THE WINGS OF THE ISOPTERA 143 



States, it may be that this is merely an instance of variation; but if so it is 

 a remarkable one. 



In these wings and in those of Leucotermes flavipes the loss of tracheae has 

 proceeded to a greater extent than in Termopsis. In fact the three genera 

 illustrated here form a good series showing the direction of specialization 

 as regards the wing tracheae in this order. In Mastotermes, the costal 

 trachea is the only one lost in the preanal area; in Termopsis, the costal 

 trachea is lost, the subcostal tracheae is reduced to a nearly unbranched 

 condition, and trachea R2+3 is represented by a single trachea; in Leuco- 

 termes both the costal and the subcostal tracheae are lost, and the radial 

 trachea is reduced to an unbranched condition. 



resume; 



The series of wings described in this chapter show the following steps 

 in specialization: 



Common features. — In all of the wings examined there is an absence of 

 regular cross-veins, the membrane of the wing being stiffened by an 

 irreg-ular network of slightly chitinized wrinkles ; the veins in the anterior 

 part of the wing are greatly thickened, those in the intermediate region are 

 reduced to thin lines, and there is a group of wide accessory veins borne by 

 cubitus near the base of the wing. 



Humeral suture. — In Mastotermes darwiniensis , the most generalized 

 of the living termites known, a complete hiimeral suture has been developed 

 in the fore wings, but in the hind wing this suture is lacking. In Termopsis 

 there is a complete humeral suture in the fore wing, as in Mastotermes, and 

 in the hind wing the anal area is crossed by a suture that appears to be the 

 beginning of a humeral suture. In Leucotermes there is a complete humeral 

 suture in the hind wings as well as in the fore wings. 



The suhcosto-radial veins of the fore wings. — In Mastotermes the veins 

 Sci, Sc2, Ri, R2, R3, R4, and R5 are preserved distinct. In Termopsis 

 angusticollis all of these veins are presented distinct in some individuals, 

 but in other individuals there is a greater or less reduction in the number of 

 veins in the subcosto-radial area of the wing. In Leucotermes there are 

 only two veins in this area of the wing : first, a very much thickened costal 

 margin of the wing, which probably was formed by the coalescence of two 

 or more veins; and second a thick vein parallel with the costal margin, 

 which is probably vein R4+5. 



The suhcosto-radial veins of the hind wings. — In the more generalized 

 members of the order, Mastotermes and Termopsis, there arc, in addition to 

 a more or less thickened costal margin, three distinct veins in front of the 

 media. These are veins Sc2, R2+3, and R44-5. In Leucotermes there is a 

 greatly thickened costal margin and what is probably vein R4+5. 



