INSECT. 



including the intestinum tenne or small gut, the 

 caecum or blind gut, and the rectum or vent gut. In 

 addition to this variously divided digestive tube, 



Figs. 146, Digestive system of Slelolontha; 147, ditto of Staphy. 

 linus (from L. Dufour). 



there are a number of slender membranous tubes 

 rilled with a fluid apparently analogous to the bile; 

 they are, for the most part, inserted into the stomach 

 near its outlet, where the secretion, which is thus 

 discharged, acts upon the already partially dissolved 

 food, and converts it into a pulpy mass or chyle, the 

 nutritive parts of which, as it passes into the small 

 gut, are taken up by the inner membrane of the 

 latter, and passed into the cavity of the abdomen, 

 lying upon and around the parts to be nourished by 

 absorption. The indigestible parts of the food are 

 collected in the caecum, and thence passed out of the 

 body, 



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 observ- 

 ations of many distinguished 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 analogous 

 to the heart, yet this name has been ordinarily given 

 to a series of large reservoirs forming a longitudinal 

 tube, extending throughout the entire length of the 

 body (commencing at the first thoracic segment) be- 

 neath the dorsal integuments and muscles, and which 

 is easily distinguished by its alternate contractions 

 and dilatations, similar to those of the heart in the 

 higher animals, which may be constantly observed in 

 the silk-worm and other naked caterpillars. In this 

 dorsal vessel, as it has been called by some physio- 

 logists, the blood is contained, and which, unlike that 

 of the vertebrata, is cold, transparent, and often en- 

 tirely colourless. It was, however, considered that 

 as no outlet had been discovered, there could be no 

 circulation, and yet it was supposed that the fluid in 

 some way or other made its escape, and was disse- 

 minated amongst the various internal organs, which 



869 



it penetrated by imbibition. Strauss-Diirckbeim, 

 however, discovered that there exists in the cock- 

 chaffer a series of orifices opening into each side of 

 the dorsal vessel, and furnished with valves. 



Figs. 148, Larva of Ephemera, showing the central dorsal vessel, 

 someof the valves being- opened, the current of the fluid being- 

 indicated by arrows (from Mr. Bowerbank) ; 14Q, lateral view of 

 the dorsal vessel of Meiolontlia. 



The dorsal vessel consists, according to Strauss- 

 Diirckheim, of eight successive chambers in the cock- 

 chaffer. Mr. Bowerbank says " they are about equal in 

 number to the sections of the body" in the larva of 

 an Ephemera, separated by converging valves, which 

 allow the blood to be propelled forward towards the 

 head, but prevent its returning. The blood abounds 

 in very minute oat-shaped particles, and is> seen cir- 

 culating in every part of the body, from whence, 

 upon each pulsation and opening of the lateral aper- 

 tures, it rushes into the dorsal vessel, and is conveyed 

 into the anterior part of the body, When the ter- 

 minal chamber is filled, the blood which it contains 

 causes the lateral valves to close, and the blood to be 

 propelled into the next chamber, which, at the same 

 time also receives a flow of blood from its own lateral 

 openings. In this manner the blood is forced towards 

 the prothoracic chamber, where it terminates, ac- 

 cording to Strauss-Diirckheim, in a single artery 

 without 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 mem- 

 branes, the external one of which is furnished with 

 numerous ramifications of the air-tubes. The various 

 air tubes also which are distributed throughout the 

 body, communicate with the blood as it is discliaiged 

 from the prothoracic chamber, and thus it receive as 

 supply of oxygen. The number of chambers 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 



