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 \\\Q fourth, where they terminated d . 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- 

 cardia). When carefully taken out of the body it is 

 found to be a membranous tube, appearing to be closed 



a PLATE XXII. FIG. 15. b Lyonet Anat. 105. 



c Ibid. 425. d Ibid. 105. e De Bombyc. 15. 

 f Reaum, i. 160. 



