MOTION. 



419 



Ermann, that the molecules approximate during 

 contraction, the magnetic force would increase 

 at the same time, and the muscular force is 

 observed to decrease, therefore both of these 

 hypotheses are inconsistent with the theory 

 of the identity of the magnetic and muscular 

 forces. 



The power of muscles, however, rapidly 

 decreases by exertion, especially in some of the 

 lower animals, such as the Ophidian and Batra- 

 chian reptiles; the ratio of decrease is in pro- 

 portion to the energy of action, until, by con- 

 tinued exertion, locomotion becomes impos- 

 sible. It has now been sufficiently demonstrated 

 that muscles are capable not only of moving 

 the levers on which they act with great force 

 and velocity, and of prolonging their action 

 for a greater or less period, but that they are 

 endowed also with a surplus of force beyond 

 what is necessary for locomotion, and which is 

 applied to the various purposes of life. 



Having now given a brief statement of the 

 mechanical principles applicable to the loco- 

 motion of animals, we shall proceed to give an 

 outline of their various modes of progression 

 according as it is performed in air, in water, 

 or on solids ; and in the first place, for example, 

 we shall select those that move by means of an 

 aeriform medium. 



SECT. II. Flying. Flying depends on the 

 power which animals possess of raising them- 

 selves in the air, and of moving through it with 

 considerable velocity in every direction. The 

 power of flight is denied to a large proportion 

 of the animal kingdom, and requires for its 

 exercise a certain configuration of body, adjust- 

 ment of parts, and modification of structure, 

 based on the most profound principles of dyna- 

 mics. In flying, as in swimming, the animal 

 moves in a medium which furnishes a suitable 

 fulcrum to its levers or locomotive organs, 

 whatever may be their kind or form. Air sup- 

 plies the medium to animals that fly as water 

 to those that swim. Air, however, being more 

 than eight hundred times lighter than water, 

 gives a proportionably diminished support to 

 the animals which move in it; consequently, 

 instead of having the whole or nearly the whole 

 weight of the body sustained, as when plunged 

 in water, the same animal weighs as much 

 more in air as corresponds to the difference in 

 the specific gravities of the two fluids, or nearly 

 as 1 to 1000. The weight of the volume of 

 air displaced by the equal volume of any 

 insect or bird indicates the amount of buoyancy 

 or force acting vertically upwards, in opposition 

 to the force of gravity on the mass of the body 

 of the animal acting vertically downwards. The 

 difference between the specific gravity of animals 

 and that of the atmosphere represents the 

 weight necessary to be overcome in flying by 

 the action of their locomotive organs ; or, in 

 other words, whatever may be the amount of 

 the force of gravity on the mass of particles 

 composing the whole animal in a direction 

 vertically downwards, and the resistance of 

 the air on its surface due to its velocity, an 

 equal force acting vertically upwards will be 



required to sustain it in the air, and a still 

 greater force to cause it to rise. It is the vast 

 preponderance of the weight of most animals 

 over that of the air they displace, which con- 

 stitutes the chief obstacle to their flight, in 

 addition to their inappropriate form and the 

 unsuitable organization of their locomotive 

 organs. 



flight of insects. The flight of insects 

 depends on the same principles as that of 

 birds, notwithstanding the dissimilar structure 

 of their bodies and wings. The skeleton of 

 insects is both light and dense, and, without 

 greatly adding to their weight, affords the 

 necessary fulcrum for the action of an elaborate 

 muscular system. The mobility of the seg- 

 ments of the abdomen upon the thorax enables 

 the insect to bend upon itself, and to adjust 

 the position of the centre of gravity, with 

 respect to the articulation of the wings during 

 flight. The attachment of the wings to the 

 trunk lies above the centres of magnitude and 

 gravity, so that the insect is kept steady whilst 

 flying. Compared with their volume, the 

 weight of insects is less than that of birds : the 

 lightness of their skeletons and the diffusion 

 through their bodies of tracheae and air cavities, 

 greatly tend to diminish their specific gravity 

 and the quantity of muscular action em- 

 ployed during their flight. The form of their 

 bodies is very irregular, but being for the 

 most part either cylindrical or ellipsoidal, is 

 well adapted to pass through the air with little 

 resistance. 



In the Diptera, the single pair of wings is 

 articulated to the meso-thorax ; in the other 

 orders with two pairs of wings, the first pair 

 is also articulated to the meso-thorax, and the 

 second to the meta-thorax. The wings are 

 composed of a duplicature of the common 

 integuments continued from those parts of the 

 body. In the Diptera and Hymenoptera the 

 ratio of the areas of the wings to their weight 

 is much less than in the Lepidoptera; and as 

 this ratio decreases, the number of the vibrations 

 of the wings in a given time proportionally 

 increases. Hence it is vastly greater in the two 

 former orders than in the latter. The neura 

 when injected with air and fluid assist in giving 

 expansion and tension to the wings ; an office 

 compared by Jurine to the support given to a 

 sail by its cordage. Insects are capable of 

 varying the area of their wings during their 

 elevation and depression, by alternately filling 

 and exhausting the tubes, which movements 

 follow synchronously the expansion and con- 

 traction of the thorax. The muscles which act 

 indirectly on the wings at the same time effect 

 changes in their surfaces, angular inclinations, 

 and ratios of velocity during their ascent and 

 descent. There is an elaborate mechanism pro- 

 vided in the structure of insects relating to their 

 flight. The surfaces of their wings, like those 

 of birds, are in general slightly convex above 

 and concave below. In the Strepsiptera, Or- 

 thoptera, and Hemiptera their figure approaches 

 that of the quadrant of a circle. In the Dipteia, 

 Coleoptera, and diurnal Sphinges it is ellip- 

 soidal. The figure of the wings varies, how- 



2 r. 2 



