QUARTERLY CHRONICLE. 273 



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 tubiforra 

 lacunae had not yet been ascertained." Having been led, by 

 general researches upon the organization of the Diptera, to 

 study the ajiparatus of circulation and respiration, I have 

 frequently examined the tracheae. 1 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 tracheae in the 

 muscles, I was too much struck by the character of this dis- 

 tribution not to dwell upon it. Having succeeded in remov- 

 ing a muscular bundle from a living Eristalis, without tearing 

 it, and brought it qviickly into the focus of a powerful micro- 

 scope, I had the surprise of seeing the blood imprisoned 

 between the two membranes of the tracheae running in this 

 peritracheal sj^ace, and penetrating into the finest arterioles. 

 I observed the course of the blood-globules with the same 

 facility as in the caj)illaries 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 

 transmitted light presents a rosy tint very favorable for 

 observation. When the blood abandons the trachea and its 

 arterioles, which I have frequently seen, they lose their 

 coloration. The trachea, recognisable by its spiral filament, 

 may always be perceived ; but it is very difficult to distin- 

 guish 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 favorable conditions, the blood 

 is seen flowing rapidly through the arterioles. For these 

 investigations a considerable magnifying power is necessary. 

 I have been singularly aided by the very perfect immersion- 

 objectives which M. Nachet was kind enough to place at my 

 disposal. 



It is necessary to give a precise explanation of the structure 

 of the arterioles and their mode of distribution. 



The tracheae, as is well known, are composed of two 

 coats: the inner coat forms the envelope of ':he aeriferous 



