306 LEPIDOPTERA 



CHAP. 



Insects it would appear that the tendency of the higher forms is 

 to shorten the pupal period, and when much time has to he 

 passed between the end of the feeding up of the larva and the 

 appearance of the imago, to pass this time as much as possible 

 in the form of a resting-larva, and as little as may be in the 

 form of a pupa ; in Lepidoptera the reverse is the case ; the 

 resting-larva period being usually reduced to a day or two. 

 Hence we can understand the importance of a hard skin to the 

 pupa. There are, however, numerous Lepidopterous pupae where 

 the skin does not attain the condition of hardness that is 

 secured for the higher forms by the chitinous exudation we 

 have mentioned ; and there are also cases where there is a pro- 

 longed resting-larva period: for instance Galleria mellonella 

 spins a cocoon in the autumn and remains in it as a resting 

 larva all the winter, becoming a pupa only in the spring. In 

 many of these cases the resting-larva is protected by a cocoon. 

 It is probable that the chief advantage of the perfect chitinous 

 exudation of the Lepidopterous pupa is to prevent the tiny, 

 complex organisation from the effects of undue transpiration. 

 Bataillon has suggested that the relation of the fluid contents of 

 the pupa to air and moisture are of great importance in the 

 physiology of metamorphosis. 



The duration of life is very different in various forms 

 of Lepidoptera. It is known that certain species (Ephestia, 

 Jcuehniella, e.g.*) may go through at least five generations a year. 

 On the other hand, certain species that feed on wood or roots 

 may take three years to complete their life-history ; and it is 

 probable that some of the forms of Hepialidae are even longer 

 lived than this. 



Lepidoptera have always been a favourite Order with ento- 

 mologists, but no good list of the species has ever been made, 

 and it would be a difficult matter to say how many species are 

 at present known, but it can scarcely be less than 50,000. In 

 Britain we have about 2000 species. 



The close affinity of the Order with Trichoptera has long 

 been recognised : Reaumur considered the latter to be practically 

 Lepidoptera with aquatic habits, and Speyer pointed out the 

 existence of very numerous points of similarity between the 

 two. Brauer emphasised the existence of mandibles in the 

 nymph of Trichoptera as an important distinction : the pupa 



