RESPIRATION OF THE PUPA.
Having glanced at these particulars in the history of the pupa, we come to the important question,—Does the pupa, in this torpid condition, still breathe, as the larva did, or not? On a careful examination we might detect, with the help of a magnifying glass, the same breathing holes or spiracles, spoken of on page 161, as appertaining to the larva; and were we sufficiently skilful to dissect the pupa, we should find much the same arrangement of air-tubes within the body. In the absence of all other means of ascertaining the fact, we should be warranted in concluding from the presence of these organs alone, that the pupa has the faculty of breathing, no less than the larva. M. Réaumur has given a beautiful description of the breathing-holes, or spiracles, of the pupa, and has represented them in an engraving, of which a copy is furnished at page 161. He found that they are protected by little valve-like contrivances, just as our mouths are protected by our lips; and these can be opened or closed at pleasure by the creature; so that if plunged into water, that fluid cannot enter the insect's body, for all its little doors—and there are no fewer than eighteen of them!—are fast shut. Singular to say, however, if plunged into oil, the oil has the power of entering them, and the pupæ may be thus drowned.
It is very easy to put the fact of the breathing of the pupa, inanimate as it appears, to a certain test. By taking a wine-glass half full of water, and putting it under an air-pump, and then exhausting the air, we shall be able to extract the air which exists dissolved in the water: or some water that has been boiled and allowed to cool will do as well. If we now put a pupa into this water, and again exhaust the air from the receiver, we shall notice, at the second stroke of the piston, a number of little jets of air come from the insect's body at the places where the spiracles are situated, thus clearly proving that the creature breathes air by this apparatus. The fact may be tested also in another way—and both this and the preceding experiment are due to the ingenious Réaumur. He took the pupa of a butterfly, and suspending it by a thread immersed it half way down in oil; on taking it out after some time it was still alive and apparently uninjured, the reason being, that the entire number of its breathing-holes were not covered. He took another of the same species, and plunged it entirely under the oil, and taking it out after a time it was found to be quite dead; in fact it had been suffocated by the air being shut out from its breathing apparatus, and that as effectually as if it had been one of ourselves lying at the bottom of the sea. We may say, therefore, with perfect accuracy, that though the pupa, this seemingly un-living object, neither moves nor eats, nor in any other way gives us a sign that it is alive, except in a few cases, and that in its advanced stages, we can, nevertheless, prove it to be living; as it can be shown that it has the power of breathing; and if it breathes, it lives.
We may take the liberty of appropriately appending to this statement a most interesting and delightfully-told anecdote, from the pages of the Baron de Geer, of an insect whose larva lives in the water, but whose pupa is an air-breathing creature, and consequently would perish if the change from larva to pupa were to take place under water, without some especial contrivance to furnish it in the pupa form with a supply of air. Yet how can this be, when the insect is under water the whole time up to its becoming a perfect moth? We shall hear:—
"At the commencement of spring, as soon as the frost and ice had disappeared, I sauntered out one day to procure some fresh plants from the bottom of a stream, in order to feed some of my caterpillars with them. Between the leaves of these aquatic plants I presently found a large number of aquatic larvæ, which had there safely passed through the rigorous season just gone by. I took a number of them, and put them in some boxes, where they eat the leaves with which I fed them. There they grew larger from day to day, although by slow degrees. I tended them until June in the same year without perceiving any other change in their appearance than that they had grown to a considerable size. But at the commencement of this month I noticed that they became very uneasy; they forsook their leaves, and wandered about the sides of the boxes, which contained water. Sometimes they would creep out of the water, and again would go into it. They seemed as if they were in search of something which they had lost. It was now evident that their object was to find some convenient place in which to undergo their transformation into pupæ, but they did not like the boxes in which they were confined, and several even died. I began to despair of seeing their metamorphosis; which gave me much regret, as I had a great desire to become acquainted with their whole history. In order to satisfy myself, I went on the 26th of June to the spot from whence I had taken them, in order to discover, perchance, some larvæ about to become pupæ, and to ascertain the places they selected in which to undergo their change. I had the gratification of being completely successful, and of discovering not only the larvæ, but even several of them which had shut themselves up in silken cocoons, and had not yet undergone their transformation.
"The month of June appears, therefore, to be the period when these larvæ prepare for their change of form. They do not leave the water to accomplish it, the change taking place under water. They attach themselves to the leaves of some of the younger aquatic plants, which are at a suitable depth from the surface of the water. By means of several silken cords the insect fastens together two, and sometimes even three leaves, between which it spins an oval cocoon, composed of very white silk. This cocoon is of a double structure; the true inner cocoon, which is very white, is placed within an outer envelope of silk, of a greyish or brownish colour. The envelope extends considerably beyond the sides of the true cocoon (which lies loosely in it); and towards one end there is a wide opening in it, the inner cocoon being perfectly closed in on every side." The cut on the next page represents the perfect insect and the pupa thus carefully provided for.
The Cut represents two leaves, between which the Worm lies, the Cocoon itself, and the Larva.
In this inner cocoon there is not so much as a single drop of water! The pupa could not exist in it if it were not full of air; and the difficulty is how to fill a little cell with air which is already buried many inches under, and on every side surrounded by, water. De Geer states that he did not find a particle of water in all the cocoons which he opened. In order to ascertain whether or not it was really necessary for the pupa to be thus carefully housed in a silken ball full of air beneath the water, he took out several pupæ and put them into some water in a saucer. At first they swam, but presently they sank to the bottom. They lived for several days, but ultimately every one died, showing plainly that the air-cell was absolutely necessary to their existence.