INSECTS AND ARACHNIDA. 237 



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 Micro- 

 scopist, than to the professed Anatomist. Hence we shall only stop to 

 mention that the ' muscular gizzard ' in which the oesophagus very com- 

 monly terminates, is often lined by several rows of strong horny teeth for 

 the reduction of the food, which furnish very beautiful objects, espe- 

 cially for the Binocular. These are particularly developed among the 

 Grasshoppers, Crickets, and Locusts, the nature of whose food causes 

 them to require powerful instruments of its reduction. 



632. The Circulation of Blood may be distinctly watched in many 

 of the more transparent larvae, and may sometimes be observed in the 

 perfect insect. It is kept up, not by an ordinary heart, but by a 'dorsal 

 vessel' (so named from the position it always occupies along the middle 

 of the back), which really consists of a succession 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 larvae, however, these partitions are very indistinct; 

 and the walls of the ' dorsal vessel ' are so thin and transparent, that it 

 can with difficulty be made-out, a limitation of the light by the dia- 

 phragm 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' corpus- 

 cles, by the motion of which its flow can be followed. The current enters 

 the ' dorsal vessel ' at its posterior extremity, and is propelled forwards 

 by the contractions of the successive chambers, 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 * dor- 

 sal vessel,' the blood is distributed in three principal channels; a central 

 one, namely, passing to the head, and a lateral one to either side; de- 

 scending so as to approach the 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 i& 

 from these also, that, in the larva of the Ephemera tnarginata (Day-fly), 

 the extreme transparence of which renders it one of the best of all sub- 

 jects for the observation of Insect Circulation, the smaller currents diverge 

 into the gill-like appendages with which the body is furnished ( 636). 

 The blood-currents seem rather to pass through channels excavated among 

 the tissues, than through vessels with distinct walls; but it is not improb- 

 able that in the perfect Insect the case may be different. In many aqua- 

 tic larvae, especially those of the Culicidce (Gnat tribe), the body is almost 

 entirely occupied by the visceral cavity; and the blood may be seen to- 

 move backwards in the space that surrounds the alimentary canal, which 

 here serves the purpose of the channels usually excavated through the 

 solid tissues, and which freely communicates at each end with the ' dor- 

 sal vessel.' This condition strongly resembles that found in many Anne- 

 lida. 1 



633. The circulation may be easily seen in the wings of many Insects 

 in their pupa state, especially in those of the Neuroptera (such as Dragon - 

 flies, and Day-flies), which pass this part of their lives under water in a 



J See the Memoirs on Corethra plumicornis, by Prof. Rymer Jones, in "Trans- 

 act, of Microsc. Soc.," N.S., Vol. xv. (1867), p. 99; by Prof. E. Ray Lankester, in 

 the " Popular Science Review" for October, 1865; and by Dr. A. Weissmann, in. 

 "Siebold and Kolliker's Zeitschrift," Bd. xvi., p. 45. 



