LIFE BEGINS IN THE EGG.
The eggs of birds are, in most instances, hatched by the warmth of the mother, who sits for a certain time covering them with her wings and downy breast. But the exception to the rule in insects is that the mother has anything to do with rearing her young brood; the cases in which this takes place will be noticed in our next chapter. Generally speaking, the eggs of insects are hatched by the increasing temperature of the air in spring. The following sketch, extracted from Mr. Darwin's interesting Journal of the Voyage of the Beagle, sets before us, in a very pleasing manner, the awakening influence of this season to all nature:—"When we first arrived at Bahia Blanca, September 7th, 1832, we thought nature had granted scarcely a living creature to this dry and sandy country. By digging, however, in the ground, several insects, large spiders, and lizards, were found, in a half-torpid state. On the 15th a few animals began to appear, and, by the 18th, (three days from the equinox,) everything announced the commencement of spring; the plains were ornamented by the flowers of a pink wood-sorrel, wild peas, œnotheræ, and geraniums; and the birds began to lay their eggs. Numerous beetles were crawling about, while the saurian tribe, the constant inhabitants of a sandy soil, darted in every direction."
As to the torpid animal and buried seed, so to the carefully laid up egg, the returning warmth of the air is the signal for the commencement of life. The winter-clouds roll reluctantly back, as the genial days of spring advance, and the changes which are to have their accomplishment in the production of a living being out of the minute object before us, are set in movement as the days grow bright and pleasant. That the hatching of the egg, in most cases, is due chiefly to the stimulating influence of heat, is now well ascertained. The school-boy who has ever amused himself with silk-worms can well assure us of this fact, for he is in the habit of hatching the insect's eggs by carefully wrapping them in paper, and keeping them in his waistcoat-pocket, where they have all the comfort and warmth of his body to bring them forward. In countries where the silk-worm is reared, women carry them in their bosom, and by this means cause the young larva to come forth from the egg in much less time than it would naturally occupy. By removing a twig of a plant upon which in the preceding autumn an insect may have been found to have deposited its eggs, into a warm room, an opportunity will be had of putting this operation practically to the test. In a short time it will be found that the eggs are all hatched, and that a number of minute larvæ are crawling actively about, while their brethren in the snow-covered fields are yet safely asleep in the shell.
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.
The frosts have disappeared, the air brightens, the sun loses its pale aspect, and glows with a more golden face. The days lengthen, the breeze has lost its penetrating chilliness, gentle showers descend and water the earth, and there is a general voice heard all over creation,—"Spring has come!" The eggs of a thousand insect species have already perceived its presence, and the newly-awakened beings within hasten to welcome it by bursting from the shell, their long occupied, but now for ever forsaken dwelling-place. Sometimes the young larva bursts through the thin walls of the shell by main force, or eats its way through by means of its jaws, which is occasionally a task of many hours' duration. "In many instances, however," write Messrs. Kirby and Spence, "the larva is spared this trouble, one end of the egg being furnished with a little lid, or trap-door, which it has but to force up, and it can then emerge at pleasure. Such lids are to be found in the eggs of several butterflies and moths. The eggs of a species of bug, besides a convex lid, are furnished with a very curious machine, as it would seem, for throwing it off. This machine is dark brown, of a horny substance, and of the shape of a crossbow; the bow-part being attached to the lid, or pushing against it, and the handle, by means of a membrane, to the upper end of the side of the egg."
But if, in our account of the various attendants on the opening of spring, we had mentioned every circumstance that takes place at that time, alas! for any poor insects, or, at least, for a large number of them, who should be hatched at that time. The warm air and gentle shower, and brighter sky, would ill satisfy them in the absence of all food, and they would be born, by a cruel destiny, only to starve and die. We well know this is not the case; but there are, probably, few persons who have ever thought much upon the admirable arrangement by which the occurrence of such a calamity to many of the insect tribes is avoided. We need scarcely remind the reader that in the opening sentence of the last paragraph there is one most important omission in the sketch of the phenomena of returning spring; that is, that there is no mention of what takes place in plants,—of the putting forth of their young and tender leaves. Now, as a majority of insects in the larva state are vegetable-feeders, we can easily understand that the unfortunate little beings if hatched before the appearance of leaves would, without doubt, quickly perish for lack of proper food. Yet the returning warmth of the air is all that is requisite to call the insect into existence, and if by the time it is ready to burst from the shell there is not food all prepared for it, it must die. The difficulty has been beautifully provided for; and perhaps, few other instances of the wisdom of the Creator in forming the insect world are so full of instructive thought as this. It has been ordained, then, that soon as winter is over, the plant is first to obey the voice of spring, and to awake; and the bursting buds on its lower boughs are already full charged with sap long before the young insect being that is to be fed therewith has left the shelter of the egg.
One of the talented authors of the Introduction to Entomology relates a pleasing anecdote in reference to this simple, yet admirable arrangement, and mutual adjustment of these two events,—the awakening of life in the plant and in the insect. "On the 20th of February, 1816, observing the twigs of the birches in the Hull Botanic Garden to be thickly set, especially about the buds, with minute oval black eggs of some insect with which I was unacquainted, I brought home a small branch, and set it in my study, in which is a fire daily, to watch their exclusion. On the 28th of March I observed that a numerous brood of aphides had been hatched from them, and that two or three of the lower buds had expanded into leaves, upon the sap of which they were greedily feasting. This was full a month before either a leaf of the birch appeared, or the egg of an aphis was disclosed in the open air." Thus showing that the coming to life of the branch and of the insects resting on it, was beautifully arranged to take place each at the proper time.