On the other hand, eggs which would otherwise be hatched the same year are arrested by the advancing cold of the winter season, and are now compelled to wait until the ensuing spring, before their time of hatching arrives. Evidently, therefore, to the commencement of the life of an insect the condition of the external temperature is an all-important consideration. Before proceeding immediately to consider the nature of the changes, it may be mentioned as an interesting fact, that although the eggs of insects are very quickly sensible of a slight increase of heat, and in consequence of its application to them very soon begin to live, yet they will endure the most severe degrees of cold almost without injury. As an illustration of this point we may transcribe a few sentences from a paper by the great Spallanzani upon this subject:--
"The year 1709, when the thermometer fell to 1° Fahrenheit," or thirty-one degrees below freezing point, "is celebrated for its rigour and its fatal effects on plants and animals. 'Who can believe,' exclaims Boerhaave, 'that the severity of this winter did not destroy the eggs of insects, especially those exposed to its influence in the open fields, on the naked earth, or on the branches of trees? Yet, when the spring had tempered the air, these eggs produced as they usually did after the mildest winters.'" He adds further on, "I have exposed eggs to a more rigorous trial than the winter of 1709--those of several insects, and among others the silk-worm, moth, and elm-butterfly, were enclosed in a glass vessel, and buried five hours in a mixture of ice and rock-salt, the thermometer falling 6° below zero. In the middle of the following spring, however, caterpillars came from all the eggs, and at the same time as from those which had suffered no cold. In the following year, I submitted them to an experiment still more hazardous. A mixture of ice and rock-salt, with the burning spirit of nitre, reduced the thermometer 22° below zero, that is, 23° lower than the cold of 1709, or 52° lower than the point at which water freezes. They were not injured, as I had evident proof--by their being hatched."
When it is known that many seeds will not endure these degrees of cold without injury, and those even of some tolerably hardy plants, it is the more surprising to find such apparently delicate and readily damaged objects as the eggs of these members of the insect tribe thus resisting an intensity of cold to which, in a state of nature, they are scarcely ever exposed. It is impossible to assign any rational explanation of these singular facts. It is undoubtedly owing to this power of resisting the generally deadly influence of extreme cold that we find insects reappear in spring, even in countries where the winter is much prolonged, and is of extreme severity. Thus, in Lapland we should have probably thought that the rigour of the climate would have been fatal to all insects in winter, in any condition, whether in the egg, or in other forms; but, as the poor inhabitants know to their cost, it is far different. The mosquitoes swarm in that country in numbers so prodigious that they have been compared to a fall of snow, or to the dust of the earth. The wretched natives cannot take a mouthful of food, or lie down to sleep in their cabins, unless they are fumigated to a degree almost dangerous to life. They fill the mouth and nostrils, and, minute though they are, render existence almost a burden by their blood-thirsty propensities. Not even thick plasters of the most offensive compounds,--tar, oil, and grease, are sufficient to shield the Laplander's skin from their attacks. The great John Hunter considered that this power of resisting cold was, in some unexplained manner, connected with the existence of a living principle in the egg, which had the effect of withstanding a degree of cold that would otherwise have been fatal to it; but, after all, this is only an apology for an explanation. When we are unable to clear up the difficulties of a natural history question like this, although we cannot explain, we are not prohibited from admiring, and can clearly perceive, that in thus endowing the eggs of insects with a self-preservative power, God has manifested his wisdom and forethought; for had it been otherwise, the lapse of a few seasons would have depopulated the insect world, leaving us, it is true, without a gnat or a mosquito to annoy us, but also without a silk-worm, or a bee, to supply us with the precious products of insect industry.