422 Mr. Newport on the Formation and Use of the Air-sacs 



course, while \n Mammalia this is not the case, the vesicles in them being only 

 at the extremities of the ramifications of the respiratory structures. 



In what way then will the anatomy of the structures lead us to a well- 

 founded inference that is supported by direct observation on the function ? 

 We must compare insects with those animals which approach nearest to them 

 in the function of these structures, — Birds. In Birds the respiratory organs 

 are not only vesicular,- but are more extensively distributed over the whole 

 body than in any other Vertebrata. These, as every anatomist knows, are not 

 confined merely to the great cavities of the body, but are extended to 'every 

 part of the skeleton, as in insects. They communicate directly with the in- 

 terior of the bones of the wings and legs, as the tracheae of the thorax are 

 extended also into these parts in insects. This distribution in both is more 

 extensive and complete in the most active species. In Birds which are unac- 

 customed to flight, as in the Ostrich, as remarked by Mr. Owen*, the com- 

 munications of the respiratory organs with the bones is imperfect ; whilst in 

 Insects, although tracheae exist in all, the vesicles are found only in those of 

 flight. This fact extends even to the sexes of the same species. Thus vesi- 

 cles exist in the male of the common Glow-worm, which is winged, and de- 

 signed to search out the apterous female, in which the respiratory organs are 

 simply tracheal. The like conditions exist in the common winter-moth, Geo- 

 metra brumaria. In the male of this insect I have found the vesicles large 

 and numerous, but not a trace of these occurs in the female. The tracheae in 

 this sex, which has only the rudiments of wings, are larger relatively than in 

 the female Glow-worm, and are precisely in that condition in which I have 

 found them in the Diurnal Lepidoptera shortly before changing to the pupa. 

 These anatomical facts are inferential of the real use of the vesicles, and are 

 supported by an observation which I have been able to make on the common 

 Dung-beetle, Geotrupes stercorarius, at the moment when it is preparing to 

 take flight. A specimen of this insect which had been in confinement for about 

 twenty-four hours, and consequently had not expanded its wings during that 

 time, when placed on a table immediately prepared to escape. After walking 

 away quickly for a short distance it began to respire freely, alternately short- 

 ening and elongating its abdominal segments at the rate of about forty respi- 



* Cyclop, of Anatomy and Physiology, art. "Aves," vol. i. p. 341. 



