314 PTILOTA : INTERNAL ANATOMY. 



branches, which carries the blood into the head, where it is 

 poured out, and thence flows back into the cavity of the body 

 (in distinct arteries, according to Mr. Bowerbank), to be 

 again taken into the heart, which consists of two membranes, 

 the external one of which is furnished with numerous rami- 

 fications of the air-tubes. The various air-tubes, also, which 

 are distributed throughout the body, communicate with the 

 blood as it is discharged from the prothoracic chamber, and 

 thus it receives a supply of oxygen. The number of cham- 

 bers in an imago appears to be smaller than in a larva ; we 

 have also seen that the number of segments in the latter are 

 more numerous than in the former. It would therefore 

 form a very interesting subject of inquiry to ascertain the 

 mode in which, and the period when, this loss takes place. 

 The circulation of the blood is most distinctly seen in the 

 aquatic larvaj, and in the wings of insects, owing to their 

 transparency. 



D. — The Respiratory System. 

 The systems of circulation and respiration are necessarily 

 dependent upon each other. In the higher animals the lungs 

 are the receptacles of the air, and the blood rushes into 

 these organs to be oxygenated. In insects we have, how- 

 ever, seen that the circulatory system extends throughout 

 the body, the great dorsal vessel equally extending from one 

 end of the body to the other : hence it is requisite that the 

 respiratory apparatus should be developed to an equivalent 

 degree, and we accordingly find that respu-ation in insects 

 is efi^ected by means of two great longitudinal vessels or 

 canals called tracheae, running along the sides of the body 

 beneath the outer integuments and muscles, and which open 

 to the outer atmosphere by means of short tubes, terminat- 

 ing in breathing pores (spiracles or stigmata). The number 

 of these spiracles is various ; those of the larva, in which 



