158 ANATOMY. 



connection lower down, in the Chapter where we speak of the sexual 

 organs ; but we must defer hinting at their hypothetical use, as well as 

 of the doctrine of a circulating system in insects, until the following 

 division, to which we consequently refer. 



122. 



IV. OF THE ORGANS OF RESPIRATION. 



We shall find the respiratory organs of insects as complex and per- 

 fectly developed, as we have found their blood-vessels simple and 

 imperfect. The relations between these systems appear to be in them 

 completely reversed, for the air-vessels intersect the insect body as 

 multitudinously as we find the blood-vessels do in the superior animals. 

 We cannot here show whence this transposition of the usual relations 

 proceeds, nor how an entirely different structure can produce a similar 

 result, this belongs to Physiology ; we are here required merely to 

 explain the structure and distribution of the air-vessels, and their 

 external orifices. Our subject thence divides itself into two portions ; 

 the first of which treats of the exterior organs attached to the respira- 

 tory organs ; and in the second, we shall describe the internal air- 

 vessels themselves. 



123. 



A. Exterior Organs of Respiration. 



The exterior organs of respiration which are found upon the surface 

 of the body, are of a triple character, namely, SPIRACLES, AIR TUBES, 

 and BRANCHI^:. The first are easily distinguished from the last, by 

 the presence of an orifice that opens directly into the tracheae, whereas 

 the branchiae are membranous leaves, throughout which tracheae are 

 dispersed, without opening anywhere. 



I. The SPIRACLES (spiracula, stigmata), which are the most fre- 

 quently found of all the exterior organs of respiration, appear as 

 incisions or small round openings at the sides of the segments of the 

 body, which are sometimes surrounded by a peculiar oval horny ring ; 

 or are encircled by merely the usual integument of the body, without 

 any apparent distinction. Both kinds of structure are supplied with a 

 muscular apparatus which opens and closes the aperture, so that the 

 insect can either open it to receive air, or close it against it. We shall 

 proceed with a description of their various forms, after this short indi- 

 cation of their differences. 



