86 



as there is no other complex structure which can present such 

 a variety of phases in some directions, and which is, at the 

 same time, so constant in others. The PapilionidcB, as 

 an instance, divide off from the Pieridcs by the absence 

 in the former, and the presence in the latter, of vein 

 1 a in the hind-wing. What other character of any kind 

 is there which can be used for such a differentiation ? 

 Yet within the PieridcB there is great variation in the other 

 veins; while in the PapilionidcB there is a great uniformity. 



Neuration, in the first place, is a complex structural 

 character, and it has a second value in that it is a more or 

 less invisible structure, and therefore not liable to be 

 affected by external influences. In some few genera of 

 butterflies it is considerably more marked and visible, such 

 as in Morpho, Lycorea and Papilio, and, of course, in all 

 those species that have developed transparent or semi- 

 transparent wings. It is worthy of note that in the most 

 transparent group of insects, the ItJwuiiiua-, there is, 

 perhaps, more variation in the neuration than in an}- other 

 family. In this remarkable South American group the 

 species are all sexually dimorphic in their neuration ; and even 

 in the same sex of the same species there is often a consider- 

 able variation in the veining. 



On the other hand, the Spliingida: are remarkabl}^ constant, 

 and do not offer characters for generic subdivision, or even 

 for sub-family subdivision. Neurational variation is very 

 different within the different families. For instance, the 

 NoctiiidcE, Notodontidce, Lyccenidcc, and Papilionidcs are all 

 very constant in their neuration, while the Syntomidce, 

 ArctiidcE, Geomctridcc, BomhycidcE, and SatiirniidcB are vastly 

 more variable. Some characters are so constant as to be of 

 family or sub-family value in certain groups, while the same 

 characters in another group var}' to such an extent as to be 

 only of generic value. The constancy of the position of 

 vein 5 of the hind-wing in the Nodnida: is such as to make 

 it one of the characters in diagnosing the famih', but this 

 same vein in the Ithomiince varies to an enormous extent ; 

 so also, and to a hardly less extent, in the Gcoinctridcs. Vein 

 8 of the hind-wing in such families as Sphingidce, Notodvntidcc, 

 Arctiida, BombycidcB, Hypsidcc. Eupterotidcv, and others, is 

 extremely constant in position, and quite of family value, but 

 in the Ithoiniincc it is very variable in both position and 

 strength, and could not be used as a family character. In a 

 large number of families the position of vein 8 of the hind- 

 wing is, however, most constant, and, in conjunction with 



