomen in 

 the third segment, somewhat similar to the constriction in the 

 Hymenoptera. Anax differs from Aeshna in being of heavier 

 build, less narrowed in the third segment, and in having a ten- 

 dency toward dorso-ventral flattening and the further develop- 

 ment of longitudinal lateral carinae. 



The typical form in the Libellulinae is much depressed dorso- 

 ventrally, with well marked mid-dorsal carina, broad, flat venter, 

 and gradually tapering width. Frequently, especially in the 

 females, the ventral portions of the terga are turned outward as 

 in the Gomphinae but to less extent. A cross-section of the 

 abdomen is often triangular. These characters hold for even 

 such minute forms as Nannothemis and Perithemis. 



The anal appendages mentioned in an earlier section are very 

 diverse in difl^erent groups and even so to the species. The}" 

 form the favorite characters for the determination of many 

 species. As their size, however, is small compared with the rest 

 of the abdomen, and they seem used exclusively in pairing, it is 

 unnecessary to discuss them here. 



