96 EVENINGS AT THE MICROSCOPE. 



employed. The air-tubes themselves are necessarily ex- 

 tremely thin and delicate; so that on the slightest pres- 

 sure their sides would inevitably collapse, and thus com- 

 pletely put a stop to the passage of air through them, 

 producing, of course, the speedy suffocation of the insect, 

 thad not some means been adopted to keep them always 

 permeable; and yet, to do so, and at the same time to 

 preserve their softness and perfect flexibility, might seem 

 a problem not easily solved. The plan adopted, however, 

 fully combines both these requisites. Between the two 

 thin layers of membrane which form the walls of every 

 air-tube, a delicate elastic thread (a wire of exquisite 

 tenuity) has been interposed, which, winding round and 

 round in close spirals, forms by its revolutions a cylindri- 

 cal pipe of sufficient firmness to preserve the air-vessels 

 in a permeable condition, whilst at the same time it does 

 not at all interfere with its flexibility; this fine coil is 

 continued through every division of the trachece, even 

 to their most minute ramifications, a character whereby 

 these vessels are readily distinguishable when examined 

 under the microscope."* 



Man has imitated this exquisite contrivance in the 

 spiral wire-spring which lines flexible gas-pipes; but his 

 wire does not pass between two coats of membrane. One 

 of the most interesting points of the contrivance is the 

 way in which the branches are (so to speak) inserted 

 in the trunk, the two wires uniting without leaving a 

 blank. It is difficult to describe how this is done; but 

 by tracing home one of the ramifications you may see 

 that it is performed most accurately; the circumvo- 

 lutions of the trunk-wire being crowded and bent round 

 above and below the insertion (like the grain of timber 

 .-around a knot), and the lowest turns of the branch- wire 

 *being enlarged so as to fill up the opening entirely. 



* "Nat. Hist, of Anim./'i. 6. 



