380 Linnaan Society. 



for the functions they perform in a normal state, and apparently 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 Heliconidce we find this struc- 

 ture, almost normal as it respects the order, in the genus Ituna, and 

 also in Ithomia. It is found in some female Ithomicp, of which the 

 males have a diflFerent 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. 



I'hus, leaving the genera Heliconia, Lycorea and their immediate 

 allies, which have the structure which is normal as regards the Di- 

 wnal Lepidoptera, though abnormal as regards the order, we find in 

 Ituna and some female Ithomia: 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 Papilionidte. 



