356 PHYLUM ARTHROPODA. 



is a marvellous internal reconstruction during the latei 

 larval, and especially during the quiescent pupal stage. 

 The more specialised larval organs are disrupted, their 

 debris being used in building new structures. In some 

 cases, such as flies, phagocytes play a very important part 

 in this metamorphosis ; in other" cases there is no true 

 phagocytosis. Parts of larval organs which have not been 

 highly specialised 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," 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 corre- 

 sponding organs of the adult. There is first a disruptive 

 process of histolysis, and then a reconstructive process of 

 histogenesis. Yet in most cases the disruption and 

 replacement of organs is very gradual. 



(Ecology. 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 



