v] TRANSFORMATIONS 47 



The gullet, crop, and gizzard of an insect, which 

 lie in front of the stomach, are lined by cells derived 

 from the outer skin (ectoderm) which is pushed in to 

 form what is called the ' fore-gut.' Similarly the in- 

 testine and rectum, behind the stomach, are lined 

 with ectodermal cells which arise from the inpushed 

 'hind-gut.' The larval fore- and hind-guts are broken 

 down at the end of larval life and their lining is 

 replaced by fresh tissue derived from two imaginal 

 bands which surround the cavity of the digestive 

 tube, one at the hinder end of the fore-gut, and the 

 other at the front end of the hind-gut. The larval 

 salivary glands in connection with the gullet are also 

 broken down, and fresh glands are formed for the 

 imago. 



A large part of the substance of an insect larva 

 consists of muscular tissue, surrounding the digestive 

 tube, and forming the great muscles that move the 

 various parts of the body, and of fat, surrounding 

 the organs and serving as a store of food-material. 

 Very many of the muscle-fibres and the fat-cells 

 also become disintegrated during the late larval 

 and pupal stages, and the corresponding tissues of 

 the adult are new formations derived from special 

 groups of imaginal cells, though some muscles may 

 persist from the larva to the adult. Similarly the 

 complex air-tube or tracheal system of the larva is 

 broken down and a fresh set of tubes is developed, 



