1815.] On the Uses of the Dorsal Fesset. 195 



I found that in general the diameter of the dorsal vessel was much 

 more equal in larvje than in perfect insects. 1 first thouglit that the 

 form of the body which in tlie larvae exhibits few contractions, but 

 is almost always cylindrical, might be the cause of this equality. , 

 But though this cause may have some influence, I do not think it 

 the principal one. It seems to he chiefly owing to the much greater 

 equality with which fat is drstributed over the bodies of larvae than 

 of perfect insects. In those insects which have no fat except in the 

 abdomen, the dorsal vessel is scarce sensible in the tliorax, or at 

 least its contractions. But in all, the contractions are always greater 

 in the abdomen, in consequence of the tracheae which pass into it^ 

 as has been already said. But as an organ never becomes larger 

 without the object which it has in view feeling ihe consequence of 

 it, the fat is much more abundant in the abdomen than any where 

 else. Though this is less evident in the perfect insects than in the 

 larvae, for reasons already explained, the abundance of adipose 

 matter in the abdomen is still very sensible in them. We shall see 

 in a future part of this memoir how far these facts will enable us ta 

 determine the functions of the dorsal vessel. 



Tlie dorsal vessel of insects receives nerves, and these in consi- 

 derable number. Lyonnet * mentions nine pair in describing the 

 dorsal vessel of the caterpillar of the willow. We cannot fix the 

 number so precisely, especially when we take a general view of tliis 

 vessel ; but it appears to us that it always receives more in the larvae 

 than in the perfect insects. Nerves seem even to enter and lose 

 themselves in the membranes of this vessel ; but we have not been 

 able to verify the observation of Lyonnet, who affirms that several 

 nerves issue from this organ. Tliis skilful observer perceiving the 

 dorsal vessel to receive a great number of nerves, thought that it 

 furnished to the nerves the substance necessary for them. It would 

 be very difficult to prove the truth of such an assertion, or to demon- 

 strate its falsehood. We think it sufficient to mention it, without 

 admitting it ; for to judge from analogy, one would not be disposed 

 to believe it. 



Before speaking of the muscles of the cellular membrane whicli 

 keep the dorsal vessel in its position, we shall say a few words on 

 the contractions and dilatations of that organ. This vessel, we have 

 said, exhibits stronger contractions and dilatations in the larvaj than 

 in perfect insects. This is true in general with this restriction, that 

 if these contractions are more distinct, they are on the contrary less 

 frequent. Thusl reckoned only Mi pulsations in the minute in the 

 dorsal vessel of the larva of tlie large butterBy (paon), while I 

 observed more than 82 in the locusta, and more than ) 40 in the 

 lifinus tcrrc'strh ; but in the larv;e the contractions were so strong 

 that the two sides of the membrane of the dorsal vessel touched one 



• Sec (he Traits Aiiatomlque de la Chenitir du Smilc chnp. xi. \i. 419, 

 |>l. xii. lig. I. 



N 2 



