GCOLOGY, 357 
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 
which must often save them; a humming-bird moth may 
resemble a humming-bird; many palatable insects and 
larvee have a mimetic resemblance to others which are 
nauseous or otherwise little likely to be meddled with. 
Many insects may be saved by their hard chitinous armour, 
by their disgusting odour or taste, by their deterrent 
discharges of repulsive formic acid, etc., by simulation of 
death, by active resistance with effective weapons. 
Many flowers depend for cross-fertilisation upon insects, 
which carry the pollen from one to another. Many insects 
depend for food on the nectar and pollen of flowers. Thus 
many flowers and insects are mutually dependent. But 
many insects injure plants, and many plants exhibit 
structures which tend to save them from attack. On the 
other hand, there may be “partnerships” between insects 
and ‘plants—as in the “myrmecophilous” (ant - loving) 
plants, which shelter a bodyguard of ants, by whom they 
are saved from unwelcome visitors. And again, the 
formation of galls by some insects which lay their’ eggs 
in plants, and the insect-catching proclivities of some 
carnivorous plants, should be remembered. 
Most insects are terrestrial and aerial; the majority live 
in warm and temperate countries, but they are represented 
almost everywhere, even above the snow-line, in arctic 
regions, in caves. Even on the sea the Challenger 
explorers found the pelagic Halobates, a genus of bugs. 
The distribution of insects is mainly limited by food- 
supplies and climate, for their powers of flight are often 
great, and their opportunities of passive dispersal by the 
wind, floating logs, etc., are by no means slight. 
Many insects are more or less parasitic, either externally 
