INTERNAL ANATOMY OF INSECTS. 83 
by which the blood is distributed in the different Classes 
of animals, I shall now confine myself to the case of in- 
sects and Arachnida, beginning with the former. 
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 a originating in the head near the mouth b , 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 c , 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 Lyonet to begin in the eleventh segment, from 
which they passed from segment to segment, till they 
arrived at the fourth, where they terminated" 1 . This 
vessel is what Malpighi, who first discovered it, termed 
a heart, or rather series of hearts 6 ; but which Reaumur, 
who injected it, regarded as a simple artery without 
striking contractions f : but to steer clear of any hypo- 
thesis, I shall merely call it the dorsal vessel {Pseudo- 
car -dia). When carefully taken out of the bod} r it is 
found to be a membranous tube, appearing to be closed 
a Plate XXII. Fig. 15. " Lyonet Anat. 105. 
B Ibid. 425. d Ibid. 105—. e Be Bombyc. 15-. 
f Reaum. i. 160—. 
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