384 LECTURK XVII. 



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 tlakes 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 Carusf ; its phenomena have been witnessed in 

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

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

 pulsations in a minute in the heart of a silkworm. Herhold 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 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 

 arterial 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 thi'ough 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-woi'k over reservoirs of 

 air. The aeriferous tubes in insects are called " trachea;,'' 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 ramifi- 

 cations of the air-tubes may be easily recognised under the mici"o- 

 scope. The spiral filament is situated between the external cellular 

 and an internal delicate epithelial lining. 



The tracheal commence either from lateral apertures, called spi- 

 racles and stigmata {fig' 153, f), or from pneumatic tubes (i), gene- 

 rally continued from the anal segment ; these latter are peculiar to 

 insects which live in water, as the Nepa and Ranatra ; and usually 

 co-exist with stigmata. 



The air is conducted by the spiracles or the pneumatic tubes, or, as 



* Vol. ii. p. 31. Mr. Bowerbank (CCXLIX. p. 239), accurately observed that 

 the blood was inclosed in distinct parietes, and did not flow in the common 

 abdominal cavity, as Cams and Wagner believed. 



t CCL. 



