CIRCULATION OF BLOOD. 669 



diversities, not only in different tribes of Insects, but in different 

 states of the same individual, that it would be utterly vain to 

 attempt here to give even a general idea of it ; more especially 

 as it is a subject of far less interest to the ordinary Microscopist, 

 than it is to the professed Anatomist. Hence we shall only stop 

 to mention, that the muscular gizzard in which the oesophagus 

 very commonly terminates, is often lined by several rows of 

 strong horny teeth for the reduction of the food, which furnish 

 very beautiful microscopic objects. These are particularly de- 

 veloped among the Grasshoppers, Crickets, and Locusts, the 

 nature of whose food causes them to require powerful instru- 

 ments for its reduction. 



389. The Circulation of Blood may be distinctly watched in 

 many of the more transparent Larvce, and may sometimes be ob- 

 served in the perfect Insect. It is kept up, not by an ordinary 

 heart, but by a "dorsal vessel," which really consists of a suc- 

 cession of muscular hearts or contractile cavities, one for each 

 segment, opening one into another from behind forwards, so as 

 to form a continuous trunk, divided by valvular partitions. In 

 many larvse, however, these partitions are very indistinct, and 

 the walls of the " dorsal vessel" (so named from the position it 

 always occupies along the middle of the back) are so thin and 

 transparent that it can with difBculty be made out, a limitation 

 of the light by the diaphragm being often necessary. The blood 

 which moves through this trunk, and which is distributed by it 

 to the body, is a transparent and nearly colorless fluid, carrying 

 with it a number of "oat-shaped" corpuscles, by the motion of 

 which its flow can be followed. The current enters the dorsal 

 vessel at its posterior extremity, and is pi'opelled by the con- 

 tractions of the successive chambers towards the head, being 

 prevented from moving in the opposite direction by the valves 

 between the chambers, which only open forwards. Arrived at 

 the anterior extremity of the dorsal vessel, the blood is distri- 

 buted into three principal channels; a central one, namely, 

 passing to the head, and a lateral one to either side, descending 

 so as to approach thfe lower surface of the body. It is from the 

 two lateral currents that the secondary streams diverge, which 

 pass into the legs and wings, and then return back to the main 

 stream ; and it is from these also, that, in the larva of the Ephe- 

 mera marginata (day-fly), the extreme transparency of which 

 renders it one of the best of all subjects for the observation of 

 Insect circulation, the smaller currents diverge into the gill-like 

 appendages with which the body is furnished (§ 393). The 

 blood-currents seem rather to pass through channels excavated 

 among the tissues, than through vessels with distinct walls; but 

 it is not improbable that in the perfect Insect the case may be 

 diflerent. In many aquatic larvse, especially those of the Culi- 

 cidce (gnat tribe), the body is almost entirely occupied by the 

 visceral cavity ; and the blood may be seen to move backwards 



