GENERAL LIFE OF INSECTS. 275 



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 distrib- 

 ution 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 internally 

 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. 



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 and 

 also of genuine intelligence. 



A Type of Insects — Periplaneta (or Blattd) — 

 The Cockroach. 



The cockroaches found in Britain are immigrants either 

 from the East {P. orienialis), or from America {P. americana). 

 They are omnivorous in their diet, and active in their habits, 

 but they hide during the day, and feed at night. They are 

 ancient insects, for related forms seem to have occurred in 

 Silurian ages ; they are average types of Insects, not being 

 highly specialised. Their position is among the Orthoptera, 

 i.e., in the same order as locusts and grasshoppers. The 

 young develop without metamorphosis. 



