Another instance of a curious contrivance for assisting the breathing of a larva, whose body is wholly immersed in water, may also be found near home. Most probably the majority of our readers have seen, or at any rate are well acquainted with the apparatus by which the diver descends and works under water. Lest some should not, we may mention that it consists of a water-tight dress, which covers the diver from head to foot, while his head is enclosed in a helmet, or hood, furnished with a pipe or hose, which is long enough to reach to the surface of the water, and is there generally connected with an air-pump, which forces down fresh air, and so enables the man to breathe almost as comfortably as if he were walking on land, instead of at the bottom of the sea. Let it be conceived now that there were no air-pump at the top of the hose, but simply a buoy of cork to keep it floating, and that the diver had the power of shortening or lengthening the hose at his pleasure, we should then have a tolerably fair representation of the contrivance with which Infinite Wisdom has supplied the humble creatures of whom we are about to speak.
By carefully examining the water running from manure heaps, or water which has become putrid, and is much loaded with mud and ooze, the reader will pretty generally be successful in discovering a number of the larvæ of a two-winged fly of the shape represented in the cut. When discovered, the singularity of their structure will amply repay him for the trifling unpleasantness attending the search after them. After washing them in a little fair water, and putting them into a tall ale-glass, or German beer-glass, which must be three parts filled with water, they will be in the best condition for our examination. When Réaumur first beheld them, he gave them the name of Vers à queue de rat; that is, larvæ with a rat's tail; and when the reader looks at them, as shown in the representation annexed, he will be inclined to think with us, that a more appropriate title could not have been found.
Rat-tailed Larvæ.
On watching them they will be seen to wriggle about until they are comfortably settled, darting from place to place, as if uneasy in their new position and in the pure element. By-and-by, however, they become reconciled to their lot, and lie perfectly quiet at the bottom of the glass in the position represented. From the tail of each will now be seen to rise a beautifully delicate and minute tube, which comes up to the surface of the water, and here, whatever be the movements of the larva below, it remains floating and free. Just as the diver may, while his companions above are pouring down torrents of fresh air, move about, and perform various duties under, water, so the larva may move from one position to another, and assume almost any variety of attitude; but its little air-pipe still retains its perpendicular direction, and does not quit the surface of the fluid.
A singular experiment may now be tried with them. Let a little water be poured into the glass by means of a funnel, the end of which dips under the water so as not to give them unnecessary disturbance, and by this means the level of the water in the glass becomes insensibly raised half an inch or so. In a few seconds it will be found that these little tails have all lengthened themselves, and are again at the surface of the water, even at its higher level. More water may be poured in until perhaps the level is raised three or four inches, and still the same thing takes place, the larvæ still prolong their curious tails, until it seems as if they would stretch to an indefinite length, for they are already eight or ten times longer than the body! But, as we might jestingly say—the longest tail comes to an end at last; for, on continuing to pour more water in, the larvæ stretch their tails to the utmost, and then, finding the level of the water still above their reach, they become uneasy, and all quit the bottom of the glass and attach themselves to its sides.
The Tail magnified.
Let us now examine more minutely this very singular apparatus. By a good lens we may readily do this, as the organ is very transparent, permitting its internal structure to be beautifully seen. Placing the creature in a convenient position for our examination, and in a good light, it will be presently seen that the tail is composed in reality of two tubes, of which one slides up and down within the other, just like the tubes of a telescope (see cut). By this means the larva is provided with an instrument capable of being made to reach to a great length, and yet also possessing the capability of being packed up again in a very small compass. The proper breathing tube is the small inner one; at its extremity is a pair of spiracles, or breathing pores, and surrounding these are placed five minute bristles, which are supposed to act somewhat as a buoy, and to retain the tail in its position. Where the tail joins on to the body of the larva, two tubes thrown into folds are placed, which are connected with the breathing tubes of the tail, at one end, and with the breathing pipes which run along the body of the larva at the other. When the tail is pushed out to its farthest, these zig-zag tubes are stretched quite straight; when it is drawn in again, they are thrown into folds. Both these appearances are shown in the plate. It has been supposed that the larva causes the tube to be pushed out by forcing air into it, and thus causing the zig-zag tubes to straighten themselves. Perhaps the tail is drawn in by the same tubes contracting again, and thus folding together and dragging back the tube to which they are connected.
