380 
Linncean Society. 
for the functions they perform in a normal state, and ap])arently does 
not render them useful for any other function, can only be explained 
by conceiving it in some way to depend on the position of the ani- 
mal in the system of Nature. 
The system of neuration of the posterior wings in the Diurnal 
Lepidoptera, which may be considered normal as regards this group, 
is abnormal as it respects the whole order ; and it would seem 
as though Nature, by a partial return to a normal structure in a 
few genera, wished to indicate to us the real homologies of these 
parts. 
In general the posterior wings of the Diurnal Lepidoptera have the 
discoidal nervure, which in these wings never branches, so placed as 
to seem to be a third subcostal nervule ; but in some genera, although 
its basal is always wanting, its real character is very evident, and it 
is united to the subcostal nervure or one of its nervules, and also to 
the median nervure or one of its nervules, by distinct upper and 
lower disco -cellular nervules. In the Heliconidde we find this struc- 
ture, almost normal as it respects the order, in the genus Ituna, and 
also in Itliomia It is found in some female Ithomia, of v^ich. the 
males have a different structure, giving indications of that change of 
position which in the next genus might lead us to mistake the discoi- 
dal nervure for a fourth median nervule, the disco-cellular nervules 
being placed more obliquely, the cell becoming thereby more elon- 
gated, and the lower disco-cellular nervule appearing almost to form 
a continuation of the median nervure. In Mechanitis both sexes have 
this character further carried out, and the wing appears to have a 
subcostal nervure dividing into two nervules, and a median dividing 
into four, so completely has the discoidal nervure assumed the po- 
sition of a branch of the latter nervure. The females of the genus 
Sais have also this character, but in the males we find a still further 
change of structure. In these the second subcostal nervule assumes 
the position of a fifth median nervule, and the subcostal nervure con- 
sequently appears simple. 
Thus, leaving the genera Heliconia, Lycorea and their immediate 
allies, which have the structure which is normal as regards the Di- 
urnal Lepidoptera, though abnormal as regards the order, we find in 
Ituna and some female Ithomioi a structure nearly normal as regards 
the whole order, but the males of the latter become abnormal in an 
opposite manner to the prevalent character of the group ; next in 
Mechanitis we find this structure common to both sexes ; and then 
in Sais, the females retaining the same structure as in Mechanitis, 
but the males varying still further from the type. 
This gradual change in the position of the discoidal nervure 
actually occurring first in the two sexes of the same species, and 
then becoming common to both sexes, is, in the opinion of the 
writer, confirmatory in the highest degree of the theory laid down 
by him in a former paper, as to the structure of the anterior wings 
of the Diurnal Lepidoptera, and leaves, he thinks, no room to doubt 
the correctness of the explanation there given of the apparent ano- 
maly of those wings in the Papilionidee. 
