Feb. 3, 1888.] 



SCIENTIFIC NEWS. 



IC5 



WILL MEN EVER FLY?— III. 



(Concluded Jrom p. 82.) 



BETWEEN the structure of the bird and that of the 

 insect we cannot but expect to find many important 

 differences. The one is a vertebrate animal, the other an 

 invertebrate. The one breathes by means of lungs, the 

 •other by means of air tubes which permeate the whole of 

 the body. The one possesses a brain and a heart, the 

 ■other possesses neither. And thus the frames of the two 

 beings are radically distinct in character, and comparison 

 between them, excepting as regards the broad principles 

 involved, is altogether impossible. 



These principles, however, are absolutely identical in 

 both cases. The strength of muscle and rigidity of 

 muscle-attachment, the provision of an increased air- 

 supply, and the removal of every unnecessary particle of 

 material, are requirements as carefully fulfilled in the 

 ansect as in the bird. Again, we find power of flight the 



the wings, which are again elevated by the flattening of 

 the back and lengthening of the thorax, due partly to its- 

 own elasticity and partly to the action of the lateral 

 thoracic muscles, which are verticle in their direction." 



Whatever the mode of their action, however, the 

 strength of these muscles must naturally be very great, 

 seeing that flight must sometimes be continuously main- 

 tained for several successive hours (as in the case of the 

 dragon-fly), and that the motion of the wings meanwhile 

 must be almost incredibly rapid. And the greater por- 

 tion of the interior of the thorax is, in fact, occupied by the 

 flight muscles. Bones, of course, being altogether 

 wanting, these vessels are attached to the inner surface 

 of the skin itseHf^vhich, firm and leathery as it is, forms, 

 in fact, a kind of external skeleton, as well as a complete 

 suit of natural armour. 



The two requirements of air-supply and lightness ot 

 frame in the insect go together ; and few things in the 

 entire range of comparative anatomy are more striking 



^r3 



I'iG. 3. — Breathing Organs of ax Iksect. 



•one paramount necessity, and all else subservient to it. 

 Again, we find every detail of the structure modified in 

 accordance with the special functions required. And in 

 the insect, just as in the bat and the bird, we notice that 

 total dissimilarity to all allied groups which is the best 

 possible evidence of man's utter impotence, future as 

 well as present, to traverse the realms of air. 



Upon the wings of insects we need not dwell, for the 

 reason already given in the case of the bat — namely, that 

 in man, could he ever sufficiently overcome his physical 

 disabilities for aerial locomotion, their place would be 

 taken by artificial substitutes. Upon their motive power, 

 however, a few words are necessary. 



For, contrary to what we might reasonably infer from 

 analogy, the muscles of flight do not operate upon the 

 wings themselves at all, but upon the thorax, from the 

 iides of which they proceed. In the words of Mr. B. T. 

 Lowne, " the wings are so attached that the flank forms 

 a kind of fulcrum, upon which they are elevated or 

 depressed by everj"^ alteration in the convexity of the 

 iback. The great longitudinal dorsal muscles, by 

 shortening the thorax, increase its convexity and depress 



than the manner in which almost all the necessary 

 modifications of structure are made to serve a double 

 purpose. 



A very important illustration of this fact we find in the 

 character of the respiratory system. 



As already stated, insects possess no lungs, the place 

 of which is taken by a series of breathing-tubes permeating 

 the whole of the body. Briefly described, these tubes 

 are found in the form, firstly, of a parent-trunk (A, B, in 

 fig. 3), so to speak, running along either side of the body, 

 and communicating with a number of spiracles, or breath- 

 ing-holes, which are generally situated at intervals upon 

 the median line ; secondly, of a number of branches and 

 sub-branches proceeding from these main-tubes, which 

 ramify, and ramify, and ramify again, until every part 

 of the body is traversed by them, even to the antenna; and 

 the claws. 



Now, these tubes, besides permitting a most complete 

 and incessant aeration of the blood, manifestly reduce the 

 bodily weight very considerably indeed. In the first 

 place, they obviate the necessity for localised lungs, which 

 are somewhat weighty organs ; in the second, they purifv 



