320 AN INTRODUCTION TO ENTOMOLOGY 



cross-veins. Other named areas are thebasal anal area (Fig. 364, ha) 

 and the cubital area (Fig. 364, ca). 



The writer has given in his "The Wings of Insects" an extended 

 discussion of the wings of Odonata, illustrated by many figures, in- 

 cluding a plate in which adjacent veins are represented in different 

 colors, so that the course of each can be easily followed. 



With the naiads of dragon-fiies there is a remarkable modification 

 of the organs of respiration, which fits these insects for aquatic life. 

 The caudal part of the alimentary canal, the rectum, is modified so 

 as to constitute a tracheal gill. It is somewhat enlarged ; and its walls 

 are abundantly supplied with tracheae and tracheoles (Fig. 365). 

 Water is alternately taken in and forced out through the anal opening; 

 by this process the air in the tracheae, with which the walls of the 

 rectum are supplied, is purified in the same manner as in an ordinary 

 tracheal gill. 



The rectal tracheal gill of the naiads of dragon-flies is an organ of 

 locomotion, as well as of respiration. By drawing water into the rec- 

 tiun gradually, and expelling it forcibly, the insect is able to dart 

 through the water with considerable rapidity. This can be easily 

 observed when naiads are kept in an aquarium. 



When the naiad of a dragon-fly is fully grown it leaves the water 

 to transform. The skin of the naiad splits open on the back of the 

 thorax and head, and the adult emerges, leaving the empty skin of 

 the naiad clinging to the object upon which the transformation 

 took place. Figure 366 represents such a skin clinging to the stem 

 of a water plant. 



The suborder Anisoptera includes two families, the ^Eschnidag 

 and the Libellulidae ; each of these families is represented in our 

 fauna by many genera and species. These are enumerated in the 

 "Catalogue of the Odonata of North America" by Muttkowski ('10). 

 The two families can be separated by the characters given below. 



Family ^SCHNID^ 



The Mschnids 



In this family the triangle (Fig. 364, t) is about equally distant 

 from the arculus (Fig. 364, ar) in the fore and hind wings; and, 

 except in the genus Cordulegdster, there is an oblique brace-vein 

 extending back from the inner end of the stigma (Fig. 364). 



The aeschnids are mostly large species ; among them are the largest, 

 fleetest, and most voracious of our dragon-flies. Some of them roam 

 far from water and are commonly seen coursing over lawns in the 

 evening twilight; but most of them fly over clear water. 



Comstock and Needham ('gS-'gg) and by Needham ('03). These conclusions 

 have been questioned by Tillyard ('22) and by Schmieder ('22); but I do not feel 

 that it would be wise to modify them before a much more extended investigation 

 of the subject has been made. 



