602 ZOOLOGY SECT. 



stage, during which the whole animal, or a considerable part of it, 

 undergoes an entire transformation. The metamorphosis is com- 

 plete in the Diptera, Lepidoptera, Coleoptera, and Hymenoptera, 

 absent or incomplete in the other orders. In the most lowly 

 organised larva? (many Diptera) the body of the larva or " maggot " 

 is completely worm-like, without any appendages, and without 

 any distinct head. In other cases (Lepidoptera, &c.), there is a 

 distinct head ; the three thoracic segments have three pairs of 

 jointed legs, and the abdominal segments short unjointed pro- 

 legs (Fig. 475). In most instances the larva? differ widely from the 

 adults in their food and mode of life ; very generally the jaws are 

 adapted for biting, even when the mouth of the adult is suctorial. 

 After a longer or shorter period passed in this larval condition, in 

 which it is usually active and very voracious, the young Insect 

 passes into a quiescent or pupa condition, 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 ima ginal 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 inclosed 

 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. Lepidopterous 

 larva? are often brilliantly coloured, and 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, inclosed within 

 which, and covered with a tough skin, they pass through a 

 quiescent or pupa condition the condition of 'the chrysalis (Fig. 

 475). 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 svn.se 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 

 Heas, 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 larva? of the Ichneumons, which develop in the interior of the 

 bodies of other insect-larva? ; also of the larva? of the Bot-flies 



