ON THE VARIATION OF INSECTS. 177 



that, owing to some peculiarities of the season, the number of 

 dwarfed forms observed is unusually great. In 1887 this seems to 

 have been the case in England, and Mr. J. T. Williams (Entom. xx. 

 388) attributed it, no doubt correctly, to the dryness of the 

 atmosphere and consequent partial drying-up of the food- plants. 

 This is dwarfing due to the inferior condition of the food, but in 

 other cases the quantity of it has to be considered. Mr. John T. 

 Carrington {in litt.) gives a good illustration of this ; he writes : — 

 " Some few Nyssia hispidaria have been taken in Richmond Park 

 this spring of 1889, but I hear they are small, and so are the 

 Hybernia leucophcearia , Last year there was a great plague of 

 larvae of Toririx viridana, which ate off nearly all the leaves from 

 the oaks, which may account for the small specimens of these 

 two species." 



Lepidoptera may be dwarfed in captivity, by giving the larvae 

 an insufficient supply of food, and it has been stated that these 

 dwarfs are generally of the male sex. Mr. T. G. Gentry (Can. 

 Ent. 1877, 50) believed he could influence the sex of insects in 

 this way, and out of twenty Samia cecropia so reared eighteen 

 proved to be males. It would be interesting if all those who 

 possess dwarfed Lepidoptera would examine tliem and report on 

 the sex. 



Some species, as the Chalcididse quoted above, and species of 

 Lycana, vary much in size when apparently under the same 

 conditions. Mr. W. H. Edwards (Can. Eut. 1876, 203), writing 

 oi Lyccena comyntas, says: — "I believe that wherever couiyntas is 

 found, two sizes of the butterfly appear, one scarcely more than 

 half the superficial area of the other. I do not know of any 

 other species in which this peculiarity is regularly found." Mr. 

 J. W. Tutt (Entom. xx. 828) relates that small Lyccena corydon 

 occur every year at Dover, and he has found very small L. icarus 

 flying together with those of normal size. But here he offers 

 an intelligible explanation, that the dwarfs are those which have 

 been reared on isolated food-plants which were insufficient for 

 their needs. 



In insects which have more than one brood in the year, the 

 small size of many vernal broods is quite well known. Presumably 

 this has to do with the length of the period of development, and 

 I have dealt with this subject under " Seasonal forms " on pages 

 27, 28. A curious instance, falling under this head, is given by 

 Mr. C. G. Barrett (Ent. Mo. Mag. 1888, 81) in connection with 

 Euchloe cardamines at Haslemere. There occurred a variety 

 about half the normal size, in both sexes, and the males of this 

 variety were invariably the earliest specimens seen, the normal 

 males appearing two or three days later. 



Geographical variation in size has been much discussed. 

 Prof. Baird pointed out that birds and mammals tend to decrease 



