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. [Illustration: _Pupa of the Ephemera, showing its gill-like Organs._] 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.