252 MANUAL OF ZOOLOGY sect. 



caterpillars, are worm-like, but with well-developed jaws, 

 three pairs of jointed thoracic legs, and a number of un- 

 jointed stumpy abdominal legs. Eventually the caterpillar 

 spins a cocoon of a silky substance, enclosed within which, 

 and covered with a tough skin, it passes through a quiescent 

 or pupa condition — the condition of the chrysalis. From 

 the interior of this the imago subsequently emerges with all 

 the parts of the adult insect fully formed. 



In mode of life there is a very considerable difference 

 between different orders and families of insects. Some 

 are parasites in the strict sense throughout life. This is the 

 case, for instance, in the Strepsiptera (bee-parasites), 

 the females of which live permanently ensconced between the 

 joints of the abdomen of their hosts. The lice and bird- 

 lice are external parasites throughout life ; bugs and fleas, 

 though not adhering to their hosts, are parasites as regards 

 their diet. Many insects are parasites in the larval condi- 

 tion, though free in the adult state. This holds good, for 

 example, of the larvae of the ichneumons, which develop in 

 the interior of the bodies of other insect-larvae, also of 

 the larvae of the bot-flies, which inhabit the alimentary canal 

 of mammalian hosts (horses, oxen, sheep, rhinoceroses, 

 tapirs). 



In accordance with the high grade of the structure of 

 their various systems of organs, insects exhibit a correspond- 

 ingly high degree of functional activity. The quantity of 

 food consumed and assimilated is great in comparison with 

 the bulk of the body, and the energy expended in muscular 

 contractions is of very considerable amount. It is estimated 

 that while the muscular force exerted by a horse bears a ratio 

 of about 0.7 to its own weight (reckoned as 1), the muscu- 

 lar force of an insect bears a ratio to its weight of from 

 about 14 to about 23. Insects are also distinguished among 



