138 The Classification of Insects 



the mean of these two lengths, but were separated 

 into two groups near the extremes with forceps about 

 3-5 mm. or 7 mm. long (82). 



Butterflies have been chosen to introduce the 

 question of variation and species-making, because 

 they, and their relations the Moths, have been made 

 the subject of experimental studies on the influence 

 of outside conditions in modifying the individual. 

 The surroundings of the larva and the pupa control 

 to some extent the appearance of the imago. It has 

 been proved, for instance, that if some butterfly- 

 pupae be subjected to a low temperature, the perfect 

 insects will be unusually dark in the colour of their 



wing-patterns. Several 

 European species, 

 which have two-life 

 cycles in the year, 

 show a dark or deeply- 

 marked spring brood 

 (derived from winter- 

 ing pups), and a pale 



Fig. 84.— Tail-forceps of three male Earwigs fpoKlv mnrkpfl 



(Forftcula auricuiariu) showing variation ^' iccuij^ iiiaiiv«:;u 

 in length and form. Magnified 3 times. summer brOod. It is 



found that artificial cold is often effective in produc- 

 ing the spring form from summer pupse, while more 

 rarely the summer form can be reared from winter 

 pupae kept in a hothouse (83, 84, 85). It is also 

 known that feebly and strongly marked forms of 

 tropical butterflies — formerly regarded as distinct 

 species — are respectively the dry and wet season 

 forms of a single species (86). It appears therefore 

 that cold and moist conditions during the pupal stage 

 have often a direct effect in darkening the colours 

 of the imago. Night-flying moths (Noctuidae) in- 

 habiting the British Islands are as a rule darker in 

 their wing-patterns than individuals of the same 



