322 



INSECTA. 



Fig. 162. 



tubes of exquisite delicacy ; in other cases they present a beaded or vesi- 

 cular structure ; and in many insects they are dilated at intervals into 

 capacious cells or receptacles, wherein air is retained in great abundance. 

 The beautiful figure given in the preceding page (fig. 161), taken from 

 Straus-Durckheim's elaborate work upon the anatomy of the Cock- 

 chafer, will illustrate this arrangement. The spiracles, situated at the 

 points respectively marked by the letters a, b, c, d, e, f, g, Ti, i, open into 

 two wide air-trunks, disposed longitudinally along the whole length of 

 the body : from these, innumerable secondary branches are given off, 

 many of them being seen to dilate into oval vesicles, from which smaller 

 tracheae proceed; while others, without any vesicular enlargement, 

 plunge at once into different textures, and supply the viscera and 

 internal organs. The muscular system, the legs, the wings, the ali- 

 mentary canal, and even the brain itself, are permeated in all directions 

 by these air- conducting tubes ; and thus the oxygen penetrates to every 

 corner of the body. 



(844.) There is one circumstance connected with the tracheae which 

 is specially deserving of admiration, whether we consider the obvious 

 design of the contrivance, or the 

 remarkable beauty of the struc- 

 ture employed. It is evident that 

 the sides of canals so slender and 

 delicate as the tracheao of insects 

 would inevitably collapse and fall 

 together, so as to obstruct the 

 passage of the air they are de- 

 stined to convey; and the only 

 plan which would seem calculated 

 to obviate this would appear to 

 be, to make their walls stiff and 

 inflexible. Inflexibility and stiff- 

 ness, however, would never do in 

 this case, where the vessels in 

 question have to be distributed 

 in countless ramifications through 

 so many soft and distensible vis- 

 cera ; and the problem therefore 

 is, how to maintain them perma- 

 nently open, in spite of external 

 pressure, and still preserve the 

 perfect pliancy and softness of their walls. The mode in which this 

 is effected is as follows : Between the two thin layers of which each 

 air-vessel consists, an elastic spiral thread is interposed (fig. 162, a), so 

 as to form by its revolutions a firm cylinder of sufficient strength to 

 ensure the calibre of the vessel from being diminished, but not at all 



Trachea! tube of an insect, highly magnified, 

 showing, at a, the elastic spiral thread. 



