MOEPHOLOGY OF THE LEPIDOPTEROUS PUPA. 251 



the female, only part of the sexual differences between pupal antennae may date from the 

 1)eginning of the rapid degeneration and development which certain species have under- 

 gone in the two sexes respectively. But it is most likely that the general and widespread 

 diflFerence between these imaginal organs in the two sexes is due to precisely similar causes 

 acting slowly and only up to a certain point — due, in fact, to the competition among the 

 males being keener than that among the females. When the two sexes seem to approach 

 most closely in respect of this competition, flying together and both apparently exer- 

 cising the powers of active selection — when courtship appears to be mutual — then the 

 differences between the antennte of the two sexes become very small, and in the cases of 

 most complete equality disappear altogether. And it must be remembered that the 

 antennae are in all probability sense-organs of very general use, although their sexual 

 function is by far the most important, and that free and active flight gives abundant 

 opportunity for their exercise in all possible directions, so that these organs may be 

 sometimes equally developed in the two sexes, when they may be more especially used in 

 courtship by the males. 



Returning to the pupal antennae of degenerate females, the observations upon the 

 wings of the same species render it likely that the former will slowly diminish in size, 

 although they have probably not done so at present, except, indeed, in the most degene- 

 rate species, viz. many Fsuclikkv.. The pupal wings may have shrunk earlier, because they 

 form organs of considerable size, and their shrinkage therefore especially favours a corre- 

 sponding increase of other parts. The degeneration of the imaginal sense-organs and 

 organs of locomotion is generally accompanied by an increase in the size of the abdomen, 

 which becomes relatively larger than in the more normal females. Thus the number of 

 eggs is often largely increased, while the additional weight is not too great a strain upon 

 the comparatively passive organism. In certain species the legs are also specially deve- 

 loped in order to bear more than their ordinary share in locomotion — e. g. in certain 

 Geometers, such as S. defoliaria (Plate XXVII. fig. 9). 



2. The Histor!/ and Causes of Degeneration in Female Imagines of Lepidoptera. — 

 It is now possible to give a more complete account of the history of degenerate females 

 than could be advanced from the consideration of the antennae alone. Starting from 

 the condition of neai'ly equal powers of flight and equally developed sense-organs in 

 the two sexes, we find the first indications of divergence in the antennae, which are 

 commonly more developed in the males because of their activity in courtship ; but in the 

 majority of moths the females remain active organisms with considerable powers of 

 flight, which are used especially in connexion with the deposition of eggs and for 

 obtaining food. The first of these important functions may be fairly looked upon as 

 balancing the chief necessity of male flight, i. e. to reach the females. Nevertheless, 

 there is this important difference, that the flight of the males tends to become very 

 rapid, because of the stress of competition, while there is no such strong tendency in 

 flight which is used for oviposition. On tlie other hand, in the flight which is employed 

 for food, the females are as suliject to competition as the males, and, accordingly, we 

 find that it is the rule among the great groups of feeding moths (the Geometers and 



