324 PERIPATUS, MYRIOPODS, AND INSECTS. 



organs which have not been highly speciaUsed form the 

 foundations of new adult structures. Of special importance 

 are certain ingrowths of the larval skin (the epi- or hypo- 

 dermis) which form what are called " imaginal discs," from 

 which arise the wings, legs, and epidermis of the imago or 

 perfect insect. The reconstruction is very thorough ; most 

 of the musculature, much of the tracheal system, part of the 

 mid gut, &c., are gradually replaced by the corresponding 

 organs of the adult. Yet there is no abruptness ; the 

 absorption and replacement of organs is perfectly gradual. 



Bionomics. 



The average insect is active, but between orders {e.g., 

 ants, bees, and wasps versus aphides, coccus insects, and 

 bugs), between nearly related families, between the sexes 

 {e.g., male and female cochineal insect), between caterpillar 

 and pupa, we read the constantly recurrent antithesis between 

 activity and passivity. 



The average length of life is short. Queen bees of five 

 years, queen ants aged thirteen, are rare exceptions. In 

 many cases death follows as the rapid nemesis of repro- 

 duction. But though the adult life is often very short, 

 the total life may be of considerable length, \\'itness some 

 Ephemerids which in their adult life of winged love-making 

 may be literally the flies of a day, while their aquatic larval 

 stages may have lived for two years or more. 



The relation between the annual appearance of certain 

 insects and that of the plants which they visit, the habits 

 of hibernation in the adult or larval state, the occasional 

 " dimorphism " between winter and summer broods of 

 butterflies should be noticed. 



The prolific multiphcation of many insects may lead to 

 local and periodic increase in their numbers, but great 

 increase is limited by the food supply and the weather, by 

 the warfare between insects of different kinds, by the 

 numerous insects parasitic on others, by the appetite of 

 higher animals, — fishes, frogs, ant-eaters, insectivores, and, 

 above all, birds. 



There is a great variety of protective adaptation. The 

 young of caddis flies are partially masked by their external 



