1 94 Oil the Uses of the Dorsal Vessel. [Marcs 



formed of longitudinal white streaks. These streaks are merely the 

 tracheae extending along the dorsal vessel, and forming on it an 

 inextricable network. It is easy to satisfy oneself that these vessels 

 are trachete, by plunging them into a coloured liquid. The whole 

 becomes coloured except the tracheje, which we see always dis- 

 tinctly, and the principal direction of which is longitudinal. This 

 fact, easily verified, proves, I conceive, that the first membrane of 

 the dorsal vessel is cellular ; since in certain species we see dis- 

 tinctly the white fibrillffi of the tracheae which compose it lose 

 themselves in it. At least we observe that the tracheae have a great 

 influence on the contractions of the dorsal vessel — contractions 

 always strongest in the points where these exist in great numbers. 

 As it is always the inferior portion of the body that receives the 

 greatest number of tracheae, so it is the extremity of the dorsal 

 vessel in which the contractions are most frequent and most sen- 

 sible.* We may say, then, in general, that the contractions of the 

 dorsal vessel are always stronger in the abdomen than elsewhere ; 

 and that on account of the great number of tracheae found there, 

 and of the direct communication of these tracheae by means oi 

 stigmata with the air. In the larvae which receive air by the anus, 

 as the libellulas, this fact is still more evident. 



If the trache.e, or, to speak more correctly, the air which they 

 introduce, have a direct influence on the contraction of the dorsal 

 vessel, this ought to be the better perceived if we examine the 

 vessel in the different transformations which the insects undergo. 

 It is known that the same organs, or the same system of organs, 

 undergo great changes in insects, according to the state in which 

 they are. When in the larva state, the digestive system is most 

 preponderant, and the tracheae passing into the intestinal tube are 

 so numerous that they form very numerous bundles round the intes- 

 tines, and very multiplied nets. This disposition is particularly 

 evident in the caterpillars. The caterpillars which inclose them- 

 selves in cocoons have likewise this peculiarity, that the silky vessels 

 become very large at tlie time that they are going to change into 

 crysalids. These vessels, then, fill all the cavity of the abdomen, 

 and the intestinal tube diminishes so much that it appears to con- 

 tract upon itself, and to be reduced almost to nothing. This is 

 particularly the case with the worms that spin a great deal, as the 

 silk-worm, the bonihyx pavonia major, media, trifolia, gjiercuSy 

 &c. In the perfect insects the organs of generation, and tliose of 

 motion, assume the greatest preponderance ; the tracheae become 

 likewise very numerous in these parts. Here is a very sensible 

 change produced in the organs of insects, a change which is the 

 consequence of their successive transformations. But does the same 

 tiling take place with the dorsal vessel ? To determine the point, 

 I compared tlie dorsal vessel of larvas with those of perfect insects. 



* Lyonnet has made that remark in his excellent Anatomie de la Cbeoille da 



Saulc, 



I 



