394 THE WINGS OF INSECTS 



dragon-flies; and in most orders, the costal margin of the wing is strength- 

 ened by a fold between the costa and the radius, the subcostal fold. In other 

 cases, the corrugations are the result of a folding of the wing when not in 

 use; this is well shown in the anal area when this part is broadly expanded. 

 It rarely happens that there is occasion to refer to individual members of 

 either of these classes of folds, except, perhaps, to the one that has just been 

 designated as the subcostal fold. 



The furrows of the wings. — There are found in the wings of many 

 insects one or more suture-like grooves in the membrane of the wing; these 

 are termed the furrows of the wing. The following furrows have received 

 distinctive names. They occur chiefly in the fore wings. 



The anal furrow. — The anal furrow is usually either between the cubitus 

 and the first anal vein or it is coincident with the first anal vein which it 

 may supplant in forms in which the venation of the anal area is reduced. 

 This is the case in the Lepidoptera and the Diptera, and is well-shown in the 



wings of Bombyx mori (Fig. 413), in 

 which a vestige of the first anal vein 

 is preserv^ed near the margin of the 

 wing. 



The median furrow. — This is a 

 longitudinal furrow which is usually 

 between the radius and the media. 

 ^ It is well-marked in many of the 



Fig. 414.— Diagram of a wing showing Hemiptera, where it separates the 

 margins and angles. . 



embolium from the remanider ot the 



coirum; and in the Hymenoptera its course is marked by a series of weak 



spots (bullae) in certain veins. 



The nodal furrow. — This is a transverse suture beginning at a point in 

 the costal margin of the wing, corresponding to the nodus of the Odonata, 

 and extending towards the inner margin of the wing. It crosses a varying 

 number of veins in different orders of insects. 



The axillary furrow. — The axillary furrow is a suture-like line extending 

 from the base of the wing to the inner margin; it ends at the axillary exci- 

 sion (a notch near the base of the wing) when this is present. 



Margins of wings. — ^An insect's wing is more or less triangular in outline ; 

 it, therefore, presents three margins; the costal margin (Fig. 414, a-b)\ 

 the outer margin (Fig. 414, b-c)\ and the inner margin (Fig. 414, c-d). 



Angles of wings. — The angle at the base of the costal margin (Fig. 

 414, a) is the humeral angle; that between the costal margin and the outer 

 margin (Fig. 414, b) is the apex of the wing; and the angle between the 

 outer margin and the inner margin (Fig. 414, c) is the anal angle. 



