1903.] 49 



medius comes a continuation of the medial area which, if regarded as distinct from 

 it, may be called the discoidal field. That part of the wing lying below and as it 

 were outside of the neuration system, i. e., that which is bounded inferiorly by the 

 actual inferior margin of the wing, and superiorly by the humerus as to its basal 

 half), and by the medius (as to its apical portion), is called the anal field. 



Owing to the disappearance of the brachius without reaching the margin, there 

 is no complete longitudinal line of division between the brachial and anal areas. 

 But for practical purposes the transverse nerve t x may be regarded as separating 

 them. 



We come now to the transverse elements of the neuration — 

 Konow's " Nerven," sensii restricto. 



Often (though not in my figui-e) the radial area is crossed by one — very rarely 

 by more than one. Then the radial area is said to be " divided," or, as some authors 

 express it, " there are two radial cells."* Similarly the cubital area shows, practi- 

 cally without exception, either as here tioo, or perhaps more often three, nerves 

 connecting the radius with the cubitus (/ »2,y p). These, as crossing the cubital 

 field, are called the cubital nerves (1st, 2nd, &c.), and the divisions into which they 

 cut up that field are the cubital cells. {Three seems to have been the original 

 number of the cubital nerves. Where only two appear, either the first or second 

 of the original three has vanished — thus, in Emphytus the first has gone, leaving 

 only the second and third ; whereas in Z'o/erw* the two surviving nerves are the 

 first and third, the second being absent). 



Although these radial and cubital transverse nervures give, both as to their 

 number and direction, obvious and easy characters for distinguishing both genera 

 and species, they are unluckily liable, as mentioned in my last paper, to considerable 

 Tariation — disappearance, duplication, displacement (within certain limits), irregular 

 (atavistic) re-appearance, &c., in particular specimens, or even in one wing of a 

 specimen and not in the other ; so that it is very unsafe to trust wholly or even 

 chiefly to them in " determinations." 



Very much more constant and trustworthy are the characters to be drawn from 

 the three nervures which cross the median field, viz., I q (perhaps the most impor- 

 tant nerve in the whole wing) the discoidalf nerve, m s and o u the medial nerves 

 (1st and 2nd) — the two latter being better known probably to English readers as 

 the 1st and 2nd '• recurrent," and the former as the "basal." The characters of 

 these nerves can hardly ever mislead us, and are of the utmost consequence in de- 

 terming not merely Grenera or Species, but Families and Tribes — such characters, 

 e. g., as whether the discoidal nerve strikes the subcosta close to the origin of the 

 cubitus (as in Fig. 3), or considerably before it (t. e., between the points b and I in 

 that Figure), or whether it strikes (e. g., in Lyda, &c.) not the subcosta at all, but 

 the cubitus : and again whether the discoidal and 1st medial nerves are convergent 

 (upwards) or subparallel, whether the two medial nerves are received in the same 

 cubital cell or in two different ones, &c. The importance of these points for " de- 

 termination " will appear abundantly when we come to construct our futui'e Tables. 



Two more transverse nerves only appear in my figure, viz., r w, the areal nerve 



* Konow has abandoned this latter expression in his most recent works. 

 t So called us crossing the " disc " or central part of the wing. 



