uf the Mule-cricket. 421 



parts on which they are respectively distributed ; but as no 

 adequate object, nor indeed any object of importance, would 

 be gained by the description of a distribution which is not 

 marked by any physiological peculiarity, I shall only insist on 

 such points as appear to me to be either new or hitherto not 

 sufficiently elucidated. 



The tracheae of insects are generally described as tubes 

 constructed of a spiral thread, the successive coils of which 

 are closely in opposition with each other : such a structure is 

 represented in Swammerdam's plates, and I have no doubt 

 from his acknowledged accuracy that he represents what he 

 observed. It has not however happened to me, with the ex- 

 ception of one equivocal instance, to perceive such a structure 

 in the mole-cricket, the character of the tracheae of which va- 

 ries in different parts of the insect ; for sometimes they resem- 

 ble the pulmonary tracheae of the higher classes of animals, in 

 having an annulated structure ; and sometimes they appear as 

 tubes of a perfectly uniform substance like cuticle, or some 

 very thin and unorganized membrane. It is generally under- 

 stood that the tracheae of insects penetrate each organ and 

 every part of the body ; and certainly the case is such in the 

 instance before us. Thus, in that brush of capillary yellow 

 tubes supposed to constitute the hepatic system, the total num- 

 ber of which amounts to 150 or 200, there is reason to believe 

 that each tube is accompanied by a distinct trachea coiled 

 round it in a long spiral. Again ; the two medullary cords 

 which connect the several ganglions of the nervous system, 

 are in their natural state united together by means of the 

 branches of a tracheal tube which runs between them, a simi- 

 lar tube being attached to the exterior edge of the cords ; and 

 the surface of what may be called the brain of this insect, is as 

 bea,utifully characterized by the ramifications of the tracheae 

 which pervade it, as the surface of the pia mater of the human 

 brain by tlie blood-vessels which penetrate that membrane in 

 evei-y direction. 



In meditating on the difficult problem of the sanguineous 

 circulation of insects, it has forcibly occurred to me that the 

 tracheae may possibly be the instruments of such a circula- 

 tion, absorbing the blood or the chyle in the first instance 

 from the internal surface of the alimentary canal, and thence 

 conveying it to the various parts of the body; nor is this opi- 

 nion, however improbable it may appear, entirely gratuitous. 

 >4o difficulty, I apprehend, attaches to the supposition that 

 such an absorption may take place, seeing that innumerable 

 minute ramifications of the trachea? penetrate the intestinal 

 canal in every part : nor does there seem any difficulty in ad- 

 mitting 



