BIONOMICS. 317 



form what are called "imaginal discs,'' i.e. embryonic 

 or germinal areas, from which arise the wings, legs, etc., of 

 the adult insect. The reconstruction is very thorough ; 

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

 of the mid-gut, etc., are gradually replaced by the correspond- 

 ing organs of the adult. There is first a thorough disruptive 

 process of histolysis, and then a reconstructive process of 

 histogenesis. Yet in most cases the disruption and 

 replacement of organs is very 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, as in 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 multiplication 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 

 cases of pebbles and fragments of stem ; many caterpillars 

 and adult insects harmonise with the colour of their environ- 

 ment ; leaf-insects, " walking sticks," moss-insects, scale- 

 insects, have a precise resemblance to external objects 



