CHAPTER III.

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.

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.

Yet, although these pupæ were air-breathing creatures, strange to say, De Geer found that if taken from their little cells out of the water, they shrivelled up and died. He put several in a dry box, and invariably found that in a short time they were dead. He imagined, with great probability, that the cause of their death was the evaporation of their fluids, which of course does not take place when they are enshrouded in their cocoons, and surrounded on all sides with water. He made one more experiment with them, which was as follows:--He took several out of their cocoons, and placed them in water in such a manner, that one of their sides was in the air, while the other was in the water; he kept them for a considerable time in this position, and he had the pleasure at its expiration of seeing them become perfect insects, just as if they had never left their cocoons. The necessity of both air and water to their well-being was thus clearly proved.

How can we sufficiently admire the ingenuity and skill displayed by this insect in the execution of its difficult task! When a human engineer wishes to build under the water, he must have recourse to the most cumbrous and powerful mechanism to enable him to effect his object. Piles must be driven into the bed of the river so closely as scarcely to allow any water to come through; a steam-engine must be fixed close by, to pump out all the water from the space thus enclosed; and a number of men must labour hard to stop up the chinks, where the water comes pouring in. Thus men, and machines of great power, and in sufficient number, are requisite to enable the most expert engineer to form a cavity at the bottom of the river's bed. What if we were to give him as a problem, to build a cell which should be perfectly water-tight and filled with air, without allowing him to employ any of these means, and to insist that it should all be done without his coming up to the surface of the water at all for anything? He would probably tell us it was impossible. Not so the insect; it, though on all sides surrounded with water, actually spins a web in the waves, and fastens it by cables of sufficient strength to adjoining plants, and afterwards forms a water-tight cell in which it shuts itself safely up secure against the invasion of enemies, or the intrusion of a drop from the element in which its cell floats and is moored.

All pupæ of aquatic habits do not, however, possess this remarkable faculty of surrounding themselves with air even in the midst of the water; and these would certainly perish unless means existed for them also to inhale the vital air. These means are found in the endowment of them with the power of swimming. Surely our readers must often have seen the wriggling movements of certain little blackish objects, which are to be found in every stagnant puddle in the summer. Many of these are the pupæ of various species of gnats; and when we watch them come darting upwards by a succession of flaps with their tail, until they reach the surface, where they remain for a short time, we see the means by which, although they live immersed in water, they are enabled to breathe the air. For if we scrutinized them a little narrowly, we might detect on each side of their largest extremity or head, a pair of minute tubes which open into the air at the surface of the water. It may sink beneath the water for a time without inconvenience, and it is soon found plunging upwards, anxiously seeking to thrust its tubes into the air. In the _larva_ state the gnat breathes by its tail, in the pupa state by its _head_! The pupæ of some aquatic insects breathe like the larvæ of the same insects by organs like gills. That of the ephemera shown on the last page is an instance.

The curious apparatus of a telescope-like air-tube of the rat-tailed insects, described in a former page, will not be forgotten by the reader. It may interest him to learn that there is another little creature which, in the pupa state, is furnished with a somewhat similar apparatus. These pupæ have not the same power of swimming with the last, and therefore require a special provision to meet the necessities of their case. They are plunged some way down in the water, and air is conveyed to them by a hollow tail-like tube, which is always found to open on the surface of the water. It is a curious thing to contemplate these little creatures, so beautifully provided for in this manner. Secure of all they need in the supplies of air furnished to them by their tube, they rest peacefully in the waters, unmoved by any of the accidents which occur to surrounding creatures, and patiently awaiting the hour which is to behold them rise from their watery bier, never more to return. Let us so likewise rest in assurance of our Heavenly Father's love and care for us, knowing that every want will be supplied to his children by Him, who has said, "I will never leave thee nor forsake thee."