M. Blanchard on the Circulation in Insects. 113 



the entire circulation in insects is reduced to this : the blood, 

 urged forwards by the dorsal vessel, bathes the organs by being 

 poured into the cavities of the body, where a retrograde move- 

 ment causes it to return into the dorsal vessel by orifices at its pos- 

 terior .part. Leon Dufour, who has contributed so much to our 

 knowledge of the organization of insects, denies, with Cuvier, the 

 existence of any circulation whatever in these animals. According 

 to that anatomist, the dorsal vessel is only a simple cord, without 

 an internal cavity. Its movement, he says, is only the result of 

 the simple contractility of its tissue, a kind of fibrillar vibration 

 common to many living tissues. 



Such are the various opinions regarding the circulation in in- 

 sects. What might perhaps have caused surprise, was that, in 

 accordance with the ordinary explanation, the circulatory and 

 respiratory apparatus were to a certain extent independent, the 

 tracheae, according to the general opinion, not coming into con- 

 tact with the nutritive liquid except by currents traversing the 

 lacunae between the organs. However, this did not attract atten- 

 tion. Their study by the aid of their transparency, not allowing 

 of accurate details being distinguished, the question has always 

 remained much in the same state. 



A very simple means, however, of tracing the whole course of 

 the circulation in insects would have been to inject coloured 

 liquids, llecourse has not been had to this proceeding ; or if so, 

 no benefit has been derived from it. Nevertheless, among the 

 invertebrate animals, there are few in which this means of inves- 

 tigation so easily yields a good result. Whether we inject by the 

 dorsal vessel or the lacunas, the entire circulatory system is im- 

 mediately filled. Nothing is more remarkable and elegant than 

 an insect properly injected : all the tracheae, which ramify 

 throughout the organs in such delicate branches, are coloured by 

 the injection; however, not even the smallest drop of liquid 

 penetrates their interior. 



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

 membranes between which there is a thread spirally coiled. The 

 sanguineous fluid penetrates between the two membranes. Every 

 part of it thus comes into contact with the air contained in the 

 tracheal tubes, and the re-organization of the blood is effected as 

 in animals furnished with lungs, although the anatomical dispo- 

 sition is so different. This observation explains the structure of 

 the tracheae. The spiral thread not only serves to give them a 

 certain solidity, it has also the purpose of keeping the two sheaths 

 of which they are composed apart, and of keeping them open 

 near the respiratory orifices so as to give passage to the nutritive 

 fluid. When the tracheae are vesicular the spiral thread disap- 

 pears, and then the very numerous and excessively fine canals 



