The immediate cause of torpidity in reptiles has been ascertained with more precision, than in the animals be longing to the higher classes with warm blood. This con dition with them, does not depend on the state of the heart, the lungs, or the brain ; for these different organs have been removed by Spallanzani, and stiil the animals became torpid, and recovered according to circumstances. Even after the blood had been withdrawn from frogs and sala manders, they exhibited the same symptoms of torpidity as if the body had been entire, and all the organs capable of action.
Cold, with these animals, is evidently the chief cause of their torpidity, acting on a frame ex'remely sensible to its impressions. During the continuance of a high tempera ture, these animals remain active an lively ; but when the temperature is reduced towards 40°, they become torpid, and in this condition, if placed in a situation when the temperature continues low, will remain torpid for au un known period of time. Spallanzani kept frogs, salaman ders, and snakes, in a torpid state ID an ice-house, where they remained three years and a half, and readily revived when again exposed to the influence of a warm atmos phere. These experiments give countenance' to those reports in daily circulation of toads being found enclosed in stones. These animals may have entered a deep crevice of the rock, and during their torpidity been covered with sand, which has afterwards concreted around them. Thus removed from the influence of the heat of spring or sum mer, and in a place where the temperature continued be low the point at which they revive, it is impossible to fix limits to the period during which they may remain in this dormant state.
Since reptiles are easily acted upon by a cold atmos phere, we find but few of' those animals distributed in the cold countries of the globe ; while in those countries whose temperature is always high, these animals are found of vast size, and of many different kinds, and in great num bers.
The torpidity of the Mollusca has not been studied with care. Those which are naked and reside on the !add, re tire to holes in the earth, under the roots of trees, or among moss, and there screen themselves from sudden changes of temperature. The different kinds of land Testacea, such as those belonging to the genera Helix, Bulimus, and Pupa, not only retire to crevices of rocks and other hiding places, but they form an operculuin or lid for the mouth of the shell, by. which they adhere to the rock, and at the same time close up even all access to the air. if they be brought into a warm temperature, and a little moisture be added, they speedily revive. In the case of the Helix nemoralis, the operculum falls off when the animal revives, and a new one is formed when it returns again to its slum bers. The first formed opercula contain a considerable portion of carbonat of lime, which is found in smaller quantity in the later formed ones. If the animal has revived frequently during the winter, the last formed oper cola consist entirely of animal matter, and are very thin. The winter lid of the Helix nomatia resembles a piece of card.
All the land testacea appear to have the power of be coming torpid at pleasure, and independent of any altera tions of temperature. Thus, even in midsummer, if we place in a box specimens of the Helix hortensis, nemoralis or arbustorum, without food, in a day or two they form for themselves a thin operculnm, attach themselves to the side of the box, and remain in this dormant state. They may be kept in this state for several years. No ordinary change of temperature produces any effect upon them, but they speedily revive if plunged in water. Even in their natural haunts, they are often found in this state dur ing the summer season, when there is a continued drought. With the first shower, however, they recover, and move about, and at this time the conchologist ought to be on the alert.
The SPIDERS pass the winter season in a dormant state, enclosed in their own webs, and placed in some concealed corner. Like the torpid mammalia, they speedily revive when exposed to intense cold, and strive to obtain a more sheltered spot.
Many INSECTS are destined to survive the winter months, become regularly torpid by a cold exceeding 40°. The common honey bee, in a small hive, when reduced to this temperature, loses all power of motion, but may be easily revived by an increase of temperature. Whcn the hive is large, there is always as much heat generated, as to protect them against this lethargic disposition. The house fly may always be found in the winter season torpid, in some retired corner ; but exposure for a few minutes to the influence of a fire recalls it to activity. Even same of the lepidopterous insects, which have been hatched too late in the season to enable them to perform the business of procreation, possess the faculty of becoming torpid dur ing the winter, and thus have their life prolonged beyond the ordinary period. These insects can all be preserved from becoming torpid by being placed in au agreeable temperature, as the following experiments of Mr Gough's (Nicholson's Journal, vol. xix.) abundantly testify. In speaking of the Hearth Cricket, (Gryllus domesticus.) he says, " Those who have attended to the manners of this familiar insect will know that it passes the hottest part of the summer in sunny situations, concealed in the crevices of walls and heaps of rubbish. It quits its summer abode about the end of August, and fixes its residence by the fireside of the kitchen or cottage ; where it multiplies its species, and is as merry at Christmas as other insects are in the dog-days" Thus do the comforts of a warm hearth afford the cricket a safe refuge, not from death, but from temporary torpidity ; which it can support for a long time, when deprived by accident of artificial warmth. " I came to the kowledge of this fact," he says, " by planting a colony of these insects in a kitchen, where a constant fire is kept through the summer, but which is discontinued from November to June, with the exception of a day, once in six or eight weeks. The crickets were brought from a distance, and let go in this room in the beginning of September 1806: here they increased considerably in the course of two months, but were not heard or seen after tile fire was removed. Their disappearance led me to con clude that the cold had killed them : but in this I was mis taken; for, a brisk fire being kept up for a whole day in the winter, the warmth of it invited my colony flout their hiding place, but not before the evening, after which they continued to skip about and chirp the greater part of the following clay, when they again disappeared ; being com pelled by the returning cold to take refuge in their lormer retreats. They left the chimney corner on the 28th of May 1807, after a fit of very hot weather, and revisited their winter residence on the 31st of August. Here they spent the summer merely, and lie torpid at present (Jan. 1808) in the crevices of the chimney, with the exception of those days on which they are recalled to a temporary ex istence by the comforts of a fire." Nothing is known with regard to the hybernation of the Those which inhabit the bodies of torpid quadrupeds, in all probability, like them, experience a winter lethargy. If they remain active, they must possess the faculty of resisting great alterations of temperature. Among the infizsory animals, numerous instances of sus pended animation have been observed, continued not for a few months, but during the period of twenty-seven years. But such instances of lethargy do not belong to our pre sent subject. Besides, they have been fully discussed under the article ANIDIALCULA, in this work.