IV. The insect, No. 4, is a pupa called the Obtected.[K] This pupa has its upper portion encased in the peculiar manner represented, the chest and lower portion being distinct. The butterfly pupa belongs to this division.
V. The insect, No. 5, is a pupa called the Coarctate.[L] In this case the pupa is enclosed within its larva skin, which forms a globular or oval case, the pupa lying loosely in it as if it had shrunk to a smaller size. The pupa of the blow-fly is an example.
Such are the five variations of pupæ, as they were recognised by Linnæus—the Complete, Half-complete, Incomplete, Obtected, and Coarctate. They are sufficiently minute for ordinary purposes: and it will possibly save the reader some confusion of ideas to endeavour to fix them in the memory; so that when looking at an insect whose pupa state may not be so very characteristic as that of the blow-fly, or butterfly, he may still be able to say with confidence, that although it is unlike these, it is nevertheless a pupa. On all subjects nothing is of so much importance as clearness of ideas. It is better to know only a few things, and to understand them clearly, than to have a confused and indistinct knowledge of a great number. It has been, therefore, simply and entirely with a view to obviate this state of things in the mind, that these definitions of the different kinds of pupæ, which may have appeared not altogether interesting, have been given.
Having fulfilled this duty, we may now proceed to the more agreeable task of ascertaining some interesting facts relative to the insect's life and age in the pupa state. We have already seen that the insect in the larva state often arrives at a very respectable old age; indeed, in this state insects live longer than either in the pupa or perfect states. But pupæ also attain to a very fair number of days, sometimes living as long as two years in that state. Often, however, they are not more than a few days in this condition, and the insect, after a short repose, springs forth a new and active being. But it has been found that insects live a longer or shorter period in the pupa state according as the temperature of the air is cold or hot. Thus, for example, when the larva of a moth has become a pupa in the early part of summer, the pupa state will generally not last beyond a fortnight. But if, on the other hand, the larva becomes a pupa late in the autumn, the pupa state will last until June in the next year: thus manifestly teaching us that according as the weather is mild and genial the pupa state will be shortened; or according as it is cold and rigorous it will be increased in duration. The ingenious Réaumur determined to put these singular facts to an experimental test; and as his results are in the highest degree interesting and important, we shall proceed to submit an abstract of them to the reader's notice.
In casting about for the means of exposing the pupæ he was about to experiment on, to a warm and equable temperature, Réaumur determined to conduct his first experiments in the Royal Conservatories, which were always carefully heated, and in which, as he with French naïveté expresses it, "summer reigned in the depths of winter." In the month of January he carried thither a number of boxes containing pupæ of different species. The result was precisely what he had expected:—in the midst of a severe winter a number of butterflies appeared in his boxes, many of which would not naturally have made their appearance until the months of May, August, or even September; thus shortening the pupa state from four, seven, or eight months, to a fortnight, or to five, or to six weeks in different instances. Five or six days seemed to be equal to a month of the natural temperature. The butterflies thus developed were in no respect different from those which are brought into activity at the natural period. They were as active and perfect, as if their time and place of birth had been the green fields, instead of amidst the strange vegetation of these splendid Conservatories. Several of the mother insects deposited their eggs, accomplishing the last act of their existence as if summer had come, and died while the frosts and snow held all external nature yet in bondage. Not only, therefore, was the duration of the pupa state in these insects shortened, but their whole life was thus abridged by several months.
In November of the same year Réaumur recommenced his experiments, and again exposed a number of pupæ to the genial influence of these hot-houses. The result was the same. In the first week in December butterflies appeared, which would not, in natural circumstances, have been developed earlier than the May of the next year. There were some pupæ, in particular, whose development he watched with great interest. These pupæ belonged to a beautiful species of moth, which has two broods in a year; that is, it lays eggs in May which become butterflies in July, and then again lays eggs which become pupæ in August or September, but do not become butterflies until the following June. He was curious to see whether this second brood, instead of waiting for several months, would, like the first, disclose its butterflies in a considerably shorter time, now that it was exposed to the warmth of the Conservatory. Such actually proved to be the case; and thus two generations of these butterflies were obtained in one year. Alluding to the depredations of caterpillars, he quaintly remarks, "This certainly is not a secret which appears very profitable at present; but who can tell whether that which is useless to us to-day, may not possibly become of value to-morrow? Could we discover some new species of larvæ which would supply us with as good a silk as that of the silk-worm, and might be more easy to rear, but which only produced one generation in each year, and if it lived upon leaves which could be found all the year through, we might avail ourselves of this means of increasing the number of its broods." This remark deserves much consideration.
