Capillary Arterial Vessels in Insects. 185 
distribution of the blood-vessels in the bodies of the higher 
animals.” 
Nearly twenty years have passed since the period when 
M. Agassiz announced these facts, which appear to have been 
but little understood ; for the authors who have written on the 
anatomy and physiology of insects have not even mentioned 
them. 
The direct observation of the phenomenon of circulation 
was wanting: no one had succeeded in detecting the move- 
ment of the blood either in the peritracheal space or in the 
capillaries ; and M. Milne-Edwards indicated as a fact to be 
regretted that ‘ the existence of currents in the tubiform lacune 
had not yet been ascertained.”’ Having been led, by general 
researches upon the organization of the Diptera, to study the 
apparatus of circulation and respiration, I have frequently 
examined the trachee. I always saw, without difficulty, the 
globules between the two coats; but, the animals being dead, 
the blood was motionless. In pursuing my investigations of 
the distribution of the trachee in the muscles, I was too much 
struck by the character of this distribution not to dwell upon 
it. Having succeeded in removing a muscular bundle from a 
living Hristalis, without tearing it, and brought it quickly 
into the focus of a powerful microscope, I had the surprise of 
seeing the blood imprisoned between the two membranes of 
the trachez running in this peritracheal space, and penetrating 
into the finest arterioles. I observed the course of the blood- 
globules with the same facility as in the capillaries of the 
mesentery or the membrane uniting the digits of a frog. I 
was, therefore, fortunate enough to see the circulation of the 
blood in the capillaries of insects. 
I have been able to convince myself of the existence of a 
system of arterial capillaries in all insects: the most delicate 
arterioles creep not only through the muscles, but also over the 
other organs. In general the blood thus observed by trans- 
mitted light presents a rosy tint very favourable for observa- 
tion. When the blood abandons the trachez and its arterioles, 
which I have frequently seen, they lose their coloration. The 
trachea, recognizable by its spiral filament, may always be 
perceived ; but it is very difficult to distinguish the arterioles, 
so delicate and transparent are their walls. 
The difficulties of the experiment are great. The insect 
must be quickly opened, a muscular bundle must be taken 
from the living animal, and this bundle conveyed under the 
microscope; and then, under favourable conditions, the blood 
is seen flowing rapidly through the arterioles. For these in- 
vestigations a considerable magnifying-power is necessary. I 
