82 INTERNAL ANATOMY OF INSECTS. 
and Arachnida, beginning with the former. As their nu- 
tritive fluid and their dorsal vessel have not been disco- 
vered to be connected, I shall consider them separately : 
but I must first observe, —that the term Circulation, with 
which this letter is headed, though not strictly applicable 
to insects, is perfectly proper when used with respect to 
Arachnida ; you will not therefore stumble at the thresh- 
old, and object to my employing it. 
I. If you examine attentively the back of any smooth 
caterpillar with a transparent skin, you will perceive in 
that part an evident pulsation, as though a fluid were 
pushed at regular intervals towards the head, along a 
narrow tube which seems to run the whole length of the 
body. Accurate dissections have proved that this ap- 
pearance is real, that there is actually present in the 
back of most insects, placed immediately under the skin 
and furnished with numerous air-vessels, a longitudinal 
vessel* originating in the head near the mouth®, running 
parallel with the alimentary canal nearly to the anus, 
containing a fluid which is propelled in regular pulsa- 
tions of from 20 to 100 per minute, more or less as the 
weather is colder or warmer‘, causing a sensible alter- 
nate systole and diastole from the anal extremity to- 
wards the head. In the Cossus these pulses were ob- 
served by Lyonnet to begin in the eleventh segment, from 
which they passed from segment to segment, till they 
arrived at the fourth, where they terminated‘. This ves- 
sel is what Malpighi, who first discovered it, termed a 
heart, or rather series of hearts®; but which Reaumur, 
* Prats XXII. Fic. 15. > Lyonnet Anat. 105. © Ibid. 425. 
4 Tbid. 105—. © De Bombyc. 15—. 
