628 THE PANORPOID COMPLEX, ii., 



the general plan of the wing-venation, it will be necessary, first 

 of all, to define the composition of this venation somewhat more 

 strictly than is usual. 



According to the well known, and b}^ now generally accepted, 

 theory of Comstock and Needham (14), the venation of an insect's 

 wing has been original^ determined by the courses of the pre- 

 cedent tracheae, which supply the growing wing-rudiment of the 

 larva or nymph. From the six main trachete that enter the 

 growing wing, tliere are thus developed six maiv i^eiiis, known 

 as the costa, subcosta, radius, media, cubitus and analis, respect- 

 ively. Some of these main veins may be branched; such primary 

 brandies are termed sectors. 



In the case of a main vein which branches and rebranches 

 several times, it is clear that the final branchlets may be botli 

 very small and very numerous. T propose to term such branch- 

 lets rciuhts. They are to be distinguished from cross-veins (see 

 below) by the fact that they are always preceded by definite 

 tracheae. 



In contradistinction to a veinlet, I propose to restrict the term 

 cr^ss-veiit to a short connecting vein which does not form part of 

 the branching system of any main vein, and is developed inde- 

 pendently of the precedent tracheation. 



It should be clearly understood that there is no reason why 

 such true cross-veins should not be developed upon any portion 

 of a wing, and at any thnc, provided the need for them arises. 

 There is thus, in reality, no difference between true cross-veins 

 and the so-called false cross-veins, which appear occasionally in 

 newly-expanded areas of the vyings of highly specialised genera; 

 as, for instance, in the enlarged humeral area of the hindwing of 

 Lasiocampa. The only difference between these newly formed 

 cross-veins and those found in older wings is one of time; where 

 the geological record is available to us, as in the case of the 

 Psychoj)sida', the rise of the true cross- veins from a similar origin 

 is clearly indicated. 



In order to appreciate the difference between veinlets and 

 cross-veins more clearly, I have figured the tracheation and oor- 



