An aquarium of the kind described stands at present on my writing-table. Many have been the inquiries as to what kind of creature lived therein; and more than one old hand at collecting has failed to find the insect. It is always safe to look round the sides of the basin for the breathing-tube; if it is not detected in this situation, a glance along the surface of the tiny pools will probably show a break in the film. The Water Scorpion will not be far off. That the insect really does cover itself with mud may be demonstrated by transferring it from the aquarium described to any shallow vessel, and allowing a fine stream of water--say, from a dipping-tube--to fall gently on it. The mud will be washed away, and in a few seconds will settle at the bottom. It is well to keep such an aquarium covered, for the Water Scorpion has no mean power of flight. A circular plate of glass, which may be bought for a few pence, makes the best cover, but a piece of fine muslin fastened round the rim of the basin will do. Swammerdam says, ‘These Water Scorpions live all the day in the water, out of which they rise about the dusk of evening into the air, and so flying from place to place, often betake themselves in quest of food to other waters.’ Then follows a deduction which shows how far he was in advance of his time: ‘This affords us a satisfactory reason for the great number of insects that immediately appear in the smallest collections of water, since they may very well get thither when it is dark, so that the opinion which ascribes to putrefaction the power of forming insects, &c., must, by this instance of the Water Scorpion’s nocturnal transmigrations, appear more and more frivolous and unnecessary.’ [Illustration: FIG. 43.--_A._ Water Scorpion. _a_, rostrum; _b_, wing-cases; _c_, wings; _d_ _d_, second and third pairs of legs; _e_, raptorial legs (the first pair); _f_, spine forming breathing-tube; _g_, parasitic mite. _B._ Portion of an oviduct containing eggs (magnified). _C._ An egg (magnified). _D._ Parasitic mite. _a_, head; _b_, eyes; _c_, legs; _d_ _d_, legs (seen from under surface). (After Swammerdam.)]