"jouZ^S^f'S'T -^^^^^ Microscopical Society. 3 
very small filaments appear to accompany the tracheae into the 
central cavity. 
The gland cells are large — ahout 3 J-cyth of an inch in diameter — 
and slightly angular by mutual pressure ; each contains a granular 
nucleus about one-third the diameter of the cell. 
In order to understand the functions of these organs it will be 
necessary to investigate briefly the structure of that portion of 
the alimentary canal which encloses them. About two lines above 
the anal orifice a sphincter, or rather a kind of muscular valve, 
closes the intestine (Fig. 1, a). A little below this the rectum 
becomes much dilated, and its muscular coat correspondingly 
attenuated ; when the insect first emerges from the pupa case this 
dilatation is filled with a semi-solid mass of uric acid. I have never 
found any of this substance above the valve I have just described — ■ 
an important fact in relation to the function of the rectal papillae. 
There is nothing new in the fact that insects excrete uric acid ; 
figures of crystals of this substance from the excrement of the 
clothes-moth and stag-beetle will be found in the * Micrographic 
Dictionary.' I have given figures of several forms from that of 
the fly (Fig. 9) ; the only question is by what structure is this 
substance eliminated. 
There are only two gland structures which open into the 
alimentary canal of the fly — the rectal papillae and the malpighian or 
liver-tubes; these latter, beside opening into the intestines more 
than two lines above the valve I have described, contain cells which 
from their contents — oil-globules and yellow pigment — are unmis- 
takably liver-cells. I have compared these carefully with cells 
from the liver of the bullock, and could see no difference except 
that those of the ktter animal contained more and larger oil- 
globules than are present in the liver-cells of the fly. Hence I 
conclude that we must not look to the malpighian tubes for the 
origin of the urinary secretion. 
On the other hand, the structure of the rectal papillae is just 
such as we might expect to find in a renal organ. If I am right in 
my behef that the central cavity is continuous with the visceral 
cavity, it affords a mean of bringing the circulating fluid into 
almost immediate contact with the secreting cells, a fine structure- 
less membrane only, being interposed — still further I believe I am 
justified in asserting that the circulating fluid is expelled from, and 
a fresh supply is drawn into, the central cavity by a rhythmic 
muscular act. 
In the female fly the rectal papillae lie fortunately between the 
second and third rings of the ovipositor when that organ is exserted, 
where it is sufficiently transparent to allow of the papillae being 
seen during life. I have repeatedly observed a movement of the 
kind I have described. I believe this is the explanation of the 
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