Sometimes on the very brink of a cataract one will see what appear like patches of loose black moss. Strangely enough, these are the larvæ of black-flies, related to the terrible black-fly of the north woods. The black-fly larvæ can live only in the swiftest water. There they pass through their transformations and succeed in emerging into their aërial stage, in spite of the rushing current. [Illustration: _Fig. 53. The May-fly sheds its nymph skin._ (_Twice natural size._)] All these things and many more are seen by those who frequent the water brooks. Observers cannot tell all they see, for some things are too deep for words. They can and do say to one and all, "Come, let us visit the brook together. The water and all that dwell in it and round about, invite us and make us welcome." [Illustration: _Fig. 54. Stone-fly, showing one pair of wings. The lower figure is a nymph._ (_Twice natural size._)] LEAFLET XII. LIFE IN AN AQUARIUM.[16] BY MARY ROGERS MILLER. [16] Teachers' Leaflet No. 11. May, 1898. [Illustration] There is no more fascinating adjunct to nature-study than a well-kept aquarium. It is a never-ending source of enjoyment, interest and instruction to students of any age. Children in the kindergarten or at home will watch with delight the lively occupants, which cut all sorts of queer capers for their amusement, and older people may read some of nature's choicest secrets through the glassy sides of the little water world. To many, the word aquarium suggests a vision of an elaborately constructed glass box, ornamented with impossible rock-work and strange water plants, or a globe in which discouraged and sickly-looking gold-fish appear and disappear, and take strange, uncanny shapes as they dart hither and thither. Such forms of aquaria have their place in the world, but they are not suited to the needs of an ordinary school-room. Every school may have some sort of an aquarium if the teacher and pupils are willing to give it some daily thought and care. Without such attention a fine aquarium may become an unsightly and disagreeable object, its inhabitants unhealthy and its beauty and usefulness lost. The great fundamental principle underlying success in making and maintaining an aquarium is this: _imitate nature_. We all know how much easier it is to formulate a principle, and even to write a book about it, than to put it into practice. Most of us have not had the time and opportunity for the close observation of nature necessary to interpret her methods and to imitate her. It is to those teachers who are anxious to learn what nature has to teach and who wish to lead their pupils to a higher and wider conception of life, that these suggestions are offered. Four things are important in making and keeping an aquarium: