The insects that frequent the waters, require predaceous animals to keep them within due limits, as well as those that inhabit the earth; and the water-spider (_Argyroneta aquatica_) is one of the most remarkable upon whom that office is devolved. To this end, her instinct instructs her to fabricate a kind of diving-bell in the bosom of that element. She usually selects still waters for this purpose. Her house is an oval cocoon, filled with air, and lined with silk, from which threads issue in every direction, and are fastened to the surrounding plants. In this cocoon, which is open below, she watches for her prey, and even appears to pass the winter, when she closes the opening. It is most commonly, yet not always, under water; but its inhabitant has filled it for her respiration, which enables her to live in it. She conveys the air to it in the following manner: she usually swims on her back, when her abdomen is enveloped in a bubble of air, and appears like a globe of quicksilver. With this she enters her cocoon, and displacing an equal mass of water, again ascends for a second lading, till she has sufficiently filled her house with it, so as to expel all water. How these little animals can envelope their abdomen with an air-bubble, and retain it till they enter their cells, is still one of Nature's mysteries that has not been explained. It is a wonderful provision, which enables an animal that breathes the atmospheric air, to fill her house with it under water, and by some secret art to clothe her body with air, as with a garment, which she can put off when it answers her purpose. This is a kind of attraction and repulsion that mocks all inquiries. STOMACH BRUSH.