4 
Transactions of the 
[Monthly Microscopical 
Journal, July 1, 1869. 
radiating bands of muscle at the base of the papillae ; tbey probably- 
open the central cavity, and at the same time press upon the 
contents of the papillae, and, assisted by the muscular wall of the 
rectum, which contracts at the same instant, they not only expel 
the contents of the central cavity, but also press the secreted fluid 
through the minute pores in the calyx into the cavity of the rectum ; 
the central cavity is probably refilled by the elasticity of the calyx 
itself. 
There is no trace of these organs in the maggot, and at present I 
cannot state at what precise period they first appear in the pupa, 
but as soon as the embryo fly, if I may be allowed the expression, 
has become so far developed that its principal parts may be easily 
recognized, about the end of the second or the beginning of the 
third week of the pupa state, the calices of these organs are already 
formed and filled with gland-cells, larger than those in the adult 
fly, but otherwise similar ; the tracheal vessels are quite rudimentary 
and very transparent, and the calices themselves are striated coarsely, 
but exhibit the nipple-like pores so characteristic of them. No 
muscular fibres are then distinctly traceable, but their position is 
marked by the presence of rapidly growing cells. 
With regard to the urinary secretion itself, that passed by the 
insect when it first emerges from the pupa case is a semi-solid mass 
of nearly pure uric acid ; that passed afterwards is a turbid fluid, 
sometimes almost clear, and very irritating when applied to any 
tender part of the skin : this fluid deposits an abundance of crystals 
of uric acid when acidulated with hydrochloric or nitric acids. Fig. 9 
represents the principal forms of these crystals. I believe the acid 
is held in solution by ammonia, but of this I am not certain. The 
excrement of the fly when heated over a lamp gives off a strong 
urinary smell. 
