154 PHILIP p. CALVERT. 



much smaller than the other two thoracic segments, which are closely 

 united to each other ; tarsi of three joints ; wings large, flat, mem- 

 branous, many-veined, of nearly equal length, furnished with an 

 opaque pterostigma on the front margin near the apex. Abdomen 

 <jf ten distinct and one anal segment, terminated with a pair of un- 

 jointed, dorsal appendages. Males with the external opening of the 

 testes on the ventral surface of the ninth abdominal segment, penis 

 and accessory genital organs on the ventral surface of the second 

 segment, and one or two terminal, inferior, abdominal aj)pendages. 

 Females with the vulva at the ventral apex of the eighth abdominal 

 segment, no inferior, terminal abdominal appendages. 



Nymph (i.e. the form from egg to imago) aquatic ; labium (mask) 

 of large size, protractile, and functioning as a seizing organ. Res- 

 piration by tracheal gills. 



The divisions here recognized are: 

 Order ODONATA. 



Suborder Zygoptera. 

 Family Agrionidse. 



Subfamily Calopteryginse. 

 Subfamily Agrioninse. 

 Suborder Anisoptera. 

 Family Aeschnidse. 



Subfamily GomphiuEe. 

 Subfamily Cordulegasterinse. 

 Subfamily Aeschninae. 

 Family Libellulidie. 



Subfamily Cordulinse. 

 Subfamily Libelluliuse. 

 They are defined in Part II. 



1. Structure of the Imago, 

 the skeleton. 



The most prominent external features of the Odonata are the large 

 size of the head and its distinctness from the rest of the body, the 

 compact thorax bearing the powerful and many-veined wings, and 

 the length and slenderness of the abdomen. 



Of the head (Pi- II, fig- H), hy far the most conspicuous parts 

 are the eyes. Even in those forms (Agrionidse) in which they oc- 

 cupy the relatively smallest area of the head, their proportionate 

 size is only occasionally equaled in other groups of animals. In all 



