342 PHYLUM ARTHROPODA. 



which must often save them ; a humming-bird moth closely 

 resembles a humming-bird ; many palatable insects and 

 larva? 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 forma- 

 tion 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 

 as adults, e.g. fleas, lice, bird-lice, plant-lice, etc., or inter- 

 nally as larvae, e.g. the maggots of gad-flies on cattle, and a 

 great number of borers within plants. 



We need only mention Hessian-fly, phylloxera, Colorado 

 beetle, weevils, locusts, to suggest many more which are of 

 much economic importance as injurious insects. On the 

 other hand, our indebtedness to hive-bee and silk-moth, to 

 cochineal and lac insects, to those which destroy injurious 

 insects, and to those which carry pollen from flower to 

 flower, is obvious. 



