636 ZOOLOGY SECT. 



quently 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 

 lodged 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, 



FIG. 541. Honey-bee (Apis mellifica). a, queen (perfect female) ; b, worker (imperfect 

 female), and c, drone (male). (After Brehm.) 



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 

 (Fig. 522), which inhabit the alimentary canal of mammalian hosts 

 (Horses, Oxen, Sheep, Rhinoceroses, Tapirs). The blood-sucking 

 Insects act in certain cases as the carriers or intermediate hosts of 

 the protozoan or bacterial parasites that are the causes of various 

 diseases in man. Thus, as was stated in the account of the malaria- 



Fio. 542. Red Ant (Formica rufa) ; male, worker, and female. (After Brehm.) 



parasite (Section II, p. 89), mosquitoes are the means of conveying 

 that disease from one person to another. 



In accordance with the high grade of the structure of their 

 various systems of organs, Insects exhibit a correspondingly 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 con- 

 siderable amount. It is estimated that while the muscular force 

 exerted by a Horse bears a ratio of about O7 to its own weight 

 (reckoned as 1) the muscular force of an Insect bears a ratio to 

 its weight of from about 14 to about 23. Insects are also dis- 

 tinguished among the Invertebrata by the keenness of their 

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



