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 lacunae 

 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 tracheae. I always saw, without difficulty, the 

 globules between the two coats j but, the animals being dead, 

 the blood was motionless. In pursuing my investigations of 

 the distribution of the tracheae 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 Eristalisj 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 trachege 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 tracheae 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 



