180 SELECTED NOTES. 



both afferent and efferent ; but, judging from the observations of 

 others, as well as my own, I must confess that I am somewhat 

 sceptical on the subject, and it is with the hope of having the 

 matter thoroughly discussed that I have placed the slide in the box. 



The minuteness of the opening of the abdominal spiracles and 

 the almost immediate branching of the large tracheal vessels are 

 eminently suited for the exclusion of dust particles ; and the quick 

 distribution of inflowing air, to which may be added the adaptabi- 

 lity of the abdomen for rhythmical expansion and contraction, 

 leaves no doubt that the supply is obtained through these spiracles. 

 To be able only to assert that the tracheae branch and re-branch 

 until they end in mere blind twigs, and that the air is constantly 

 being changed in them, without being able to discover the cause, 

 is, to say the least, unsatisfactory. 



By mounting fresh insect muscle in strong glycerine, I have 

 succeeded for a time in retaining air in the smallest tracheal vessels, 

 and believe that I have traced a connection through them to and 

 from the larger tracheae. My difficulty in following them was so 

 great and perhaps uncertain that I should be glad to have my con- 

 jectures confirmed by more able observers before it can safely be 

 stated as a fact. 



On the other hand, the thoracic spiracles in the Blow-fly and 

 in many other insects are large and open outwards, so that there 

 is little or no protection against the entrance of dust particles ; the 

 various parts also of the thorax are so firmly soldered together as 

 to make it almost rigid. These spiracles appear to me to be pur- 

 posely and peculiarly adapted for carrying off the expired air and 

 vapour from the body. 



There is yet another purpose which these spiracles may pro- 

 bably serve — namely, the production of sound. The well-known 

 buzzing of the fly can be varied in tone or may cease altogetfier, 

 as circumstances require. Is it not conceivable that by varying the 

 size of the opening of the valve of these spiracles, and thereby 

 increasing or diminishing the pressure in the large air-tubes con- 

 nected with them, such variations of sound may be accounted for? 

 When the wings and halteres of the fly are removed ,the sounds 

 produced are as loud as before removal, and the muscular exertion 

 is always great when sounds are produced. 



