THE WINGS OF ODONATA 237 



SO. These two cross-veins and the transverse section of the cubitus outHne 

 the triangle (Plate IV, Fig. i). 



The triangle in Gomphus consists of a single cell; but frequently this 

 area is divided by one or more cross- veins into two or more cells (Fig. 237). 



The supertriangle . — This term is applied to an area of the wing lying 

 immediately in front of the triangle and extending towards the base of the 

 wing to the arculus (Fig. 230, s). Like the triangle, this area of the wing 

 may consist of a single cell, as shown in Figure 230, or may be divided by 

 one or more cross- veins (Fig. 237). 



The anal veins. — In the course of the evolution of the anal area of the 

 wings of insects of this order, there has been a specialization by reduction, 

 which has resulted in the preser\^ation of a single anal vein. 



There is available no data showing the cotirse of this reduction. In the 

 youngest nymphs that we have studied there is a single branched vein in the 

 anal area of each wing; and it is impossible to determine whether this 

 represents one, two, or all three of the primitive anal veins. For this 

 reason this vein is designated as the anal vein, without any predicate, and 

 its principal branches as Ai, Ao, A3, and A4. The use of the terms ist A, 

 2d A, and 3d A, for this purpose would indicate homologies that we do not 

 know to exist. 



The anal crossing. — The anal vein coalesces with vein Cu for a short 

 distance at the base of the wing (Fig. 230, Cu + A) ; and then separates 

 from vein Cu and extends another short distance transversely to the length 

 of the wing (Fig. 230, ac) ; it then makes a second abrupt turn and extends 

 outward. The short section of this vein that extends transversely has 

 been designated by Tillyard ('14) the anal crossing (ac). See also Plate IV, 

 Fig. i). 



The anal crossing has the appearance of a cross-vein, and has been 

 regarded as one; it is the first cubito-anal cross-vein of Needham's paper 

 ('03). Needham, however, furnished the data upon which the conclusions 

 regarding the course of the anal vein, as stated above, were based. His 

 figures of the tracheation of the n^-mphal wing of Lanthiis parvulns (Plate 

 VII, Fig. 2) and of the nymphal wings of Gomphus descripius (Plate V) were 

 what first suggested these conclusions. 



Although Needham recognized the course of the anal trachea, he 

 believed, at the time he first figured it, that it did not indicate the course of 

 the anal vein ; but that the basal part of the anal trachea had been shifted 

 fon\'ard from the position in which the anal vein is later developed, and that 

 the vein marked A' in Figure 230 is the basal part of the anal vein. 



Tillyard ('14) has shown, however, that the vein A' is a secondarily 

 developed bridge between the anal crossing and the base of the ^^ang ; a 

 conclusion that was independently reached by the writer before Tillyard's 

 paper was received in this countr\'. 



