for the introduction of air into the chrysalis, a state in which the animals have the appearance of being almost totally dead, and, of course, would seem to have little use for respiration. It is likewise discovered that these horns, which pierced the hard exterior covering, terminate in as many trachea in the body of the animal. This fact affords a strong example of the necessity of air for sustaining the principle of life, even in its lowest condition. After these animals pass from the chrysalis state to that of flies, they are deprived both of their tails and horns. Nature, in this last stage of their existence, has not left them without proper resources for the introduction of air into their bodies. Instead of protuberant trachea in the form of tails or horns, they now, like other flies, receive air by means of stigmata, or holes, variously disposed over different parts of the body.

But

'The nymphs of the libellula, or dragon-fly, respire water by an aperture at the termination of their bodies. These nymphs sometimes throw out the water, at certain intervals, with such force that the stream is perceptible at the distance of two or three inches from their bodies. When kept some time out of the water, the desire or necessity of respiration is augmented, and accordingly, when replaced in it, inspirations and expirations are repeated with unusual force and frequency. If you hold one of these nymphs in your hand, and apply drops of water to the posterior end of its body, it instantly, by an apparatus similar to the piston of a pump, sucks it in, and the dimensions of its body are visibly augmented. This water is again quickly thrown out by the same instrument. But though this insect respires water, air seems to be not the less necessary to its existence; for, like other insects, the whole interior part of its body is amply provided with large and convoluted tracheæ; and, externally, there are several stigmata destined for the introduction of air.

'The worms and nymphs of the ephemeron flies merit attention. They have received the denomination of ephemeron because very few of them survive the day in which they are transformed into flies. Many of them live not one hour after their transformation. When in the worm and nymph states, they generally live in holes near the surface of the water; and under

these two forms, continue to grow till they are mature for passing into the last and shortest period of their existence. Swammerdam informs us that some of them remain three years under water, others two, and others one only.

'On each side of their bodies there are six or seven protu berances, which have the appearance of so many oars. With these instruments the animals describe arches in the water, first on one side, and then on the other, with astonishing rapidity. This circumstance led Clutius, and some other authors, to think that these protuberances were fins, or instruments of motion, and that the animals were fishes. But Réaumur remarked, that they moved these fins with the same rapidity when the animals were at rest as when they were in motion; and that, instead of fins, when examined by the microscope, he discovered them to be gills through which the creatures respire. Each gill consists of a short trunk, and two large branches or tubes, which give off on all sides a number of smaller ramifications, and are perfectly similar to the trachea of other insects. At the origin of every gill, two trachea penetrate the trunk, and are dispersed through the body of the animal.'

The Crustacea, the Worms, and Mollusca, respire by means of gills, which, although they differ in some measure from those of fishes, are formed upon the same plan. In a few instances they respire air by itself, but in general through the medium of water alone. In some animals of these classes the gills are situated upon the outside of their bodies, but commonly within. Many of the Radiata have no distinct organs for respiration; yet the air seems, in some way or other, absolutely necessary for their existence also, and probably penetrates their bodies, and acts upon their blood by means entirely unknown. These animals are all cold-blooded.

CHAPTER III. (W.)

ON THE CONNECTION OF ANIMALS WITH HEAT, LIGHT, AND ELECTRICITY.

THE power possessed by animals of maintaining a temperature independent of the medium in which they live, has been already referred to. A certain degree of this power is probably universal, but it varies much in different classes, being least in the lower and greatest in the higher. In its most limited degree, it is shown by the fact that the animal resists the influence of cold and is not frozen so speedily as the substances around it, or as it would be itself after being deprived of life. But in the lower species, vitality is not destroyed even by freezing, as it is, for the most part, in the higher. In Fishes the temperature is usually that of the water in which they reside, except when this falls very low. It is then maintained at a few degrees above it. A few species, distinguished by unusual muscular activity, are capable of a still greater heat as compared with that of the water; even ten or twelve degrees. Some in the Arctic regions are known to have retained life even after having been imbedded for some time in ice.

