ARTHUR dp:witt whedok 429 



would be as stout as possible. But in the Zygoptera the males 

 are almost invarial^ly not only longer but more slender. The 

 latter peculiarity could be accounted for especially as there seems 

 to be no difference in the size of the muscles of the two sexes, by 

 the presence of the large ovaries in the female, but there is no 

 explanation thus far for excess of length in the male. It will 

 be seen from the figures that the sternal muscles of the Zygoptera 

 and the Aeshninae, in which copulation occurs during flight and 

 the male retains his hold during oviposition, are relatively 

 stronger than in the Libellulinae. The lifting of the female, who 

 is often passive, during the transfer of sperm capsules from the 

 ninth segment to the copulative organs of the male on the second 

 segment, possibly requires stronger sternal and inferior tergal 

 muscles. (See Wesenberg-Lund, page 204.) 



3. Respiration. As a final suggestion, it may be said that the 

 elongated form may have some value in the respiratory processes 

 of so exceedingly active an insect as the adult dragonfly. No 

 part of the internal structures is far from the great tracheal 

 trunks. Numerous air sacs and spaces are to be found, though 

 they have not been worked out, in the ends and dorsal regions 

 of the aljdomen (plate XXV, figure 24). Packard (1898) states 

 that such sacs are reservoirs for the storage of air for respiration 

 and that they do not aid in flight and the l)Uoyancy of the 

 body. Regarding these matters there is still room for difference 

 of opinion, but these sacs certainly bear some vital relation to 

 the life of the dragonfly. 



After all the present evidence has been collected it is perfectly 

 clear that the general question of the adaptation of the abdomen 

 in various groups and in the sexes can only be solved, if at all, 

 by very close studies of a large number of species in the field 

 representing different groups and especially different habits and 

 life activities. The anatomical features set forth in this paper 

 may then be apphed. It is not unlikely that such study will also 

 involve insects of other groups, as marked cases of abdominal 

 elongation are to be seen in many representatives of the orders 

 Hymenoptera, the Neuroptera (Mijrmelcon, etc.), the Diptera 

 (Tipula, the Asilidae, etc.), and some others. In some of 

 these there is a most remarkable difference in the sizes of the 

 sexes. 



TRANS. AM. ENT. SOC, XLIV. 



