INSECTA. 223 



culation, there is this heart answering both purposes" (viz. the cor- 

 poreal and puhnonary circulations) ; and again, " with respect to its 

 use, it is, in the most simple kind of heart, to propel the blood through 

 the body, immediately from the veins, which blood is to receive its 

 purification in this passage, when the lungs are disposed throughout 

 the body, as in the flying Insect." In the note at p. 221. he alludes 

 to the animals in which the veins are entirely cellular ; and expresses 

 his idea more definitely in the following passage from his manuscript 

 Observations on Insects: — " Of the veins. The veins of the Insect would 

 appear to be simply the cellular membrane ; but they are regularly 

 formed canals, although not so distinctly cylindrical canals as in the 

 quadruped, &c., nor branching with that regularity. They would 

 appear to be, or to fill up, the interstices of the flakes of fat, air-cells, 

 muscles, &c., and therefore might be called in some measure the 

 cellular membrane of the parts." * 



The chief merit of the rediscovery of the circulation of the blood 

 in Insects is due to Carus; its phenomena have been witnessed in 

 the appendages of Insects by other observers, as Ehrenberg, Wagner, 

 Burmeister, Bowerbank, and Tyrrell. Hunter counted thirty-four 

 pulsations in a minute in the heart of a silkworm. Herholt counted 

 from thirty to forty pulsations of the heart in a minute in a full- 

 grown caterpillar : Suckow observed thirty per minute in a full- 

 grown caterpillar of the pine moth, and only eighteen in its pupa 

 state. 



The action of the heart is accelerated in Insects, as in other 

 animals, by muscular exertion and excitement ; and Mr. Newport has 

 counted as many as 142 pulsations in a minute, in a species of wild 

 bee so excited. 



Although the anatomist searches in vain for that profusion of ar- 

 terial and venous vessels which pervade the body of most animals, 

 the insects are not without their systems of capillary tubes, which 

 ramify as richly over all the organs and through every tissue, and 

 which connect together the different parts of the body. These ves- 

 sels, however, carry air instead of blood ; the relations between the 

 sanguiferous and respiratory systems are reversed, and the air is dis- 

 tributed by a vascular system over the reservoirs of blood, instead of the 

 blood being distributed by a capillary net- work over a reservoir of air. 

 The aeriferous tubes in insects are called " tracheae," having their 

 parietes strengthened by an elastic cartilaginous filament, not indeed 

 disposed in a series of distinct rings, but in a continuous close spiral 

 coil. By this structure the most delicate and invisible ramifications 

 of the air-tubes may be easily recognised under the microscope. The 



* Physiol. Catalogue, vol, ii. p. 31. 



