150 INSECT TRANSFORMATION 



food material. In the adult insect the fatty tissue is less 

 extensive, a very large proportion of the larval fat is therefore 

 absorbed during the pupal period and some of its transformed 

 substance may go to supply the material needed for other 

 tissues. 



The breaking-down (or histolysis) of the various larval 

 structures is believed to be due, in great measure, to the 

 activity of special cells the phagocytes which, like the white 

 blood-corpuscles of vertebrates, have the power of ingesting 

 the cells destined for dissolution. In many cases, however, 

 internal chemical changes within the doomed cells themselves 

 are believed to be the cause of the destructive process, and 

 the activity of the phagocytes will then be displayed in 

 devouring the products of disintegration. 1 A feature of much 

 interest in the microscopic structure of transforming insects 

 is the small size of the cells of various tissues in the winged 

 adult, compared with their comparatively large size in the 

 corresponding tissues of the larva. This contrast (Fig. 85) 

 is particularly noticeable in the remarkable cells known as 

 oenocytes of relatively great size in insect larvae which are 

 found in close association with the fatty tissue of the blood- 

 spaces and the finest branches of the air-tube system (Fig. 

 ii oc), and are believed to be concerned in the functions of 

 metabolism and excretion. 



The air-tubes of an insect are derived, as previously men- 

 tioned (p. 19), from inpushings of the skin or ectoderm and 

 are lined with a sheet of chitin continuous with the outer 

 cuticle, shed like the latter at every moult. But at the pupal 

 stage the change in the respiratory system is much more 

 profound than this ; much reconstruction of the cellular wall 

 of the tracheal tubes is then effected, because the breathing 

 organs of a flying insect must needs differ in many respects 

 from those suited to a crawling larva, while in the case of 

 larvae living in water special modes of breathing must be 

 adapted to the aquatic life. In a fly-maggot there are but 

 two effectively acting spiracles at the tail end, whereas in the 

 pupal stage the prothoracic spiracles alone are functional, 

 and in the winged insect there may be six laterally-situated 

 pairs. Hence much of the larval tracheal system must be 



1 See L. F. Henneguy's " Les Insectes ". Paris, 1904. (pp. 677-687). 