In Reptiles the power of resisting the influence of cold is somewhat greater. The frog maintains a temperature of between thirty and forty degrees, when enveloped in ice reduced some degrees below the freezing point. In such cases the animal is surrounded by a thin covering of water, which the heat of its body prevents from being frozen.

The bodies of Insects are so small that it is difficult to observe their temperature accurately, except where they can be collected together in large quantities. There are some facts which seem to show that they have considerable power in this respect. From observations upon the temperature of bees in winter, it has been found that, when the inhabitants of a hive are in a quiescent state, their degree of heat does not vary much from that of the air. If, however, they are roused and become active, it rises immedi

ately to seventy or eighty degrees, and this continues as long as the state of activity continues. This increase is probably owing to the greater energy of respiration and circulation attendant upon muscular action.

The circumstances according to which the temperature of insects varies, are still not perfectly understood. In the common caterpillar, for example, its increase of heat is not always connected with increased activity of function. While in an inactive condition, their temperature will sometimes rise above that of the air, though usually it is the same.* Many more and minute observations are necessary, before this subject can be fully comprehended.

The power of producing and keeping up the heat of animals, is closely connected with their life and health. This is particularly the case with the warm-blooded. This power is least vigorous at birth, and increases with age, till it is most perfect in the adult. It then again diminishes, and is deficient in old age. In order to prevent the disastrous effects of cold, animals in a state of nature usually produce their young in spring and summer, and in our own species, when this is not the case, with all the precautions taken, the mortality of very young infants is greater during the cold months than the warm. When life is not directly destroyed by causes diminishing the heat of the body, their continued influence depresses the vital powers and injures the constitution, laying the foundation for future disease. The idea, so prevalent, that children may be hardened by exposure, and by dressing as thinly as possible, is most pernicious,

* In examining the temperature of the common caterpillars which infest the apple-tree, it was found that at 6 A. M., the sun being bright, when the thermometer in the shade stood at 44° and in the sun at 63°, if introduced into a large mass of them collected together on the outside of their nest it rose to 87°; at 1 P. M., the sun being obscured, and the air being at 54°, the thermometer, within the same nest, into which the animals had retired, rose to 70°: at 4 P. M., the air at the same degree, the temperature ❤ithin was only 65°. On several other days the result was mainly the same.

case were in a state of complete inactivity.

The animals in each

So far as such limited observations suggest any conclusion, they indicate that the generation of heat is greatest in the cool of the morning, while the animal is in a state of inactivity, but has crawled abroad to bask in the sun, and declines gradually in the course of the day.

with regard to their physical management. Those who have a robust constitution and an originally strong heat-making capacity, survive the ordeal, and the vigor which is natural to them is regarded as the result of the discipline to which they have been subjected. As a general rule, infants and children require to be more warmly clothed than adults, because not only is their heatmaking power less, but their bodies, being smaller, are more rapidly cooled.

The heat-making power also varies with climate and season. It is greater in cold climates than in warm, and in winter than in summer. Repeated exposure to cold is best borne by those who, in the intervals of exposure, avail themselves of artificial heat; and those bear the cold best, who go into it well warmed and clothed. It is a mistake to suppose that those who live in houses insufficiently heated are best able to bear exposure abroad. This truth is well understood by the inhabitants of very cold climates. We are informed by Dr. Kane that the Esquimaux, who are frequently exposed to a temperature fifty or more degrees below zero, live in cabins whose heat is raised to more than ninety degrees. From this they go abroad at once into the external air, and are able to endure an almost incredible amount of hardship and cold.

By repeated exposure to cold, particularly when the body is not well warmed in the intervals, and under unfavorable circumstances as to clothing and food, the power of generating heat is diminished, and a person suffers from a less amount of exposure. Hence, the diseases produced by cold are more prevalent toward the latter part of winter and in spring, than in the autumn and early winter months. More protection from clothing is necessary to guard against injury in the former season than in the latter.

The power of generating heat is probably at its minimum at the beginning of summer, having been exhausted by the continued demand made upon it during the cold months. It accumulates during the summer, probably because the demand for it is so small, and is at its maximum in the beginning of winter. Hence it is safer, so far as health is concerned, to defer precautions against cold in autumn, both in our houses and clothing,