672 CIRCULATION OF BLOOD IN INSECTS. 



between the chambers, which only open forwards. Arrived at the 

 anterior extremity of the Dorsal Vessel, the blood is distributed in 

 three principal channels ; a central one, namely, passing to the 

 Head, and a lateral one to either side, descending 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 is from 

 these also, that, in the Larva of the Ephemera marginata (Day- 

 fly), the extreme transparence 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 (§ 526). 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 different. In many Aquatic Larvse, 

 especially those of the Cidicidce (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 Dorsal Vessel. This condition strongly 

 resembles that found in many Annelida. * 



523. The Circulation may be easily seen in the Wings of many 

 Insects in their Pupa state, especially in those of the Neuro- 

 pterous Order (such as Dragon-flies and Day-flies) which pass this 

 part of their lives under water in a condition of activity ; the Pupa 

 of Agrion pitella, one of the smaller Dragon-flies, is a particularly 

 favourable subject for such observations. Each of the ' Nervures' 

 of the wings contains a 'Trachea' or Air-tube (§ 524), which 

 branches-off from the Tracheal system of the body ; and it is in a 

 space around the trachea that the Blood may be seen to move, 

 when the hard framework of the nervure itself is not too opaque. 

 The same may be seen, however, in the wings of Pupa? of Bees, 

 Butterflies, &c, which remain shut-up motionless in their cases ; 

 for this condition of apparent torpor is one of great activity of 

 their Nutritive system, — those organs, especially, which are peculiar 

 to the perfect Insect, being then in a state of rapid growth, and 

 having a vigorous Circulation of Blood through them. In certain 

 Insects of neai-ly every Order, a movement of fluid has been seen 

 in the Wings for some little time after their last Metamorphosis ; 

 but this movement soon ceases, and the Wings dry-up. The 

 common Fly is as good a subject for this observation as can be 

 easily found ; it must be caught within a few hours or days of its 

 first appearance; and the Circulation may be most conveniently 



* See the Memoirs on Coretlira plumicorius, by Prof. Rymer Jones, in 

 " Transact, of Microsc. Soc," Vol. xv. (N.S.), p. 99 ; by Mr. E. Ray 

 Lankester in the " Popular Science Review" for October, 1865; and by 

 Dr. A. Weissmami in "Siebold and Kolliker's Zeitschrift," Bd. xvi., p. 45. 



