6o THE LARVA OF THE BLOW- FLY. 



externally by a reticulum of stellate niesoblast and tracheae, and 

 lined by a very regular pavement epithelium consisting of 

 large hexagonal cells. These glands, in the recent condition, 

 are most beautiful microscopic objects. They lie one on 

 either side of the alimentary canal, and measure nearly a 

 centimetre in length, and "6 mm. in diameter in their widest 

 part. Each has a narrow duct, which joins its fellow beneath 

 the pharynx. The conjoined duct opens through a papilla, which 

 represents the ligula (Figs. 7 and 8). 



The condition of the secreting cells varies greatly at different 

 periods. In the young larva the cells are small and increase in 

 size with the growth of the insect. In the active feeding adult 

 they measure 100" to 150'^ in diameter, and form a thin pave- 

 ment 25" thick, enclosing a large cavity, which is filled with 

 clear fluid. In the resting larva the cells rapidly increase 

 in thickness, until at length they reduce the cavity of the 

 gland, so that it appears as a mere stellate fissure in sections. 

 In this condition they are cubical. 



A chitinous or cuticular intima between the cells and the 

 lumen of the gland has been frequently described in the 

 salivary glands and sericteria of various insects. In the active 

 gland in the fly larva I find no trace of such a membrane, but 

 in the resting larva a thin cuticular intima is often present. 



The gland-cells exhibit many exceedingly interesting appear- 

 ances. They have a distinctly reticular structure. Their 

 vesicular nuclei can often be separated in the recent condition 

 of the organ. In the resting larva the cells stifl have numerous 

 granules of secretion towards their inner surface, and are 

 firmly united externally, where they have a distinctly laminated 

 structure, the laminae passing from cell to cell, an indication 

 that the lamellated cuticle is formed from the cells themselves, 

 and not by a secretion poured out on their surface (PI. I., Fig. 9). 



Towards the gland-duct the cells are considerably reduced in 

 magnitude, and in the duct itself they form a cuticular intima 

 on their inner surface, which exhibits a spiral thickening 

 similar to, but coarser than, that of the tracheal vessels. 



A ring of embryonic cells intervenes between the cells of the 



