384 MORPHOLOGY OF THE ABDOMEN IN THE ODONATA 



female; the length of the thorax is here the same in both sexes 

 with an abdominal variation of at least 3 mm. Aeshna multicolor, 

 a very close relative of nnitata, shows tendencies in the same 

 direction. The same is true of A. interrwpta interrupta. 



Cordulinae: Didy7nops appears to have the females the 

 longer. Williamson's (1909) revision of the North American 

 species of Macromia, though based upon limited collections, is 

 as yet the most extensive work upon the genus. He gives full 

 measurements. In l)ut two of his nine species, M. avstralensis and 

 M. pacifica, are the females the sJiorter. Usually the difference 

 is not great but in M. ivabashensis, of which but one female was 

 taken, it is greater: male 54.4 mm., female 58 mm. Much the 

 same size relation exists l^etween the sexes of Epicordulia prin- 

 ceps. If wc may judge from Muttkowski's "Studies in Tet- 

 ragoneuria'' (1911) and from a half-dozen specimens in the 

 writer's possession, Tetragonenria shows nothing but the nor- 

 mally shorter female. The genus Somatochlora seems, though 

 it has not been well worked up, on the other hand to reverse 

 the condition, having females perhaps a millimeter the longer; 

 5. Jilosa, however, has lengths of 41 mm. for the male and 48 mm. 

 for the female. 



Libellulinae: The most noticeable characters of this group 

 are the reduced length and increased width of the abdomen, and 

 its uniformity in shape. The normal relation between sexes is 

 but seldom changed and then only slightly as in Syynpetrum 

 vicinum and corruptum, Pantala flavescens and hymenaea, and 

 Tramea onusta. 



Size is a matter of much less variation in the larvae than in 

 the adults. There are but two general types: the tapering cylin- 

 der of the Z^^goptera and the oval ventrally flattened forms of 

 the Anisoptera. The measurements of a few representatives of 

 the groups are given in the table (page 388) . These were taken 

 from material in hand. In addition use has been made of Gar- 

 man's (1917) measurements of Zygoptera, and of figures on Mecis- 

 togaster furnished by Dr. Calvert. So far as comparisons have 

 been made the ratios of thorax to abdomen have fallen close to 

 1:3.5 in the Zygoptera, 1:3 in the Gomphinae and Aeshninae, 

 and 1 : 2.5 in the Libellulinae. Very few species have, however.. 



