strike the air. But in the bird the wing plays also a passive role, i. e., it receives the pressure of the air on its under side when the bird is projected rapidly onward by its acquired swiftness. In these conditions the whole animal is carried onward in space; all the points of its wing have the same velocity. The neighboring regions of the body are useful to press upon the air, which acts as on a paper kite. The base of the wing also, in the bird, is broad, and provided with feathers, which form a broad surface, on which the air presses with a force and method very efficacious in supporting the bird. Fig. 12 gives an idea of this disposition of the wing at the active and passive time in a bird.

11. Trajectory of an insect's wing.

12. Trajectory of a bird's wing.

The inner half of the wing is the passive part of the organ, while the external half, that which strikes the air, is the active part. A fly's wing makes 330 revolutions in a second, executing consequently 660 simple oscillations; it ought at each time to impress a lateral deviation of the body of the insect, and destroy the velocity that the preceding oscillation has given it in a contrary direction. So that by this hypothesis the insect in its flight only utilizes fifty to one hundred parts (or one-half) of the resistance that the air furnishes it.

13. A bird on the wing.

In the bird (Fig. 13), at the time of lowering the wings, the oblique plane which strikes the air, in decomposing the resistance, produces a vertical component which resists the weight of the body, and a horizontal component which imparts swiftness. The horizontal component is not lost, but is utilized during the rise of the wing, as in a paper kite when held in the air against the wind. Thus the bird utilizes seventy-five out of one hundred parts of the resistance that the air furnishes. The style of flight of birds is, therefore, theoretically superior to that of insects. As to the division of the muscular force between the resistance of the air and the mass of the body of the bird, we should compare the exertion made in walking on sand, for example, as compared with walking on marble. This is easy to measure. When a fish strikes the water with its tail to propel itself forward, it performs a double task; one part consists in pushing backwards a certain mass of water with a certain swiftness, and the other in pushing on the body in spite of the resistance of the surrounding fluid. This last portion of the task only is utilized. It would be greater if the tail of the fish encountered a solid object. Almost all the propelling agencies employed in navigation undergo this loss of labor, which depends on the mobility of the point d' appui. The bird is placed among conditions especially unfavorable.

The Senses of Insects. The eyes of insects are sometimes so large as to envelop the head like an Elizabethan ruffle, and the creature's head, as in the common house fly, seems all eyes. And this is almost literally the case, as the two great staring eyes that almost meet on the top of the head to form one, are made up of myriads of simple eyes. Each facet or simple eye is provided with a nerve filament which branches off from the main optic nerve, so that but one impression of the object perceived is conveyed to the brain; though it is taught by some that objects appear not only double but a thousand times multiplied. But we should remember that with our two eyes we see double only when the brain is diseased. Besides the large ordinary compound eyes, many insects possess small, simple eyes, like those of the spider. The great German anatomist, Johannes Müller, believed that the compound eyes were adapted for the perception of distant objects, while those nearer are seen by the simple eyes. But it may be objected to this view that the spiders, which have only simple eyes, apparently see both near and remote objects as well as insects.

14. a Larva, b chrysalis of a butterfly.

The sense of touch is diffused all over the body. As in the hairs of the head and face of man, those of insects are delicate tactile organs; and on the antennæ and legs (insects depending on this sense rather than that of sight) these appendages are covered with exquisitely fine hairs. It is thought by some that the senses of hearing and smell are lodged in the antennæ, these organs thus combining the sense of feeling with those of hearing and smelling. And the researches of anatomists lend much probability to the assertion, since little pits just under the skin are found, and even sometimes provided with grains of sand in the so-called ear of the lobster, etc., corresponding to the ear bones of the higher animals, the pits being connected with nerves leading to the brain. We have detected similar pits in the under side of the palpi of the Perla. It seems not improbable that these are organs of smell, and placed in that part of the appendage nearest the mouth, so as to enable the insect to select its proper food by its odor. Similar organs exist on the caudal appendages of a kind of fly (Chrysopila), while the long, many-jointed caudal filaments of the cockroach are each provided with nearly a hundred of these little pits, which seem to be so many noses. Thus Lespès, a Swiss anatomist, in his remarks on the auditory sacs, which he says are found in the antennæ of nearly all insects, declares that as we have in insects compound eyes, so we have compound ears. We might add that in the abdominal appendage of the cockroach we have a compound nose, while in the feelers of the Perla, and the caudal appendage of the Chrysopila, the "nose" is simple. We might also refer here to Siebold's discovery of ears at the base of the abdomen of some, and in the forelegs of other kinds, of grasshoppers. Thus we need not be surprised at finding ears and noses scattered, as it were, sometimes almost wantonly over the bodies of insects (in many worms the eyes are found all over the body), while in man and his allies, from the monkey down to the fish, the ears and nose invariably retain the same relative place in the head.

How Insects Grow. When beginning our entomological studies no fact seemed more astonishing to our boyish mind than the thought that the little flies and midges were not the sons and daughters of the big ones. If every farmer and gardener knew this single fact it would be worth their while. The words larva and pupa will frequently occur in subsequent pages, and they should be explained. The caterpillar (Fig. 14, a) represents the earliest stage or babyhood of the butterfly, and it is called larva, from the Latin, meaning a mask, because it was thought by the ancients to mask the form of the adult butterfly.

When the caterpillar has ended its riotous life, for its appetite almost transforms its being into the very incarnation of gluttony, it suddenly, as if repenting of its former life as