994 INSECTS AND ARACHNIDA 



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. The blood-currents 

 seem rather to pass through channels excavated among the tissues 

 than through vessels with distinct walls. In many aquatic 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 exca- 

 vated through the solid tissues, and which freely communicates at 

 each end with the dorsal vessel. This condition strongly resembles 

 that found in many Annulata. 1 



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 condition of activity, the pupa of Agrion puella, one of 

 the smaller dragon-flies, being a particularly favourable subject for 

 such observations. Each of the ' nervures ' of the wings contains a 

 ' trachea ' or air-tube, which branches off from the tracheal system 

 of the body ; and it is in a space around the trachea that the blood 

 maybe 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 

 pupse of bees, butterflies, etc., 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 nearly every order a movement of fluid may 

 be seen in the wings for some little time after their last meta- 

 morphosis ; 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 

 brought into view by inclosing it (without water) in the aquatic- 

 box, and pressing down the cover sufficiently to keep the body at rest 

 without doing it any injury. 



The respiratory apparatus of insects affords a very interest- 

 ing series of microscopic objects ; for, with great uniformity in its 

 general plan, there is almost infinite variety in its details. The 

 aeration of the blood in this class is provided for, not by the trans- 

 mission of the fluid to any special organ representing the lung of a 

 vertebrated animal or the gill of a mollusc, but by the introduction 

 of air into every part of the body, through a system of minutely 

 distributed trachece, or air-tubes, which penetrate even the smallest 

 and most delicate organs. Thus, as we have seen, they pass into 

 the haustellum, or ' proboscis.' of the butterfly, and they are minutely 



1 See the memoirs on Corethra^lumicornis, by Professor Rymer Jones, in Trans. 

 Microsc. Soc. n.s. vol. xv. 1867, p. 99 ; by Professor E. Ray Lankester, in the Popular 

 Science Beview for October 1865 ; and by Dr. A. Weismann, in Zeitsclir. f. wiss. Zuol. 

 Bd. xvi. p. 45. On the circulatory system of insects consult Graber, ' Ueber den pro- 

 pulsatorischen Apparat der Insecten,' Arcli.fur mikr. Anat. ix. p. 129. 