He was now anxious to try whether, by applying a more equable method of warming, he could succeed in hatching pupæ as he had done in the hothouse. The idea occurred to him of endeavouring to hatch them under a hen. He concluded that the warmth of the mother's breast would quite as easily hatch the insects, as it does the eggs. But there was this obstacle in the way: How could he prevent the fragile and tender bodies of the insect pupæ from being crushed and killed by the weight of the hen's body? and, as we would also suggest, How could he ensure that the bird would not actually have eaten up the objects of his care? Aware of the fact, that, when a hen is in the humour to sit, she will often allow smooth stones to be placed among her own eggs, he anticipated no difficulty on that score, and he hit upon the following ingenious experiment:—He procured some hollow glass balls which he had caused to be made as nearly as possible similar in size and shape to the eggs themselves. Into these, by an opening at one end, he introduced seven or eight pupæ, and stopped the mouth up with a cork, but so as to allow a free communication with the external air by paring off a piece from the side of the cork.
Thus prepared, he put the glass egg together with the others in the nest. The hen was a little more sensible than Réaumur had given her credit for; and though she did not thrust the egg out of her nest, she removed it to the outside, where she was so obliging as to permit it to remain; and as it was here just as warm as if it had been in the centre of the eggs, Réaumur did not attempt to interfere with her arrangements. A great deal of moisture arose from the bodies of the pupæ, and condensed like dew on the sides of the glass; but after a day or two this disappeared. The reader may now be anxious to learn the result of this experiment. It was equally successful; indeed, it was more so than the preceding, for in the afternoon of the tenth day a pretty little butterfly was seen within his glass egg, being the first that had appeared of the eight pupæ, and the first ever hatched under the bosom of a hen! The remaining pupæ, all but two, appeared soon after; these two died. Perhaps the warmth of their glassy cell was too violent for them, for it was found by the thermometer to be two or three degrees above blood heat. The whole six pupæ were born in less than six days, while others of the same species in a box in a window-seat were not developed until twelve days later. As the heat thus obtained seemed too violent for pupæ to be artificially reared with success, Réaumur suggests that many variations might be made in the experiments, which would have the effect of moderating its amount. These experiments decided in the most satisfactory manner the quickening influence of increased warmth upon the pupæ of insects. Réaumur now became anxious to try the effects of the opposite state of temperature, and to ascertain whether exposure to cold would exercise any effect upon the pupæ. It was reasonable to imagine that as warmth had hastened forward their development, cold would retard it.
Réaumur determined to try what would be the result of putting his pupæ in their boxes in a cellar, and taking proper care to preserve them from the damps of such a situation. He put them there about the end of January. In ordinary circumstances these pupæ would have become butterflies in the month of July in the same year. July came, and we can conceive the curiosity with which the ingenious experimenter went down, as he tells us, to his cellars, to see if any change had taken place in the pupæ. July passed away; August also passed by, yet the pupæ still slumbered on in their original form. Réaumur left Paris in September, and did not return until the November following. He immediately went in quest of his pupa-charge, and found them still unaltered. Were they dead? Placing one in his hand, it soon began to exhibit such symptoms of motion as plainly showed that it was alive. Winter closed over them still in the pupa form. The spring of the next year dawned upon them, but they were insensible to its influences. "And even now," cries Réaumur, in the month of August, just two years from the time they left the larva form and became pupæ, "they are in perfect health, in excellent condition, and would all become butterflies very soon if I were only to expose them to a warm summer's influence."
"These extraordinary facts," observe Messrs. Kirby and Spence, "lead us to a very singular and unexpected conclusion,—that we have the power of lengthening or shortening the life of many insects at pleasure—that we can cause one individual to live more than twice as long as another of the same species, and vice versâ. Had Paracelsus made this discovery, it would have led him to pursue his researches after the elixir of immortality with redoubled confidence, and would have supplied him with an argument for the possibility of prolonging the life of man beyond its usual term, which his sceptical opponents would have found some difficulty in rebutting. Even the logical Réaumur seems inclined to infer from it, that this object of the alchemist's was not so chimerical as we are wont to conclude. He confesses, however, that, if it were to be attained only by the same process as effects the extension of an insect's life,—by prolonging its state of torpor and insensibility,—few would choose to enjoy it on such conditions. The man of pleasure might, perhaps, not object to a sleep of a hundred years, in the hope of finding something new under the sun when he awakened; and an ardent astronomer would probably commit himself with scientific joy to a repose as long and as sound as that of the Seven Sleepers, for the chance of viewing his predicted return of a comet on stepping out of his cave. But ordinary mortals would consign themselves to the perils of so long a night with reluctance, apprehending a fate no better than that which befel the magician who ordered himself to be cut in small pieces and put in pickle, with the expectation of becoming young again."