_Crawfish or crayfish._--These can be collected with nets from under stones in creeks or ponds. They can live very comfortably out of the water part of the time. There is small chance for the unsuspecting snail or water insect which comes within reach of the hungry jaws of the crawfish, and the temporary aquarium is the safest place for him. Many who live near the ocean can obtain and keep in sea water the lobster, a cousin of the crawfish, and will find that the habits of either will afford much amusement as well as instruction. The school boy generally knows the crawfish as a "crab." [Illustration: _Fig. 66. A useful net for general collecting._] _The frog._--The study of the development of the common frog is accompanied with little or no difficulty. To be sure there are some species which require two or three years to complete their growth and changes, from the egg to the adult, yet most of the changes can be seen in one year. Frogs are not at all shy in the spring, proclaiming their whereabouts in no uncertain tones from every pond in the neighborhood. The "frog spawn" can be found clinging to plants or rubbish in masses varying in size from a cluster of two or three eggs to great lumps as large as the two fists. The "spawn" is a transparent jelly in which the eggs are imbedded. Each egg is dark colored, spherical in shape, and about as large as a small pea. The eggs of the small spotted salamander are found in similar masses of jelly and look very much like the frog's eggs. If a small quantity of this jelly-like mass be secured by means of a collecting net or by wading in for it, it may be kept in a flat white dish with just enough clean, cool water to cover it, until the young tadpoles have hatched. As they grow larger a few may be transferred to a permanent aquarium prepared especially for them in a dish with sloping sides, and their changes watched from week to week through the season. The growing polliwog feeds on vegetable diet; what does the full grown frog eat? [Illustration: _Fig. 67. The predaceous diving-beetle._] _Insects that can be kept in aquaria._--Insects are to many the most satisfactory creatures that can be keep in aquaria. They are plentiful, easy to get, each one of the many kinds seems to have habits peculiar to itself, and each more curious and interesting than the last. Some insects spend their entire life in the water; others are aquatic during one stage of their existence only. Those described here are a few of the common ones in ponds and sluggish streams, of the central part of the state of New York. If these cannot be found, others just as interesting may be kept instead. One can hardly make a single dip with a net without bringing out of their hiding places many of these "little people."