1S15.J On the Uses of the Dorsal Vessel. 1921 



tractions are so distinct as to be perceptible on the outside of the 

 body when the skin is not thick, and has a certain degree of trans- 

 parence. The contractions of tliis vessel are irregular, and scarcely 

 ever isochronous. This irregularity shows that they are not pro- 

 duced by a liquid in circulation. We shall find hereafter that they 

 do not depend upon that liquid, and that they appear even inde- 

 pendent of the organization of the dorsal vessel. These contrac- 

 tions are not equally strong, nor equally numerous, in all the 

 species. Though it is difficult to determine any thing precise oa 

 this head, yet they seem more distinct in the larvffi than in the 

 perfect insect. This difli-iencc is least distinct in the voracious 

 larvas, as those of the geotrupa nasicornis, and certain species of 

 bpliinx and bombyx. 



As to the liquid contained in the dorsal vessel, its colour has 

 always a relation to that of the adipose substance which surrounds 

 this same vessel; consequently it is not uniform in the different 

 species. We observe in fact that the fat, which surrounds the 

 dorsal vessel, has always a colour analogous to that of the liquid 

 contained in the dorsal vessel. This constant similarity of colour 

 may lead to the suspicion that the dorsal vessel is destined for the 

 secretion of that matter so necessary in animals, whose parts increase 

 rapidly when they are transformed into new organs different from 

 those which existed before. 



The humour of the dorsal vessel, then, exhibits various shades 

 of colour, ic is deep brown in most caleopterie, greenish in certain 

 orthoptcrae, yellow in the silk-worm, orange in the caterpillar of the 

 willow, transparent in the larvae of the great butterfly {paon), and 

 of a very light colour in most of the lepidopteraj. V\'hcn examined 

 before tlie microscope, this humour appears composed of a great 

 number of globules, the transparency of which depends upon the 

 colour of the humour itself. This organization announces an analogy 

 of this humour with fat. Like it, we see it composed of small 

 grains, which before the microscope appear each to consist of otlier 

 smaller grains. These grains are merely little globules of fat, which 

 swim, or are contained in small spherical and membranous sacks. 

 The fat is contained in membranous sacks ; for of itself it is fluid, 

 and runs out easily when the sacks are i)ierced with the point of a 

 fine needle. When thus extracted, it renders water muddy, and 

 divides into small masses. The liumor of the dorsal vessel put into 

 water easily mixes with that liquid. A drop of it spread upon a 

 piece of porcelain hardens by evaporation, and then resembles gum. 



The coats of the dorsal vessel are in general very thin. It would 

 be diflicuU to say to what class they belong. As far as 1 can judge, 

 the external membrane is cellular, and llie internal nuiseular. Tlie 

 dorsal vessel is kept in its position by numerous trachea;, several of 

 which lose themselves in it. It is jjiobable that these traehcaj, by 

 their crossing eaeli other in every direction, form the outer coat of 

 the vessel. What proves this is that, in certain species, as the 

 lurvH; of the lornh/x pavonia mnjoTf we sec the dorsal vessel us if 



Vol. V. N° 111. N 



