22 THE IMAGO [CH. 



modified to receive the reduced and highly specialized anal 

 appendages of the male, in such a way that a firm interlocking 

 of parts may be secured (see p. 37). 



The Synthorax (fig. 1). The remarkable form of the synthorax 

 in Dragonflies is correlated with special conditions of the legs and 

 wings. The former have moved forwards towards the mouth, 

 carrying the sterna with them. The wings, however, have moved 

 backwards away from the head, carrying the terga with them. 

 The reduction in size of both terga and sterna is more than counter- 

 balanced by an enormous development of the pleura. To under- 

 stand the significance of these changes fully, the student should 

 trace the development of the thorax in the larva, from the time 

 that the wing-buds appear (p. 74). 



The well-known obliquity or "skewness" of the Dragonfly 

 thorax has been ingeniously measured by Needham and Anthony 

 [107]. If the wings be spread out into a single plane, as if for 

 flight, that plane practically represents the notal plane of the 

 synthorax, and its obliquity to the long axis of the body may be 

 measured by the slant of a line drawn through the two axillaries 

 (c, ax 2 , ax 3 ) or other suitable points. This angle of obliquity is 

 much smaller in the Anisoptera than in the Zygoptera, the approxi- 

 mate ranges being from 19 to 38 in the former, and from 35 to 

 62 in the latter. Thus we see that, in this character, the Anisoptera 

 have remained in much the more primitive condition. The ability 

 of the Zygoptera to fold their wings back above the abdomen is 

 of course correlated with the increase in the angle of obliquity. 



The obliquity of the synthorax is brought about chiefly by the 

 great development of the mesepisterna (eps 2 ), which grow forwards 

 and upwards in front of the tergum, so as to meet in a long middle 

 line, forming the dorsal carina or ridge (dr). Thus they form, in 

 their new position, not only the humeral portion of the thorax, 

 but also all the dorsal part in front of the ante-alar ridge (air). 

 The true terga are pushed backwards and lie between the wing- 

 bases. Further, there is a correlated growth of the metepimera 

 (epm 3 ) downwards and backwards, which pushes the sterna and the 

 legs forwards, so that the latter come into position close behind 

 the mouth, and are used for holding the prey during flight. In 

 most Dragonflies (G) the metepimera actually meet and fuse 



