118 Journal New York Entomological Society. [Voi. xi. 



type of venation, and the remarkable structure of the labium — espe- 

 cially, of the nyniphal labium — have been frequently noted. But this 

 skewness of the thorax, hitherto almost unstudied, is the external evi- 

 dence of the most profound alterations of the whole bodily organiza- 

 tion. As for the skeleton, the legs have moved forward and the 

 wings backward, greatly increasing the areas between the sternum of 

 the metathorax and the abdomen, and between the tergum of the 

 mesothorax and the prothorax respectively, and these areas have been 

 overgrown by neighboring lateral sclerites (mesepisternum in front and 

 metepimeron behind). The unusual proportions and the new (dorsal 

 and ventral) positions thus attained by these sclerites were long a 

 puzzle to many eminent entomologists. The question of their homol- 

 ogies was finally set at rest by a study of the segmental muscles and 

 sutures made by Dr. Calvert for his well-known catalogue, published 

 in 1893. He showed that the muscles have retained fully their seg- 

 mental arrangement, the wing muscles becoming enormously enlarged 

 and taking on the general inclination of the thorax. The mid-lateral 

 suture is completely and the others are almost obliterated. 



This fusion of sclerites is doubtless an accompaniment of the in- 

 creasing power of the wing muscles. The skeleton is further strength- 

 ened by the development of a unique system of carina, the strongest 

 of which is the mid-dorsal thoracic carina, formed at the junction of 

 the mesepisterna along the dorsal line, forking above and ending in 

 an antealar crest, ending below in a transverse collar-like ridge abut- 

 ting against the prothorax. There are also carinae along the upper 

 ends of the lateral sclerites about the wing bases, and others trussing 

 the iloor of the metathorax between the bases of the hind legs and the 

 abdomen. Doubtless these all contribute to the strength of the tho- 

 racic skeleton, and enable it to withstand the pull of the enormously 

 large and powerful wing muscles. If in a dragonfly that has newly 

 emerged from the nymphal skin and that has not yet had time for the 

 hardening of the skeleton, the muscles be stimulated artificially to 

 contract (as by putting in alcohol or cyanide bottle) they draw the 

 thorax into a crumpled and contorted condition. Doubtless a careful 

 study of this system of carinte, and of the external topography of the 

 thoracic skeleton in general would yield good results : but it is a less 

 ambitious undertaking that this paper records. 



Impressed by the differences in degree of skewness in the thorax 

 of a number of dragonflies that were lying before him one day, the 



