162 
ME TAMORPHOSIS. 
become transparent membranes of the wing. The 
nervures are hollow, and through them run trachese, 
for it is the air contained in them which assists in the 
expansion of the wings, still soft when the adult bee 
leaves the cell. Blood surrounds these tracheae during 
the period of development of the wings. 
The development of the silk glands was specially 
studied by Schiemenz (144) in 1883. Portions of 
these glands are shown in Fig. 59. They are situated 
on each side of the alimentary canal, and are in the 
shape of spiral tubes, which unite in the head, con- 
tinuing as a single tube, having its outlet at a conical 
papilla in the under lip. 
There are two distinct structures in these glands : 
the one found in the anterior portion from the outlet, is 
shown in section at c, Fig. 59, and the other as in section 
Fig. 59. — Silk Glands. 
B, is continued to the blind extremities of the glands. 
The anterior portion (c) is a reservoir, whose intima, the 
continuation of the epidermis, is surrounded by a cell 
layer, which thickens towards the blind ends, and the 
cells then take the form shown in Fig. 59, b, surrounding 
