INSECTA. 



267 



elastic spiral thread is interposed {Jig. 119, a), so as to form by 

 its revolutions a firm cylinder of sufficient strength to insure the 

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

 fering with its flexibility, or obstructing its movements ; and this 

 fibre, delicate as it is, may be traced with the microscope, even 

 through the utmost ramifications of the tracheae, a character 

 whereby these tubes may be readily distinguished. 



(308.) We must now consider Fi 119> 



the mechanism by which air is per- 

 petually drawn into the body of 

 the insect, and again expelled. If 

 the abdomen of a living insect be 

 carefully watched, it will be found 

 continually performing movements 

 of expansion and contraction that 

 succeed each other at regular in- 

 tervals, varying in frequency, in 

 different species, from twenty to 

 fifty or sixty in a minute,* but 

 occurring more rapidly when the 

 insect is in a state of activity than 

 when at rest. At each expansion 

 of the abdomen, therefore, air is 

 sucked in through all the spiracles, 

 and rushes to every part of the o/^ 

 body ; but, when the abdomen 

 contracts, it is forcibly expelled through the same openings. 

 Burmeister even supposes that the humming noises produced 

 by many insects during their flight, must be referred to the vi- 

 bration caused by the air streaming rapidly in and out of the 

 spiracular orifices. Insects which live in water are obliged, at short 

 intervals, to come to the surface to breathe, at which time they 

 take in a sufficient quantity of air to last them during the period 

 of their immersion ; but if the spiracles are closed by any acci- 

 dent, or by the simple application of any greasy fluid to the ex- 

 terior of their body, speedy death, produced by suffocation, is the 

 inevitable result. 



(309.) A moment's reflection upon the facts above stated, con- 

 cerning the respiration of insects, will suggest other interesting views 

 connected with the physiology of these little creatures. It is evident, 



* Sorg, Disquisitio Phys. circa Resp. Insectorum et Verminum. 



