404 PHYLUM ARTHROPODA 



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 repre- 

 sented almost everywhere, even above the snow-line, in 

 arctic regions, in caves. Even on the sea the Challenger 

 explorers found the pelagic Halohates, a genus of bugs. 

 The distribution of insects is mainly limited by food- 

 supplies and climate, for their powers 

 fof 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-Hce, etc., or internally as 

 larvae, e.g. the maggots of bot-flies in 

 sheep, and a great number of borers 

 within plants. 

 We need only mention Hessian-fly, 

 phylloxera, Colorado beetle, weevils, 

 locusts, to suggest many more which 

 Fig. 225. — Anurida are of much economic importance as 

 maritima (after injurious insects. On the other hand, 

 imms), one oy^e ^^j. indebtedness to hive-bee and silk- 

 Colkmbol™* ^^^ moth, to cochineal and lac insects, to 



those which destroy injurious insects, and 

 to those which carry pollen from flower to flower, is obvious. 

 Finally, we must at least mention that in ants, bees, 

 wasps, and termites we find illustration of various grades 

 of social life, and marvellous exhibitions of instinctive skill 

 as well as some intelligence. 



Pedigree. — Insects must have appeared relatively 

 early, for remains of a cockroach-like form have been found 

 even in Silurian strata. The higher forms with complete 

 metamorphosis appear much later {e.g. beetles in the 

 Carboniferous ages) ; but it seems that the Palaeozoic 

 insects were mostly generalised types, prophetic of rather 

 than referable to the modern orders. 



