1NSECTA. 265 



state; for in tlie larva and pupa condition, where flight is not 

 possible, various additional organs, frequently of considerable bulk, 

 are provided, that we shall speak of in another place. If we 

 examine the external skeleton of any large insect, a beetle for ex- 

 ample, we shall find between the individual segments of the body 

 minute apertures or pores (spiracles) through which the air is 

 freely admitted ; these openings, ten in number, on each side of 

 the body, are situated in the soft membrane interposed between 

 the different rings, and not in the rings themselves, a provision for 

 the purpose of allowing their orifices to be opened or closed at 

 pleasure, instead of being rigid and motionless. The margin of the 

 spiracle is frequently encompassed by thick horny lips, which may 

 be approximated by muscles provided for the purpose, so that the 

 opening can be shut at pleasure, in order to exclude any extraneous 

 substances that might otherwise obtain admission : in many insects 

 indeed, especially in beetles which crawl upon the dusty ground, 

 an additional provision is necessary to prevent the entrance of foreign 

 matter, and in such cases the spiracles are seen to be covered with 

 a dense investment of minute and stiff hairs, so disposed as to 

 form a sieve of exquisite fineness ; a beautiful contrivance, by which 

 the air is filtered, as it were, before it is allowed to pass into 

 the breathing-tubes, and thus freed from all prejudicial particles. 

 From every spiracle is derived a set of extremely delicate tubes 

 (trachea), that pass internally, and become divided and subdi- 

 vided to an indefinite extent, penetrating to every part of the 

 body, and ramifying through all the viscera, so that air is thus 

 supplied to the entire system. Upon more minutely inspecting 

 these air-tubes, they are found to assume various forms in different 

 parts of the body, being sometimes simple tubes of exquisite deli- 

 cacy ; in other cases they present a beaded or vesicular structure, 

 and in many insects they are dilated at intervals into capacious cells 

 or receptacles, wherein air is retained in great abundance. The figure 

 in the following page (Jig. 118), taken from Strauss Durckheim's 

 elaborate work upon the anatomy of the cockchafer, will illustrate 

 this arrangement. The spiracles, situated at the points respectively 

 marked by the letters a, c, e?, e,jf, g, h, 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 en- 

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



