and Dilated Trachece in Insects. 421 



and elongated, and mainly assist in inducing a rush of blood into these struc- 

 tures, which are thus expanded on the sides of the new pupa as the rudi- 

 mentary wings. This elongation of the small trunks at the sides of the lon- 

 gitudinal tracheae in the thorax, relieves them of a portion of that tension 

 which results from the powerful respiratory efforts of the insect in effecting 

 its change ; and which, with a tendency to enlarge by the natural forces of 

 growth in these structures, results in the dilatation first of the tracheae at 

 the base of the abdomen, and afterwards of those of the thorax, and the sides 

 of the abdomen, into distinct sacs. This is the manner in which the air-sacs 

 are formed in all insects. After the main trunks have become dilated their 

 ramifications also are enlarged in like manner, and this enlargement con- 

 tinues from the time when the insect enters its pupa, to that of its appearance 

 in the perfect state. 



But although we are enabled to show the manner in which the vesicles are 

 formed, it is difficult to give direct proof of their immediate use. It is assumed 

 from the facts I have mentioned, that they are most numerous and large in 

 volant insects, and entirely absent in apterous, that they are connected with 

 the power of flight ; but in what way, has never yet, so far as I am aware, been 

 actually proved. It was assumed by John Hunter that they are for the pur- 

 pose of enabling the insect to alter the specific gravity of its body, — a view 

 which I have myself long ago adopted, — but the facts on which the opinion is 

 founded are only those of anatomy, and not of direct observation or experi- 

 ment, which are required to confirm it. Anatomical facts alone, as will be 

 seen in this instance, although the safest guides, are not always sufficient. 

 Thus we find that a vesicular structure of the organs of respiration exists in 

 the whole of the air-breathing Vertebrata. The lungs of Man and the Mam- 

 malia are formed of multitudes of vesicles, which are more numerous than in 

 any other class ; those of Birds exhibit a like condition, but in these the vesi- 

 cles are distributed more extensively over the whole body. A vesicular con- 

 dition of the respiratory organs exists also in the Reptiles and Amphibia, but 

 far less extensively than in Birds. Thus each of these classes agrees with in- 

 sects in possessing a vesicular form of the respiratory organs. The whole of 

 them also, excepting the Mammalia, agree in the fact that the vesicles are 

 parietal, that the tracheal structures are dilated in different parts of their 



