XXIV 
THE HABITS AND STRUCTURE OF MOTHS. 
vures which may be absent, are counted as if present, in 
order that the numbers may correspond as far as possible. 
The subcostal and median nervures each divide into three 
branches, the subcostal nervure of the hind wings into two, 
in such a way that the separate branches spring successively 
from the main stem. On the hind wings no further division 
takes place, all the branches running out into the hind mar¬ 
gin, like those of the median nervure of the fore wings. On 
the fore wings, on the other hand, the first two branches of 
the subcostal nervure run to the costa (numbers 11 and 10); 
the third extends to the hind margin as nervule 6, but forks 
at or beyond the transverse nervule, and nervule 8 which here 
arises from it ends usually at or near the apex of the wing, 
after having previously split up and given off nervule 7 to the 
hind margin and nervule 9 beyond to the costa. Nervule 10 
is also frequently forked, and sends a branch towards the 
hind margin, which usually touches nervule 8 at the point 
where 7 arises, so that it is the continuation of the latter 
which then intersects 8. There are, however, many modifi¬ 
cations of the system of neuration, thus, owing to a different 
forking of the subcostal nervure of the fore wings and its 
branches, the fore wings of Microlepidoptera are more simply 
constructed. It is plain that the costa and apex of the fore 
wings, on which the greatest strain is brought to bear in flight, 
are strengthened by the more complicated arrangement of their 
nervures; whilst the hind wings require it less, since they fit 
close to the fore wings, and are provided with additional sub¬ 
median nervures toward the inner margin, which are sufficient 
to keep the broader surface extended. The submedian ner¬ 
vure of the fore wings and the costal nervure of the hind wings 
appear to be correlated to one another in accordance with 
their relative positions. They have both a tendency to bifur¬ 
cate towards the base. Thus when the costal nervure unites 
with the subcostal beyond its origin, a fork is formed, one 
branch of which is formed by the subcostal nervure itself. 
The spaces between the nervures are called cells, and are 
numbered like the nervures from behind forwards, in such a 
way that each cell bears the same number as the nervure below 
it. Thus the cell between nervures 2 and 3 is cell 2, that 
between nervures 3 and 4, cell 3, &c. The cells between the 
inner margin and nervure 2 are called cells la, lb, 1 c, and 
Id, counting from the inner margin, in such a way that cell 
la is that between the inner margin and nervure la, cell lb 
the next, and so on, the cell bearing the same number as the 
nervure above it. The cell which lies on the inner side of 
the transverse nervule, between this and the two principal 
