MOTION. 
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 arc 
endowed also with a surplus of force beyond 
what is necessary for locomotion, and which is 
peered to the various purposes of life. 
aving 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. 
Secr. II. Flying.—Flying depends on the 
power which animals pos 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 
fulerum 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 
419 
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 trachee and air cavities, 
greatly tend to diminish their specific gravity 
and the quantity of muscular action em- 
presen during their flight. The form of their 
odies 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 neure 
when injected with air and fluid assist in giving 
expansion and tension to the wings; an office 
compared by Jurine to the support given toa 
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 Diptera, 
Coleoptera, and diurnal Sphinges it is ellip- 
soidal. The figure of the wings varies, how- 
2E2 
