No. 39] ODONATA OF CONNECTICUT 27 



Still further back into the Carboniferous and they may have arisen 

 at approximately the same period. In view, however, of the evi- 

 dence available at the present time, the original order of the 

 families, Agrionidae and Coenagrionidae, is reversed, but this does 

 not mean necessarily that either is more generalized. 



The abdomen is composed of ten complete rings or segments 

 variously modified and pigmented. The spiracles of the segments 

 (except the first) are hidden completely by the overhanging terga 

 and the pleura reduced to small membranes beneath the lateral 

 edges. The sterna or sclerites of the ventral surface are often 

 reduced to a line, or hidden entirely by the approximated terga, 

 while on segments two and nine the sterna are modified to form 

 the genitalia. The male genitalia of segment two, known as acces- 

 sory, are unique, being found in no other order of insects. Their 



Fig. 5. Nymph of Zygopteron (Ischnura verticalis) showing parts. 



structure, represented in Fig. 20, affords an important means of 

 classification. 



The ovipositor of the female, situated on segment nine, is shown 

 in Fig. 26. The ovipositor, however, is frequently reduced to two 

 small plates on the ventral surface (see Fig. 19). Attached to 

 the tip of the tenth segment are the anal appendages. Their 

 structure varies a great deal in the male sex and offers another 

 important feature of classification. 



Nymph 



The nymph differs from the adult in many features. In the 

 head, the appendages surrounding the mouth are essentially the 

 same, but the labium is different, especially the shape of the labial 

 palpi (Fig. 7). The marginal setae, mental setae, setae of the 

 labial palpi and general form of the mentum are important in 



