94 



longitudinal veins, which are connected by many cross-veins, 

 the area R^ (which is strictly homologous with cell ^? 2 ) being 

 composed of a large number of secondary cells. 



The furrows of the wing. The wings of comparatively 

 few insects present a flat surface ; in most cases we find that 

 the membrane is thrown into a series of folds or corruga- 

 tions. This corrugating of the wing in some cases adds 

 greatly to its strength. This is well shown by the wings of 

 dragon flies ; and in most orders the costal margin of the 

 wing is strengthened 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 ex- 

 panded. 



It rarely happens that there is occasion to refer to indi- 

 vidual members of either of these classes of folds, except, 

 perhaps, to the one that has just been designated as the sub- 

 costal fold. But there are three other furrows which it is 

 necessary to designate, as they are frequently referred to. 

 These are the anal furrow, the median furrow, and the 

 nodal furrow. Only the first of these is well-marked in 

 the Diptera ; but for sake of completeness all are defined 

 here. 



The anal furrow. This is a longitudinal furrow which is 

 usually between the cubitus and the first anal vein. In the 

 figure of the wing of Rhyphus (Fig. 2), it is represented by 

 a dotted line immediately behind the cubitus. In the order 

 Diptera it is always in this position if present. 



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 Hemiptera, where it separates the 

 embolium from the remainder of the corium ; and in the 

 Hymenoptera its course is marked by a series of weak spots 

 (bullae) in certain veins. 



