312 PTILOTA : INTERNAL ANATOMY. 



C. The Circulatory System. 



It has been generally considered that insects were destitute 

 of a real and direct circulation analogous to that of the 

 higher animals ; but the recent observations of many distin- 

 guished authors (Comparetti, Carus, Bowerbank, Burmeister, 

 &c.), appear to have fully proved that they do possess such a 

 system. Although there is no part in insects strictly analo- 

 gous to the heart, yet this name has been ordinarily given to 

 a series of large reservoirs forming a longitudinal tube, ex- 

 tending throughout the entire 

 length of the body (commencing 

 at the first thoracic segment) be- 

 neath the dorsal integuments and 

 muscles, and which is easily dis- 

 tinguished by its alternate con- 

 tractions and dilatations, similar 

 to those of the heart in the higher 

 animals, which may be constantly 

 observed in the silkworm and other 

 naked caterpillars. In this dorsal 

 vessel, as it has been called by 

 some physiologists, the blood is 

 contained, and which, unlike that 

 of the vertebrata, is cold, trans- 

 parent, and often entirely colour- 

 less. It was, however, consider- 

 ed, that as no outlet had been 

 discovered, there could be no cir- 

 culation, and yet it was supposed 



that the fluid in some way or other made its escape, and 

 was disseminated amongst the various internal organs, which 

 it penetrated by imbibition. Strauss-Diirckheim, howrvor, 

 discovered that there exists in the cockchafer a series of on- 



