254 MR. E. B. POULTON ON THE EXTERNAL 



appears in February and March : the larva feeds on oak. H. aiirantiaria appears in 

 October and November : the larva feeds on whitethorn, birch, and oak. H. progemmaria 

 appears in February and March : the larva feeds on hornbeam ; and H. defoliaria 

 appears in October : the larva feeds on many trees. In the genus Anisopteryx, A. cescu- 

 laria appears in April, and the larva feeds on many trees. Lastly, in the genus 

 Cheimatobia, C. brumata appears in October, November, and December : the larva feeds 

 on nearly every kind of tree ; while C. boreata appears in October, and the larva feeds 

 on birch. No one of these species appears on the wing before September or after April, 

 while most of them assume the perfect state in the coldest part of the year. Further- 

 more, the difficulties in connexion with oviposition do not obtain here ; for the female 

 moths leave their cocoons and crawl upon the food-plants of the larva, depositing their 

 eggs in appropriate situations as readily, although not as rapidly, as if they were able 

 to fly. The legs of many of these females are specially developed, thus facilitating loco- 

 motion (see Plate XXVII. fig. 9). Furthermore, the moths are not large, and generally 

 lay their eggs upon large trees or upon food-plants such as yarrow, which are very 

 numerous, and which grow in close proximity to one another, so that there is little fear 

 of starvation, even when the eggs are laid near together. 



Considering all these facts, it appears most probable that the wings became functionless 

 in certain female moths which appeared in the colder months of the year and did not 

 require the use of these organs for feeding, for courtship, or for oviposition. Further- 

 more, the wings when useless would be encouraged to shrink by a variety of causes, of 

 which a probably important one is the fact that the females would be much more con- 

 spicuous during ovijjosition if they crawled about with a surface immensely extended by 

 means of these organs. But there has also been the same cause acting thoughout 

 which was pointed out as of the first importance in the Bombycince — the advantage 

 which the species gains from the possession of a large abdomen by the female. The 

 difference in size between the male and female abdomen in these moths is far greater 

 than in any other species in the Geometrince. The last is doubtless the great cause of 

 degeneration in all cases, and among this large group of feeding moths the temperature 

 has indirectly produced such results in a few species by the discouragement of feeding, 

 so that the chief cause could come into operation, receiving accessory aid from the 

 positive protective gain which would result from the loss of functionless wings. 



It is hardly necessary to point out that the degenerate condition of the females 

 renders certain a keen competition among the males, which ensures the continued 

 possession of the power of flight in this sex, notwithstanding the low temperature and 

 indifference to food. It is well known, however, that another condition may render the 

 wings functionless, even in the male sex, i. e. existence upon islands in very stormy areas 

 where high winds recur constantly. But this cause is not efficient in Great Britain, 

 and no such effect is witnessed in our Lepidopterous fauna. 



It is quite clear that the initial stages of diminution, however caused, in functionless 

 wings would not affect the corresponding pupal organs, because the latter are very much 

 smaller than those of the imago, the difference in size being due to expansion imme- 

 diately following emergence from the pupa. We should not therefore expect any 



