It is a really wonderful fact that the insect which, for three stages in life--namely, an egg, larva, and pupa--lived in the water, should in the fourth not only be incapable of aquatic life, but should employ its old skin to protect it from that very element in which it was living only a minute or two before. Should the reader wish to examine for himself either the egg or skin boat of the Gnat, he can easily procure them by searching any quiet pond, or even an uncovered water-butt. They are, of course, very small, averaging about the tenth of an inch in length, and are nearly always to be found close to the side either of pond or tub, being drawn there by the power of attraction. I may here mention that there are other dipterous insects belonging to the genus Stratiomys, which undergo their metamorphosis in a very similar fashion. In these insects, the larva breathes through the tail, and when it attains its pupal condition, the actual insect is very much smaller than the pupal skin, only occupying the anterior and enlarged part. Indeed, the difference of size is so great, that several entomologists believed the future Stratiomys to be but a parasite on the original larva. The beautiful Chameleon-fly (_Stratiomys chamæleon_) is a familiar example of these insects. NAUTICAL. CHAPTER II. THE OAR, THE PADDLE, AND THE SCREW. Propulsion by the Oar.--Parallels in the Insect World.--The “Water-boatman.”--Its Boat-like Shape.--The Oar-like Legs.--Exact mechanical Analogy between the Legs of the Insect and the Oars of the human Rower.--“Feathering” Oars in Nature and Art.--The Water-boatman and the Water-beetles.--The Feet of the Swan, Goose, and other aquatic Birds.--The Cydippe, or Beroë.--The Self-feathering Paddle-wheel.--Indirect Force.--The Wedge, Screw, and Inclined Plane.--“Sculling” a Boat.--The “Tanka” Girls of China.--Mechanical Principle of the Screw, and its Adaptation to Vessels.--Gradual Development of the Nautical Screw.--Mechanical Principle of the Tail of the Fish, the Otter, and the sinuous Body of the Eel and Lampern.--The Coracle and the Whirlwig-beetle.