Chambers's Edinburgh Journal, No. 425 / Volume 17, New Series, February 21, 1852

by Various

TENACITY OF LIFE IN INSECTS.

However useful insects may be in the general economy of nature, it is but too true that farmers and gardeners often find them a pest, and with each returning summer the pages of agricultural journals abound with remedies, offensive and defensive, against the obnoxious invaders. In such cases, it becomes desirable to know what remedial means are the most efficacious, and we are glad to find that the question has been taken up by persons competent to discuss it. Among these, Dr J. Davy has given the results of his inquiry in a paper, 'On the Effects of certain Agents on Insects,' which has just been published in the Transactions of the Entomological Society, and is well worth reproduction in a condensed form. The experiments were begun in the winter of 1850, the season, as will be remembered, being so mild that insects were readily met with. Their objects were threefold—to test the effects of temperature, of gases, and of vapours. In the former, recourse was had to extremes of heat and cold. A bee placed in a temperature of 32° became at first more active, but the next morning was found torpid, as if dead; a register-thermometer shewing that 25° had been the lowest temperature during the night. Transferred to a temperature of 52°, the bee revived in half an hour, and on the following day exhibited the same results under the same conditions. A fly which, on December 8, was lively on the wing, in a temperature of 52° indoors, was disinclined to move at 40°; and still more so, stirring only when touched, at 33°, but did not become torpid, as in the case of the bee, even at 23°, signs of life being distinctly visible. Several trials made with different species of flies all gave the same result—a remarkable power of sustaining life. The method adopted was to enclose the insects in a glass tube, and place them out of doors all night; and though the tube was frequently covered with frost, they soon revived in the warm temperature of a room. It is perhaps scarcely possible to estimate the degree of cold which insect life will bear without destruction, since many of these creatures survive the terrible winters of the arctic regions. Still, a knowledge of the effects of reduction of temperature will be valuable, as affording data by which to judge of the effects and probable duration of visitations of insects, and of the nature of the precautionary measures to be adopted. In an experiment of alternate temperature from 40° to 65° tried for five days on a bee, the creature at last 'ceased to give any sign of vitality.'

The influence of heat appears to be much more rapid than that of cold: a fly exposed to a temperature of 120°, died in two or three minutes; and 113° proved fatal to another; while a third, placed in a temperature increased gradually to 96°, remained alive for more than an hour. Others bore from 80° to 90° for two hours; and in one instance, a fly survived from 86° to 100° for several hours, but became uneasy with a slight rise, and died at 105°. A bee, taken on March 15, from a temperature of 45°, was exposed to 80° without any apparent diminution of activity; at 90° it ceased to buzz; and at 96°, ceased altogether to move, and did not revive. Although these results are too few to enable us to determine the laws with respect to the influence of temperature on insects, they may serve a purpose, in shewing that the effect is not that gradual one of hybernation, where activity and torpor succeed each other but slowly.

In the series of experiments with gas, it was found that flies placed in carbonic acid gas became instantly motionless, and died if left for any length of time. Some revived after an hour's immersion; others, after two or three hours—the revival being slow in proportion to the time of exposure to the gas. Somewhat similar results were obtained with flies and bees in hydrogen and azote. To try the effect of deprivation, a fly was shut up in a tube with but a small quantity of common air, on the 5th February, in a temperature varying from 52° to 60° during the whole time of the experiment. The insect manifested no uneasiness until the 25th day, and was found dead on the 28th. Another fly, enclosed in a similar tube, with a quantity of air not more than a few times its own volume, became languid on the second day, and motionless on the twelfth, but revived on being taken out.

Flies immersed in oxygen were found dead the second day, with a diminution of the quantity of the gas. Coal-gas produced almost immediate insensibility, with a few feeble attempts at revival, but in no case effectual. Sulphuretted hydrogen also proved especially fatal—an instant's immersion was sufficient to destroy life; though withdrawn at once, not one of the flies recovered. It was the same when the portion of gas diffused in the air of the tube was so minute as to be scarcely appreciable. On bees, too, the effect was similar; the deadly nature of the gas on their delicate organisation being invariably destructive. Like results were obtained with chlorine.

In the class of vapours, ammonia proved fatal in one case, and harmless in another; muriatic acid stupified in two, and killed in twenty-four hours. The vapour of nitric acid was equally fatal with sulphuretted hydrogen; and, in alcoholic vapour, at a temperature of 74°, 'for a few minutes the fly shewed increased activity; in a few more, it became nearly motionless; after about a quarter of an hour, it appeared to be torpid. Now, exposed to the air of the room, in a few minutes a slight motion of its feet was seen; after a couple of hours, it was nearly as active as before the experiment; two hours later, it was found dead.' The same effects, with slight variations, were produced on other flies. With ether, cessation of motion was almost instantaneous, followed, however, by revivification, except in one instance: brief immersion in chloroform did not prevent revival, but an exposure of eight minutes killed: camphor and turpentine were both fatal: with attar of roses, musk, or iodine, no ill effect was perceptible.

The experiments with prussic acid are worthy the attention of entomologists, with whom it is often a matter of importance to kill an insect with the least possible amount of injury. In these instances, the plan pursued was to charge a small tube with the acid, and place it inside that containing the insects. The vapour of 1-16th of a grain was sufficient to destroy bees and flies; and that of seven grains proved fatal to large beetles, and the largest kind of bees. Although as yet the investigation has taken but a limited range, it will be seen that it opens a wide field of research: the next step will be to group or class those agents which appear to have produced similar effects. It is remarkable, as Dr Davy observes, 'that most of the substances which, even in minute portions mixed with common air, prevent the slow combustion of phosphorus, as indicated by its shining in the dark, have the effect, on the insects on which they were tried, of suspending animation.'

He says further: 'Some of the results may not be undeserving notice for practical purposes—as those in the instances of sulphuretted hydrogen, oil of turpentine, and camphor, in relation to the destruction of parasitical insects, whether infesting plants or minerals, or to the preservation of substances from the attacks of insects. To be applicable to the preservation of plants, of course it is necessary that the agents to be used should not exercise on them any materially injurious effects. This must be determined by experiments made expressly for the purpose. The few trials I have yet made on seeds seem to shew, that the steeping them in a solution in water of sulphuretted hydrogen has not prevented their germination. The seeds tried were mignonette, cress-seed, and that of a Nemophila: analogy—namely, that of steeping the seed of the cerealia in a solution of the white oxide of arsenic, is in favour of the same conclusion. Further, for the preservation of articles, whether of clothing or furniture, it is hardly less necessary that the substances to be employed should have no offensive odour. Judging from the effects of attar of roses, and from what we know of scented woods not being liable to be attacked by insects, the probability is, that any volatile oil of agreeable perfume will answer the purpose required, and prove a true instance of the utile et dulce combined.

'As carbonic acid gas, and some of the other agents mentioned, produce merely a temporary torpor, it may be a question whether this gas, or simple immersion in water, may not be advantageously substituted for the fumes of burning sulphur, destructive of life, at the yearly gathering of honey; the former, indeed, may be said to be in use in the Levant, where the smoke of the fire of leaves, in which the carbonic acid generated may be considered as chiefly operative, is employed to stupify the bees preparatory to the spoiling of their hives.'