Upon an average, a water-beetle remains from fifteen to twenty minutes without requiring to breathe; this period being capable of considerable extension should occasion arise. I have forced one of these insects, for instance, to stay beneath the surface for nearly an hour and a half, by alarming it as often as it attempted to rise. Generally speaking, however, before the first half hour is over, the beetle allows itself to float to the surface, protrudes the tips of the wing-cases, and expels the exhausted air from the cavity beneath them; a fresh supply is then taken in, and the insect again dives, the entire operation occupying barely a second of time. The Water Scorpion affords us an instance of a perfectly different structure. Here we have a being, feeding upon living prey, which it must capture for itself, and yet sluggish and slow of foot. By stratagem alone, therefore, can it hope to succeed, and it accordingly lies hidden among the dead leaves, sticks, &c., at the bottom of the water until some luckless insect passes within reach of its jaw-like fore-limbs. But this may not occur for hours, and it is imperatively necessary that no alarm should be given by frequent journeys to the surface in search of air. So, the extremity of the body is furnished with a curious organ consisting of two long filaments, which are, in reality, tubular, and which serve to convey air to the spiracles. The extreme tips of these project slightly above the surface when the insect is at rest at the bottom of the pond, so that respiration can be carried on without difficulty, and without necessitating the slightest change of position. A still more curious structure, although of very much the same character, is afforded us by the grubs of the common Drone-fly. These are inhabitants of the thickest and most fetid mud, dwelling entirely beneath its surface, and consequently cut off from all personal communication with the atmosphere. But from the end of the body proceeds a long tube, which can be lengthened or shortened at will, somewhat after the manner of a telescope, and which conveys air to the spiracles just as do the tail filaments of the water scorpion. Unable to change their position, these “rat-tailed maggots,” as they are popularly called, are yet independent of any alteration in the depth of the water above them, for the air-tube can be instantly regulated to the required length, and so insure an uninterrupted supply of air.