JOURNAL 



J}f\a igopfe ^Intomologiral Horiptg. 



Vol. XL SEPTEMBER, 1903. No. 3. 



THE SKEWNESS OF THE THORAX IN 

 THE ODONATA. 



By Jamks G. Needham and Maude H. Anihonv. 

 (Plate VIII.) 



Any one looking carefully at a dragonfly sees that the legs are at- 

 tached far forward and the wings far back upon the thorax, and that 

 the side plates of the latter are decidedly aslant. This arrangement 

 of parts is an adaptation to perching on the sides of vertical stems 

 without much alteration of the position maintained in flight. It 

 makes for celerity in stopping and starting again. The legs are 

 thrown forward where they readily reach and grasp the vertical stem, 

 and the wings are shifted backward and tilted so that their cutting 

 edges are directed oblicjuely upward, in which position a simple scul- 

 ling action lifts the body instantly from its support. 



In the jumping Orthoptera exactly the reverse inclination of the 

 lateral thoracic sclerites has taken place : the legs have been shifted 

 backward — especially the large hind ones used in jumping — and the 

 side pieces are aslant with the opposite inclination. Doubtless these 

 lateral sclerites (episternum and epimeron) were primitively placed at 

 right angles to the axis of the body, so that the sutures between them 

 were vertical, as they still are when first developed in dragonfly and 

 grasshopper alike. 



Among the orders the Odonata are extremely isolated, and, in 

 their own way, undoubtedly highly specialized. As marks of their 

 isolation the accessory genitalia of the males developed in an isolated 

 position on the ventral side of the second abdominal segment, the 



