THE MICROSCOPE AND ITS REVELATIONS. 



condition of activity; the pupa of Agrion puella, one of the smaller dra- 

 gon-flies, being a particularly favorable subject for such observations. 

 Each of the 'nervures' of the wings contains a ( trachea' or air-tube ( 

 634), which branches-off from the trached 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 pupae of Bees, Butterflies, etc., 

 Avhich 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 he caught within a few hours or days of its first appear- 

 ance; and the circulation maybe 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. 



634. The Respiratory apparatus of Insects affords a very interesting 

 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 ( 692) 

 or the gill of a Mollusk ( 586), but by the introduction of air into every 

 part of the body, through a system of minutely-distributed trachea or 

 air- tubes, which penetrate even the smallest and most delicate organs. 

 Thus, as we have seen, they pass into the haustellium or f proboscis ' of 

 the Butterfly ( 630), and they are minutely distributed in the elongated 

 Idbium or ' tongue ' of the Fly (Fig. 428). Their general distribution is 

 shown in Fig. 431; where we see two long trunks (f) passing from one 

 end of the body to the other, and connected with each other by a trans- 

 verse canal in every segment; these trunks communicate on the one hand, 

 by short wide passages, with the ' stigmata,' 'spiracles,' or ' breathing 

 pores' (g}, through which the air enters and is discharged; whilst they 

 give off branches to the different segments, which divide again and again 

 into ramifications of extreme minuteness. They usually communicate 

 also with a pair of air-sacs (fi) which is situated in the thorax; but the 

 size of these (which are only found in the perfect Insect, no trace of them 

 existing in the larvae) varies greatly in different tribes, being usually 

 greatest in those insects which (like the Bee) can sustain the longest and 

 most powerful flight, and least in such as habitually live upon the ground 

 or upon the surface of the water. The structure of the air- tubes reminds 

 us of that of the 'spiral vessels' of Plants, which seem destined (in part 

 at least) to perform a similar office ( 362); for within the membrane 

 that forms their outer wall, an elastic fibre winds round and round, so as 

 to form a spiral closely resembling in its position and functions the spiral 

 wire-spring of flexible gas-pipes; within this again, however, there is 

 another membranous wall to the air-tubes, so that the spire winds between 

 their innner and outer coats. When a portion of one of the great trunks 

 with some of the principal branches of the trachea! system has been dis- 

 sected- out, and so pressed in mounting that the sides of the tubes are 

 flattened against each other (as has happened in the specimen represented 

 in Fig. 432), the spire forms two layers which are brought into close 



