48 THE LARVA OF THE BLOW-FLY. 



the cuticular tunica propria and the cells in the silk-glands of 

 caterpillars, a view from which I must dissent ; from his figures 

 I conclude that his tunica propria is the peritoneal coat, and 

 that the capillaries do not perforate the basement membrane as 

 he supposes. I have never found any instance of such a dis- 

 position ; they are everywhere confined to the mesoblast, and 

 neither perforate the myolemma of the muscle fibres nor pene- 

 trate between the epithelial elements of the hypoblast or epiblast. 



No organs undergo so many changes during the life of the 

 larva as the arborescent tracheae ; in the newly-hatched insect 

 they are few, but increase rapidly in number as growth pro- 

 gresses, until at length every organ, except the fat bodies, 

 is richly supplied with a network of tracheal capillaries. 

 Before the first moult the main tracheae communicate with the 

 exterior by a single pair of stigmatic slits on the fourteenth 

 segment. After this, at first two, and then three, such slits are 

 found in each posterior spiracle, and a second pair of spiracles 

 appear on the fourth or prothoracic segment, at the second 

 ecdysis. It is usual to speak of the larvae of the Muscidse as 

 being in the first, second, or third stage of development in 

 relation to the number of slits in the posterior spiracles. 



The Structure of the Tracheae (Fig. n, ^). The main trunks 

 and larger vessels exhibit an external coat of thin polygonal 

 cells closely united by their edges. Indeed, in some prepara- 

 tions the boundaries of the cells are not visible, so that the 

 tracheae appear to be covered by a continuous layer of nucleated 

 protoplasm, and were so described by Weismann [2, p. 117]. 

 This appearance is, however, delusive, and the edges of the 

 individual cells are perfectly distinct in properly preserved 

 preparations. These cells do not exhibit indications of division, 

 and increase in size with the growth of the larva, like the cells 

 of the hypodermis. In the largest tracheal trunks of the adult 

 larvae they measure from 60^ to 8o M ( T ^ 17 inch) in diameter. In- 

 ternally to the cells there is a thick cuticular intima, with a 

 distinct spiral structure, which gives the vessels their well- 

 known appearance. Sections do not show a spiral thickening 

 of the cuticular membrane. 



