68 BPJTISH LEPIBOPTERA. 



instances, among insects, in which the female is more brilliantly 

 tinted than the male must be exceedingly rare. Scudder mentions 

 one South American genus where this is so ; perhaps Zephyrus quercus, 

 Z. betulae and Thecla ilicis may also be cited, but the cases are com- 

 paratively few. Darwin considers that the excessive beauty on the 

 part of the males is due to sexual selection, the females having, 

 through a long period of time, selected the more attractive males. 

 He further thinks that the various forms of beauty originated as casual 

 variations, and that the special characters were then intensified by the 

 selection exercised by the female. Wallace, on the other hand, con- 

 siders that the sober colours of female insects are due to natural 

 selection, and have been the means of their preservation, since the 

 operation of natural selection has eliminated those individuals of the 

 latter sex that are most gay, and, therefore, conspicuous to their 

 enemies. Darwin starts from inconspicuous forms, from which he 

 derives the conspicuous ones, whilst Wallace starts from conspicuous 

 forms, and from them derives the inconspicuous ones. We have al- 

 ready* shown that, among the Lepidoptera, facts distinctly bear out 

 Wallace's view. There is no need to give any special examples of 

 sexual dimorphism, for, as a matter of fact, it would be difficult to 

 find among our British lepidoptera many species that do not exhibit 

 tbis phenomenon to a greater or less extent. 



Another marked form of variation that occurs in certain species is 

 that known as seasonal dimorphism. In those countries which have 

 a very distinct difference between the summer and winter temperatures, 

 certain species produce one form of the imago in the spring, after 

 the pupa has been exposed for some months to the climatic conditions 

 of winter, and another form of the imago in the early autumn, after 

 the pupa has been exposed for only a few weeks, or even days, to 

 the climatic conditions of summer. The differences between these 

 two broods are usually marked in two ways — (1) Size. (2) Colour. 

 It frequently happens that the summer or autumn-emerging brood is 

 the smaller, and this is undoubtedly due to the difference in the 

 quantity of food eaten, since the larval state of this brood lasts a much 

 shorter time than that of those that emerge in the spring, the larvas, 

 indeed, often missing a moult in order to come to maturity more 

 quickly. The difference in colour is probably due, in different species, 

 to two distinct causes : (1) The less energy at disposal for the purpose 

 of pigment formation in the quickly- feeding individuals. (2) The 

 direct influence of the temperature on the pigment during its 

 formation. 



Standfuss asserts {Causes of Variation, etc., p. 5) that, in some ex- 

 periments that he made on lepidopterous larvae, the more the period 

 of larval feeding was shortened by the raising of the temperature, the 

 better marked was the reduction in size of the imago. This was the 

 regular, and almost invariable result. A pair of Eutricha quercifolia, 

 of which the male measured 58 and the female 89 mm. across the 

 wings, produced offspring of which, after a sojourn of 70-85 days in 

 the larval, and 12-15 days in the pupal, condition, the males measured 

 only 35-37 and the females 36-39 mm. across the wings. Arctia 

 fasciata, male 46 mm., female 48 mm. across the wings (from pupae 



* British Noctitae, etc., vol. iii., pp. xvii. et seq. 



