and Life History of Psychoda sexpunctata, 299 



of the bean. They are placed one on each side of the 

 alimentary canal, which they partly encircle. The two 

 ducts run forward beneath the alimentary canal, and unite 

 in the posterior region of the head. The structure of the 

 glands is very simple. They are hollow, the wall being 

 only one layer of cells thick. The cells are very large, 

 and possess very large nuclei, in which by suitable methods 

 a distinct chromatic filament and nucleus can be made 

 out. I have not been able to detect the elaborate 

 nuclear structure of the salivary cells of the Chirononms 

 larva.* Outside the cells there is a thin basement- 

 membrane. 



Respiratory System of the Larva (Figs. 4-6). 



The respiratory system of the larva is well developed. 

 The air is renewed mainly, if not altogether, by means of the 

 large posterior spiracles (Fig. G). These are situated at the 

 apex of the last segment, close to the bases of the minute 

 anal appendages. Each spiracle communicates with a large 

 longitudinal trachea. Immediately within the spiracle the 

 trachea is slightly dilated into an oval chamber, and the 

 centre of this is occupied by an ovoid mass of chitin, attached 

 to the walls of the chamber by chitinous threads. The 

 whole arrangement forms a kind of loose spongy plug. In 

 addition to this pair of large posterior spiracles, there are 

 an anterior pair borne by two short processes on the sides 

 of the prothorax (Fig. 4). They are not open, and pro- 

 bably not functional, since they are always immersed in 

 water or mud. Leading into each spiracle is a trachea, 

 which, as it passes into the process on which the spiracle 

 is borne, gradually becomes spongy and takes on a black 

 appearance as seen by the microscope, probably due to 

 contained air. 



There are two very large trachese (Fig. 4), each arising 

 from one of the posterior spiracles, and running forward 

 along the back. They are connected by a series of 

 commissures, one of which occurs at the hinder end of 

 every segment, except the last. In many cases small, 

 forward-directed branches arise from the commissures. 

 Some of these are connected with the alimentary canal, 

 and on this account the commissures are very slack, 

 especially those near the middle of the body. During the 



* Balbiani (1881). 



