THE HIDDEN TYPE OF WING-GROWTH 149 



the digestive tract in many insects is formed from a forward 

 and a hinder inpushing of the outer skin lined with chitinous 

 cuticle. This cuticle is, of course, shed at the crisis of trans- 

 formation, and the cellular layer (epithelium) is broken down 

 in order that a new fore- and hind-gut, adapted to the needs 

 of the adult, may be formed from special bands of cells which 

 surround the food-canal at the hinder end of the proventriculus 

 and at the front end of the intestine (Fig. 78). The salivary 

 glands opening into the mouth also undergo destruction and 

 rebuilding but the excretory (Malpighian) tubes, given off at 

 the front end of the intestine, survive from the larval to the 

 adult state with comparatively slight change. 



Muscular tissue forms a large proportion of the mass of an 



FIG. 85. SECTION THROUGH WALL OF INTESTINE OF LARVA 



OF WASP (Vespa). 



ct, chitinous lining ; le, larval epithelium ; m, muscular coat ; 

 ie, small cells of imaginal epithelium. After Anglas, Bull. 

 Set. France et Belg., XXXIV. 



insect's body both in larval and adult life ; movement of the 

 segments of body and limbs on one another, of the wings and 

 of the digestive tube, is always due to the contraction of bundles 

 or sheets of muscle fibres. The arrangement of many of these 

 muscles necessarily differs in larva and imago respectively on 

 account of the difference in the body-form, and consequently 

 much of the larval muscular system is broken down during 

 the pupal stage, while the most important muscles of the 

 winged adult are then built up from special groups of imaginal 

 cells. 



In an insect larva there is usually much fat distributed in 

 masses of cells in the walls and cavity of the pericardial 

 blood-space and in the great blood-space surrounding the 

 digestive and other organs. These fat-cells form a reserve of 



