570 INSECTS AND ARACHNID A. 



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. In some larvae, whose development 

 is yet less advanced, even the dorsal vessel appears to be want- 

 ing, although the fluid of the visceral cavity (in which corpuscles 

 abound) is in a state of continual oscillatory movement. 



390. The Circulation may be easily seen in the wings of many 

 insects in their Papa state, especially in those of the Neurop- 

 terous order (such as dragon-flies and clay-flies) which pass this 

 part of their lives in water, in a condition of activity ; the pupa 

 of Agrion puella, one of the smaller dragon-flies, is a particularly 

 favorable subject for such observations. Each of the "nerves" 

 of the wings contains a "trachea" or air-tube ( 391), 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 nerve itself is not too opaque. 

 The same may be seen, however, in the wings of the pupae 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 pecu- 

 liar 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 has 

 been 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 con- 

 veniently brought into view, by enclosing it (without water) in 

 the animalcule cage, and pressing down the cover sufficiently to 

 keep the body at rest, without doing it any injury. 



391. The Respiratory Apparatus of Insects aflbrds a very in- 

 teresting 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 

 transmission of the fluid to any special organ representing the 

 lung of a Vertebrated animal ( 438) or the gill of a Mollusk 

 ( 353), 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 haustellium or " pro- 

 boscis" of the Butterfly ( 386), and they are minutely distributed 

 in the elongated labium or " tongue" of the fly (Fig. 287). Their 

 general distribution is shown in Fig. 290 ; where we see two 

 long trunks (/) passing from one end of the body to the other, 

 and connected with each other by a transverse canal in every 

 segment; these trunks communicate, on the one hand, by 



