HYMENOPTEllA — SAW-FLIES, ETC. l7l 



the more important, its direction and the point at which it is re- 

 ceived in the cell above it giving several useful characters. 



" We have now, I believe, dealt with all the nervures which are 

 regularly present in the upper wing of a saw-tly. But certain others 

 which appear only in particular genera or families are for that very 

 reason especially useful for 'determinations.' Thus, between the 

 costa and sub-costa may lie a 6th longitudinal nervure, and this may 

 ultimately either run simply into the sub-costa somewhere near I, or 

 be forked at its apex into two branches, one joining the sub-costa and 

 the other the costa. Or, in the same region (the intercostal field), 

 there may be a transverse nerve stretching from the sub-costa to the 

 costa, either before or after the point where the former receives the 

 discoidal nerve. Or, as in Arge, the costa, instead of lying wholly on 

 the margin of the wing, may quit it just before its apex, and bend 

 down to meet the radius, thus cutting off from the rest of the radial 

 area a little subtriangular apical cell (ceUula radialis ajixjendkulata). 

 Lastly, in the humeral area (' lanceolate cell ') we have a number of 

 important characters for determining genera depending partly on the 

 presence or absence of transverse in that field, and partly on certain 

 modifications in the structure of the humerus itself, especially in its 

 basal part. These characters we have now to examine. 



"It will be seen by reference to fig. 159 that the humerus (quite near 

 its base) shows a strong inclination to unite with the brachius, long 

 before it ultimately does so, at g. Generally this inclination is, if we 

 may say so, suddenly checked : the Jiumerus, though approaching very 

 near the brachius, starts off at a tangent, and gradually recedes to a 

 respectful distance from it, before taking the final curve by which it 

 ultimately reaches it. Sometimes, however, the inclination is not 

 checked : the humerus continues its approach to the brachius till 

 it actually reaches it, and so the two veins for a while coincide, — it 

 may be only for a moment or for a considerable distance, — but always 

 separate again, so as to enclose a spindle-shaped space between them 

 before their final point of union. Again, sometimes the humerus, 

 without quite reaching the Ijrachius at the (sub-basal) point alluded to 

 above, all but does so, and throws a 'short perpendicular' nerve 

 across the narrow interval which separates it from its companion vein. 

 Yet again the humerus, soon after its origin, seems to vanish and pres- 

 ently to reappear emerging from the brachius, though it has never 

 been seen to join the latter. I^astly, sometimes — though never, I 



