356 PHYLUM ARTHROPODA. 
is a marvellous internal reconstruction during the later 
larval, and especially during the quiescent pupal stage. 
The more specialised larval organs are disrupted, their 
débris 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,” ze. 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. 
CEcology.—The average insect is active, but between 
orders (¢.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 
