xi PHYLUM ARTHROPODA 651 



passes into a quiescent or pupa stage, during which it remains 

 passive, enclosed in a tough integument, while a more or less 

 complete reconstruction of the organs goes on, resulting in the 

 development of all the parts of the perfect Insect. The develop- 

 ment of the new parts takes place from certain patches of cells, 

 the imagined discs, present in the larva. 



In the Diptera the larva or maggot is sometimes completely 

 devoid of jaws. In some Diptera, however, the jaws are well 

 developed, and there is a distinct head. After frequent moultings 

 the maggot passes either into a quiescent or pupa stage enclosed 

 in a hard skin, or into the stage of an active aquatic pupa, which 

 swims about actively in water and may possess tracheal gills. 



In the Lepidoptera the larvae ("caterpillars") are worm-like, 

 but with well-developed jaws, three pairs of jointed thoracic legs, 

 and a number of unjointed stumpy abdominal legs (pro-legs). 

 Lepidopterous larvae are often brilliantly coloured, are very 

 active, and feed with voracity, chiefly on leaves and other succulent 

 parts of plants. Eventually they spin a cocoon of a silky substance, 

 enclosed within which, and covered with a tough skin, they pass 

 through a quiescent or pupa condition the condition of the 

 chrysalis (Fig. 513). From the interior of this the imago subse- 

 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, 

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

 the larvse of the Ichneumons, which develop in the interior of the 

 bodies of other insect-larvae ; also of the larvae of the Bot-flies 

 (Fig. 512), 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-parasite (Section II, p. 86), 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 system 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 0*7 to its own weight 



