90 INTERNAL ANATOMY OF INSECTS. 
Whatever be the functions of the dorsal vessel, this 
seems the most proper place to state to you what further 
is known respecting it. Its construction is nearly alike 
in insects in all their states, except that in the imago it _ 
is shorter and narrower. Reaumur has affirmed, and 
before him Malpighi made a similar observation, that in 
chrysalises newly disclosed from the larva, and yet trans- 
parent, the motion of the included fluid is the reverse of 
what it has been in that state, it being propelled from 
the head to the tail, which he found to be the case also 
in the imago?. If this be true, and there is no reason to 
doubt his accuracy, when they are more advanced, it re- 
sumes its old course, as Lyonnet observed, from the tail 
to the head’. But probably it is not always uniformly 
in the same direction, since Malpighi states that a very 
slight cause will change its course, and that the pulsa- 
tions differ in quickness in different portions of the heart °. 
If its course were really always the same, and in one di- 
it is more analogous to gum or varnish. He saw indeed a few 
globules, which appeared ten times as big as the others, which swam 
upon the water, but which he did not regard as componeit parts of 
the fluid, but as little drops of grease extravasated by dissection. The 
fluid of the vessel itself easily mixed with water, and appeared to sink 
in it to the bottom!. ‘These circumstances seem to indicate that it 
is not of a fatty or oleaginous nature. Further experiments however 
seem necessary to ascertain the nature of the fluid and its object: 
but I think it is a fair and reasonable conjecture, that as the vessel 
in question is in many respects analogous to the real heart in Arach- — 
nida and some Crustacea, it so far performs the functions of a heart 
as to produce an important effect in the nutrition of the animal. A 
more satisfactory elucidation of the uses of this vessel may be ex- 
pected from the able pen of Mr. W. S. MacLeay. 
1 Lyonnet Anat. 426—. 
* Reaum. i. 409, 643—. Malpigh. De Bombyc. 38. 
* Lesser L. ii, 87 note *. © Ubi supra. 
