ON the left hand of the same illustration are shown the curious Rat-tail Maggots, as they are popularly called. They are the larvæ of the common Drone-fly (_Eristalis tenax_), which is so common towards the end of summer, and looks so curiously like a bee. [Illustration: RAT-TAILED MAGGOTS.] [Illustration: DIVER WITH AIR-TUBE.] These creatures pass their larval life buried in the mud and below the surface of the water, and yet are obliged to breathe atmospheric air. This they do by means of the long appendages which have gained for them the name of Rat-tails. These “tails” are very elastic, and are capable of elongation and contraction to a wonderful extent. When the creature is undisturbed, it lies buried in the mud with its head downwards, and its tail extended so that it reaches the surface of the water. Within this tail are two air-tubes, which are connected with the principal tracheæ, which have already been mentioned. They are wonderfully elastic, and, when the tail is extended to its utmost limit, are nearly straight. When, however, the tail is contracted, the tubes become self-coiled by their own elasticity, and shrink into the base of the tail. As the tail is very transparent, it is easy to see how these movements are conducted. The larvæ, which may be found in almost any stagnant water, should be placed in a tall and narrow glass. Some mud should be placed at the bottom of the glass, which should then be filled with water to the depth of three inches or so. When the mud has quite subsided, and the water become clear, the long slender tails of the larvæ will be seen so elongated that their tips reach just above the surface of the water. A magnifying-glass will easily show the two tubes within the tail. Let the glass be but slightly tapped, and all the tail is withdrawn in a moment, so as to be out of reach of external danger. The magnifying-glass will then show the two tubes lying contracted in the base of the tail, and taking astonishingly little space, considering the amount of elongation which they can sustain. And, on examining the various bends and curves of the tubes, the value and power of the spiral spring will at once be seen. True, they are very small, but in Nature all things go by comparison, and our whole earth itself is as a grain of sand upon the seashore among the grandeurs of the visible universe. THE LEAPING SPRING. The last of the springs which can be mentioned in this work are those which are used for leaping purposes. The figure on the right hand represents the common Spring-jack or Skip-jack with which children are always so much amused. It consists of a flattened piece of wood called the “tongue,” which is inserted into a twisted string, so that it forms a tolerably powerful spring. When twisted round, and then suddenly released, it strikes against the ground with such force that the whole machine is thrown into the air.