THE WINGS OF ODONATA 



241 



Fig. 241. — Base of wing of Heliocharis 



abrupt bends in these veins in the region of the triangle, which are so 

 characteristic of the Anisoptera, are only slightly developed here. This 

 results in the areas corresponding to the triangle and supertriangle of the 



Anisoptera being in a direct 

 line and forming a quadrangu- 

 lar area, which is termed the 

 quadrangle (Plate IV, Fig. 2, q). 

 In a large part of this order, 

 the cross-vein separating the 

 parts of the quadrangle cor- 

 responding to the triangle and 

 the supertriangle of the Anis- 

 optera is lacking, in which case the quadrangle consists of a single cell 

 (Plate IV, Fig. 2, q). In some members of this suborder, as Heliocharis, 

 it is present. In Figure 241 representing the base of a wing of a species of 

 this genus, the two cells of the quadrangle are labeled t and s to facilitate 

 comparison with figures of wings of Anisoptera. In certain other members 

 of this suborder the quadrangle is divided into several cells by cross-veins 

 (Fig. 242). 



The suhquadrangle of the Zygoptera. — The cubital area of the wing is 

 usually quadrangular in outline in the Zygoptera, and is termed the sub- 

 quadrangle, as it lies immediately behind the quadrangle (Plate IV, Fig. 2, 

 sq) . Like the quadrangle, it may consist of a single cell or it may be divided 

 by cross- veins (Fig. 242). 



(c) THE METHODS OF SPECIALIZATION OF THE WINGS OF THE ODONATA 



The more striking of those features that distinguish the wings of the 

 Odonata are easily observed ; and most of them do not occur in the wings 

 of other insects. They are the following : the invading of the area of the 

 radial sector by one or more of the 

 branches of media, this probably occurs 

 also in the Ephemerida; the peculiar 

 arrangement of the veins that has 

 resulted in the formation of the triangle 

 of the Anisoptera and the quadrangle 

 of the Zygoptera ; the presence of the 

 nodus, this is very distinctive in ap- 

 pearance, but a nodal furrow occurs 

 in certain other insects, as in Cicada, 

 for example; the development of a secondary anal vein; and the most 

 perfect development of the spread fan-like type of wing. An approach 

 to the latter is seen in the Ephemerida; and may have existed in the 

 Protodonata, a group of insects of the Carboniferous period, that are 



Fig. 242. — Base of wing of Iletaerina. 



