ARTHUR DEWITT WIIEDOX 385 



been available for study. In Zygoptera the relation of greatest 

 width to length of abdomen is 1: 5 or 1: 6, and there seems very 

 little variation in size between species that differ nmch as imagoes. 

 In the Gomphinae the abdomen is widest at about the fifth or 

 sixth segment and is ciuitc or even very wide. The ratio for 

 G. cor7iutv.s is about 1 : 3.4 for thorax and abdomen, and 1 : 3 or 

 1: 4 for width and length of abdomen. The njanph of Ilayeniu.s 

 (plate XXVI, figure 30) is a verj^ remarkable one, having a flat- 

 tened nearly orl)ic'ular abdomen four-fifths as wide as long. The 

 larvae of the Libellulinac are thick bodied. 



When the relation of larval to adult abdomen is observed, the 

 ratio of 1:5 for .l/ec/.s/o(/n.sfer is unapproached l)y anj^thing else; 

 the next in line being 1: 3.7 to 1: 4.3 in Arcjia (apicalis 1: 3.8, 

 moeata p^itrida 1:3.7, and tibialis 1:4.3). In the Lestinae, 

 Lestes rectangularis is 1: 2.5, and in the Coenagrioninae the Enal- 

 lagmas arc from 1:2.3 to 1:3. The Anisoptcra show differences 

 running from 1:4 in certain Gomphi and 1 : ]. 7 in Aeshna to 1:2.6 

 in Sympetnini. Such figures are but the crudest approximations, 

 as exact ratios jnust be obtained from a series of larvae or exuviae 

 and the adults they produce. Except in the cases of Mecistogas- 

 ter incdestus and Coniphus coniutKS these are not at hand. The 

 general conclusion that less difference exists in abdominal dimen- 

 sions between n>-mph and adult in the higher groups than in the 

 lower is certainly safe. 



Modifications in Shape 



A description of minute details of comparative shape is not 

 necessary for the purposes of this paper. A few general conditions 

 should be mentioned. 



The slender cylindrical form of Calopterjjx with its slight dila- 

 tion in the first and second segments and at the tip is scarcely 

 changed throughout the Zygoptera. Between the sexes there are 

 only such difference's as are necessary to accommodate the sex 

 organs, and to allow for the attachment of the muscles activating 

 the anal appendages in the male or the ovipositor in the female. 

 Carinae are not strongly developed, and accessory carinae are 

 never present. 



In the Gomphinae and many of the Cordulinae the slender, 

 •cylindrical form is very pronounced, especiallj^ through the greater 



TRAXS. AM. ENT. SOC, XLIV. 



