449 . 



The Salivary Glands. 



Tlie main changes undergone by the salivary glands 

 during larval life are a great growth in the size of the con- 

 stituent cells. In the adult larva they are as much as 57/i, 

 in length, 28/x in breadth; they are highly vacuolated and 

 have gigantic granular nuclei 30/x long, 17/x broad, and usually 

 contain several small nucleoli. 



The duct cells also grow largely in size; their spiral 

 intima is shed and reformed at each larval moult. 



While the glands do not disappear till early in the pupal 

 period, the median duct is actively disintegrating already in 

 the defaecating larva. The larval cells have met the same 

 fate as those of the oesophagus, i.e., they have degenerated 

 and the products of degeneration have been cast, in part, at 

 any rate, into the blood stream. Renovation of the median 

 duct quickly ensues (fig. 117); embryonic cells growing in- 

 wards and downwards from the regenerating epithelium of 

 the mouth and pharynx pass among the disintegrating cells, 

 nourishing themselves perliaps, in part, at their expense. 



The cells, however, do not grow back along the duct 

 beyond its point of bifurcation; forsaking the old larval 

 salivary duct here (the larval ducts at this point do not 

 appear to be dead yet) they grow as a slender, hollow column 

 up the back of the head and terminate in the neck. From 

 the duct at a point about one-quarter its length from the 

 mouth, the salivary gland of the imago develops in the second 

 day of pupal life. I have not observed the process,^" but it 

 seems unlikely that the gland should be formed in any other 

 way than by a thickening of the duct. 



Meanwhile, the remainder of the larval salivary glands 

 disappear. In the larva shortly before pupation the 

 greatly hypertrophied cells are to be observed undergoing 

 obvious degeneration. Numerous minute globules are to be 

 seen oozing out from the gland cells (fig. 149) into the cavity 

 of_the gland, in the same manner as I have described above 

 in theTlepatic caeca. The cytoplasm is even more highly 

 vacuolated than usual. But about six hours later (four-hour 

 pupa) the cells have entered into a state of granular dis- 

 integration. Parts of the cells are in a condition of fine 

 debris; other part-s seem to have till now maintained their 

 structure. The whole organ is very fragile, and I have 

 observed a case in which the tracheae of the forewings, grow- 

 ing downwards from the main trunks, have torn off a portion 

 of the disintegrating tissue and carried it along with them! 



(fig- 88), \ 



Lying within the disintegrating salivary glands are great * 

 numjbers of leucocytes, actively engaged in clearing away the 



