INSTINCT. 551 



material required for its future changes, in the form of the enormously 

 developed fat body, it is ready to enter on the pupal stage. The 

 larvae of many insects prepare above or below the ground, by means 

 of their spinning glands, a protective web, in which, after casting 

 their skin, they enter the pupal stage (Chrysalis). The external 

 parts of the body of the winged insect either lie against the common 

 horny skin of the pupa, so that they are recognizable as such 

 (Lepidoptera, pupa obtecta), or they already stand out freely from the 

 body (Coleoptera, pupa libera). This distinction is, however, an un- 

 important one, since in the first case the limbs are free just after the 

 ecdysis, and are only cemented afterwards by the hardening cuticular 

 layer. If the pupa remains enclosed by the last larval skin (Mus- 

 cidce) it is termed pupa coarctata. 



In all cases the body of the winged insect lies with its external 

 parts sharply marked in the pupa, and the special object of the pupal 

 life is to complete the changes of the internal organisation and the 

 maturity of the sexual organs. When this is accomplished the 

 winged insect bursts the pupal skin, forces its way out by means cf 

 antennae, wings and legs, and expands those parts which have been 

 folded together, under the influence of violent inspirations, by which 

 the tracheae become filled with air. The chitinous covering becomes 

 harder and harder, the urinary secretion which has accumulated 

 during the pupal sleep is ejected from the rectum, and the insect 

 is capable of performing all the functions of the sexually adult 

 animal. 



The mode of life of insects is so varied that it is hardly possible 

 to give a general account of it. The diet is both animal and vege- 

 table, and is taken in the most varied forms, either solid or fluid, and 

 fresh or decaying. Plants are especially, subject to the attacks of 

 insects and their larvae, and there exists, perhaps, no Phanerogam 

 which does not afford nourishment to one or more species of insects. 

 On the other hand, insects seem useful or even necessary to the well- 

 being of the vegetable world, for in many cases e.g., many flies, 

 bees, and butterflies they bring about fertilization by carrying the 

 pollen to the stigmata of flowers. 



The complex, often marvellous, and apparently intelligent actions 

 performed by insects correspond to the perfection with which the 

 vegetative organs discharge their functions. Such actions are 

 largely carried out instinctively by the mechanism of the organi- 

 sation, but they certainly in part depend upon psychical processes, 

 since they presuppose memory and judgment, in connection with 



