EXTERNAL FRAMEWORK OF INSECTS. 297 



vigorous and active description, and a correspondent strength and firm- 

 ness in the skeleton upon which the muscles act. It is sufficient 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 ener- 

 getic as would be indispensable to wield the instruments used in flying, 

 or raise the body above the surface of the ground. 



(771.) Similar changes, therefore, to those which we found requisite 

 in order to convert the aquatic Annelid 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 reduced ; 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 gathered into masses of increased power, suffi- 

 cient to animate the more vigorous muscles with which they are in 

 relation. 



(772.) Such changes are precisely those which are most remarkable 

 when we compare the external appearance of a centipede with that of 

 a winged insect : the entire number of segments, and consequently 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 con- 

 nected by a moveable joint. The three anterior segments 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 rings only, the organs of locomotion are ap- 

 pended. The remaining segments of the body are much less firm in 

 their texture, especially in insects with hard or horny wing-covers, in 

 which indeed they are almost of a membranous consistence, so as to 

 increase, as far as possible, the lightness of the animal in parts where 

 strength is not required. Here, then, is an annulose skeleton adapted 

 to flight ; dense and unyielding where support is required for the at- 

 tachment of the locomotive organs, but thin and flexible elsewhere. 



(773.) The above conditions being required in the arrangement of 

 the pieces which compose the outward framework of the body in insects, 

 we may easily conceive that the mode of union between the various 

 segments above described is by no means a matter of indifference, inas- 

 much as very different degrees of motion are required between the in- 

 dividual rings. In the Annelida and Myriapoda a very simple kind of 



