250 MANUAL OF ZOOLOGY SECT. 



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 

 condition, though free in the adult state. This holds good, 

 for example, of the larvae of the Ichneumons (belonging to 

 the order Hymenoptera), which develop in the interior of the 

 bodies of other insect-larvae ; and also of the larvae of the 

 Bot-flies (members of the order Diptera)> which inhabit the 

 alimentary canal of mammalian hosts. 



In accordance with the high grade of complexity 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 07 to its own weight (reckoned as i), the 

 muscular force of an Insect bears a ratio to its weight of 

 from about 14 to about 23. Insects are also distinguished 

 among the Invertebrata by the keenness of their senses. 

 The sense of sight is, as we should expect from the 

 elaborate character of the optic organs, the most highly 

 developed, many Insects having been shown by experiment 

 to have a keen sense of colour ; but a sense of smell, the 

 seat of which is in the antennae, can also be shown to exist in 

 a high degree, and the parts about the mouth bear nerve- 

 endings concerned in a well-developed sense of taste. A 

 sense of hearing does not appear to be universally present, 

 but is well marked in such forms as produce sounds. At 



