Phryganeida. There are a great many of them, and the larvae of each have usually a different plan of constructing their well-known tubes, by which they may be identified. Some select minute shells of a species of Planorbis or Pisidium; others use grains of sand. The genus Limnephilus prefers pieces of rush or other aquatic weeds. But all of them are interesting, and Fig. 63. seem to be perfectly aware that they are regarded as choice and dainty bits by other larger and more active water animals. All of them hold on to the interiors of their frail defences by means of a series of hooks, so that it is somewhat difficult to drag them out forcibly. Before they pass into the quiescent state, previous to changing into their image condition, they protect themselves by making gratings at the ends of their tubes. The insects into which these larvæ even Nat. size. tually pass, in many respects (notably in their having scales on their wings) resemble butterflies and moths. The larvæ of another insect, belonging to the Ephemera, is usually very abundant in ponds. It is shown in its natural size at b, Fig. 67, as well as Fig. 67. Fig. 68. Water Spider (Argyoneta aquatica). Larvæ of Ephemera. enlarged, to indicate the breathing leaflets along the sides of the body. Nor should we forget the exceedingly interesting water spider, not only because its habits depart so extremely from those of its kind, but also on account of its prettiness and intelligence. This species (Ar gyoneta aquatica) is not uncommon. In the water it looks as if its body were covered with a film of quicksilver. This is in reality a film of air which it entangles on the surface and carries below, so as to fill the diving-bell-like nest it has spun with it, and which is air-tight. This nest is filled by successive journeys of the spider to the surface, to store up the air for subsequent breathing. These air-nests may be seen in most ponds, especially where there is an abundance of Anacharis and water-crowfoot leaves. There are several species of the order Arachnida which live in water, but they are usually of the kind we call "ticks." Some of them pass through very interesting metamorphoses, being parasitic on plants and insects alternately before they attain their fully developed condition. It will be seen that the objects required for stocking ordinary fresh-water tanks are not difficult to find; and, if proper precaution as to the habits and voracity of the different kinds be taken, there need be little fear as to ultimate and continued success. Every aquarium, large or small, ought to be as perfect an imitation of natural conditions as possible, and success usually depends on the degree to which this is carried out. MICROSCOPIC RESEARCH. 113 CHAPTER VIII. THE AQUARIUM AS A NURSERY FOR THE To those who keep aquaria for the sake, not merely of being amused, but of learning the higher lessons which animated nature is ever so ready to teach, both fresh-water and marine parlour-aquaria may easily be converted into nurseries for microscopic research. Here may be reared with the utmost ease thousands of minute forms of life, whose ephemeral history of various conditions may be actually seen enacted upon the stage of the microscope. Human eyes can thus look down upon and witness the evolutions of these lower forms of life, just as it is possible other eyes look down upon our own terrestial career. The fresh-water tank especially is worth supporting, even for the sake of its microscopical animals and plants alone. Mere littleness does not detract from the interest of their microscopical study, but rather throws a romantic glow about it. By the true naturalist magnitude is not taken specially into account, nor is minuteness of size regarded as in itself a sign of low organisation. It is true that most of the lower forms of life are microscopic, but this is because it is I an advantage to them in the peculiar conditions by which they are surrounded. Not unfrequently their small size is more than compensated for by the enormous rapidity with which individuals are produced. Many of the lowest types of vegetable life with which every tarn, pond, and stream is crowded, and which may be kept with the utmost ease ready for inspection in the aquarium, are single-celled. But these single cells are constantly splitting into two parts, as in the Desmids and the Diatoms, each of which becomes a new individual, and goes through the same mysterious self-division. The main difference between these peculiar objects and vegetable species of a higher organisation and greater magnitude seems to us to consist in the fact that in the former the cells are detached as fast as they are formed, whereas in the latter they adhere together, and thus produce objects of large volume. This is proved by the fact that all species of desmids and diatoms are not single cells. Not unfrequently we find them living in colonies, either for the whole or part of their lives. Few objects are prettier than the microscopic plants we are now referring to. Seen by the naked eye their presence is perhaps only revealed by the green film covering the inside of the glass, to which aquarium keepers who are not microscopists strongly object. Desmids and diatoms often cover the stems and leaves of aquatic plants with a greenish or olivecoloured slime, such as Hyalotheca dissiliens (Fig. 69) |