236 THE AMERICAN NATURALIST. [VoL. XXXII. 
is strictly homologous with cell 2) being composed of a large 
number of secondary cells. 
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 corrugations. 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 costa and 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 expanded. 
It rarely happens that there is occasion to refer to individual 
members of either of these classes of folds, except, perhaps, to- 
the one that has just been designated as the subcostal fold. 
But there are three other furrows which it is necessary to 
designate, as we shall have frequent occasion to refer to them. 
These we term the anal furrow, the median furrow, and the 
nodal furrow, respectively. They may be defined as follows: 
The anal furrow. This is a longitudinal furrow which is 
usually between the cubitus and the first anal vein (Fig. 6, F). 
It has been referred to by many writers, but the variableness 
of its position has not been pointed out. 
The median furrow. This is a longitudinal furrow which is 
usually between radius and 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 (bullze) in certain veins. 
The nodal furrow. This is a transverse suture beginning at 
a point in the costal margin of the wing, corresponding to the 
nodus of the Odonata and extending towards the inner margin 
of the wing. It crosses a varying number of veins in different 
orders of insects. 
The furrows of the wing are in no sense homologous or even 
analogous to veins. More than this, as will be shown repeatedly, 
the relative positions of the furrows and of the wing-veins are 
not constant; for it frequently happens that the course of a vein 
has been so modified that it crosses the line of a furrow and 
the relative positions of the two are thus reversed. If this fact 
