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STONE AGE, the term employed by anthropologists to describe the earliest stage of human civilization when man had gained ms knowledge of metals, and his weapons and utensils were formed of stone, horn or bone. The term has no chronological value, as the Stone Age was earlier iii some parts of the world than in others, and even to-day races exist who are still in their Stone Age. This first period of human culture has been subdivided by Lord Avebury into Palaeolithic and Neolithic, words which have been generally accepted as expressing the two stages of the rough, unpolished and the finely finished and polished stone implements. (See ARCHAEOLOGY.)
STONE-FLY, the name given to medium-sized, neuropterous insects of the family Perlidae with long flexible antennae, wide thoracic sterna and with the wings resembling, as regards size, shape and the fan-like folding of the posterior pair, those typical of the Orthoptera except that the anterior pair is membranous and not coriaceous. The immature forms, which are aquatic, carnivorous and active, are very like the adults except in the absence of wings and in their method of respiration, which is either cutaneous or effected by means of variously placed integumental tufts richly supplied with tracheae. By some authors the Perlidae are regarded as a special order, Plecoptera; by others as a sub-order of an order Platyptera, which contains the Termitidae and some other insects as well.