INSECT A. 



(79.) Insects, examined generally, differ from all other articu- 

 lated beings in one remarkable circumstance they are capable of 

 flight can maintain themselves in the air by means of wings : it is 

 true, indeed, that some species are met with in all the orders de- 

 scribed above, which are apterous, being destitute of such organs ; 

 but these form exceptions to be noticed hereafter. Such a mode of 

 progression, through so rare a medium as that of the atmosphere, 

 necessarily demands an exercise of muscular power of the most 

 vigorous and active description, and a correspondent strength and 

 firmness in the skeleton upon which the muscles act. It is suffi- 

 cient to cast a glance at the external construction of any of the 

 Annelidans or Myriapoda, which have come under our notice, to be 

 convinced that in such animals flight would be impossible under any 

 circumstances : their long and flexible bodies present no point to 

 which efficient wings could be appended ; neither is any part of their 

 divided skeleton possessed of sufficient strength to support the action 

 of muscles so forcible and energetic as would be indispensable to 

 wield the instruments used in flying, or raise the body above the 

 surface of the ground. 



Similar changes, therefore, to those which we found requisite in 

 order to convert the aquatic Annelide into the terrestrial Myriapod, 

 must be still further carried out before the animals last mentioned 

 could be adapted to become inhabitants of the air. The number of 

 segments composing their elongated bodies must be materially re- 

 duced ; certain parts of the skeleton must be strengthened in order 

 to sustain the efforts of muscles sufficiently strong to raise the 

 weight of the animal ; and, in the last place, the nervous ganglia, 

 by a like concentration of hitherto separated parts, must be gather- 

 ed into masses of increased power sufficient to animate the more 

 vigorous muscles with which they are in relation. 



(280.) Such changes are precisely those which are most remark- 

 able when we compare the external appearance of a centipede with 

 that of a winged insect : the entire number of segments, and conse- 

 quently the proportionate length of the latter, is obviously reduced. 

 The head is seen to be more distinct from the rest of the body, to 

 which it is connected by a moveable joint. The three anterior seg- 

 ments of the trunk become largely developed, and, from the density 

 of their substance, form by far the strongest part of the skeleton, 

 constituting what is called the thorax of the insect ; they are, 

 moreover, generally united together, especially the two posterior, 

 so as to be consolidated, as it were, into one piece ; and to these 



