THE WINGS OF ODONATA 229 



it serves this purpose well. In the oldest member of this order known, 

 Diastatomma Hasina (Fig. 231), this intercalary vein is well-developed. It 

 may be regarded, therefore, as one of the primitive veins of this order. Its 

 appearance antedates the development of the nodus, and also that of the 

 secondary'' anal vein, two of the most distinctive features of modern dragon- 

 fly wings. In most of the recent Odonata, however, this vein is less 

 distinct than it is in Diastatomma. 



This table will be found useful for the purpose of identifying the veins 

 when in doubt; as the nature of a vein, whether convex or concave, and its 

 relative position with regard to other more easily recognizable veins will 

 serv^e as sure criteria. 



The corrugations of a wing can be best seen by examining it from a point 

 a little above or below its outer end. When viewed in this way, the wing 

 is seen to present a series of prominent, alternating ridges and furrows. 



(6) THE SPECIAL FEATURES OF THE WINGS IX THE ODONATA 



In general the veins and areas of the wings of the Odonata are desig- 

 nated as in the accounts of the wings of other orders of insects; but there 

 are certain features in the wings of insects of this order that are peculiar to 

 them, and that have made necessar}^ the adoption of a set of special terms, 

 which are here defined. The. following list indicates the abbreviations used 

 to designate these special features of the wings of the Odonata in the 

 illustrations. 



Abbreviations. — a', secondary anal vein'; ac, anal crossing; al, anal loop; 

 al', supplemental anal loop; an, antenodal cross- veins; ar, arculus; aspl, 

 apical supplement; at, anal triangle; ba, basal anal area; br, bridge; ca, 

 cubital area; cu-a, chief cubito-anal cross- vein; cual, cubi to-anal loop; 

 cuspl, cubital supplement; I Mi, IR^, etc., intercalary^ veins; m, mem- 

 branule; mspl, median supplement; n, nodus; 0, oblique vein; pn, post- 

 nodal cross- veins; q, quadrangle; rspl, radial supplement; s, super- 

 triangle; sn, subnodus; sq, subquadrangle ; st, stigma; t, triangle; t', 

 subtriangle; x, trigonal supplement. 



The stigma. — The stigma of the Odonata, like that of certain other 

 insects, is a limited area on the costal margin of the wing, between the 

 middle of its length and the apex of the wing, which is more dense and 

 usually darker in color than the other parts of the wing (Fig. 2 2g, st). The 

 stigma is termed the pterostigma by many writers. 



"The stigma is developed upon the cutting edge of the wing at the point 

 of greatest impact against the air. It would seem to serve the double 

 purpose of firmly uniting the veins of the front margin and of increasing the 

 efficiency of the wing stroke by adding weight at this striking point. Its 

 shape and extent vary considerably and are often characteristic of groups ; 

 but the stigma seems not to contain in itself such characters for the critical 



