RESPIRATORY APPARATUS OF INSECTS. 673 



brought into view by enclosing it (without water) in the Aquatic 

 Box, and pressing-down the cover sufficiently to keep the body at 

 rest without doing it any injury. 



524. The Respiratory Apparatus of Insects affords 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 (§ 583) or the Gill of a Mollusk 

 (§ 481), 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 ' Pro- 

 boscis' of the Butterfly (§ 520), and they are minutely distributed 

 in the elongated Labium or ' Tongue' of the Fly (Fig. 343). Their 

 general distribution is shown in Fig. 346 ; 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 seg- 

 ment ; 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 (h) 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 (§ 295) ; 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 inner 

 and outer coats. — "When a portion of one of the great Trunks with 

 some of the principal Branches of the Tracheal system has been 

 dissected-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. 347), the Spire forms two layers 

 which are brought into close apposition ; and a very beautiful 

 appearance, resembling that of "Watered Silk, is produced by the 

 crossing of the two sets of fibres, of which one overlies the other. 

 That this appearance, however, is altogether an Optical illusion, 

 may be easily demonstrated by carefully following the course of any 

 one of the fibres, which will be found to be perfectly regular. 



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