The creatures skipping, dry-shod, on the surface of the river or pond, though often called water-spiders, are true insects. The real water-spider, _Argyroneta_, which, though air-breathing, spends most of its time below the surface of the water, is not to be found everywhere, but there are many riparian species which are semi-amphibious in their habits and have no objection to a wetting. Finally, turn into the wood and look carefully on the ground, especially where last year’s leaves are still lying. You are certain to see a few—and may very likely see countless myriads—of sober coloured, rapidly moving “wolf-spiders” (Lycosidae), roaming in quest of food. No stay-at-homes, these, but rovers, trusting to speed and agility, and not to guile, for their food supply. All the spiders we have observed so far are in active pursuit of their daily business, but if we turn over stones, or logs, or look under sheets of loose bark, we shall find others, quiescent for the moment, but waiting for nightfall to begin their operations. But we have probably seen enough to show that a pretty wide field for investigation lies immediately at hand, and that a detailed study of what we have cursorily glanced at will occupy us so long that we shall have little time for considering the spiders of other lands. In the first place, however, we had better make quite sure of what is meant by a spider. CHAPTER II WHAT IS A SPIDER? Not many years ago the group Insecta was held even by Zoologists to include numberless small creatures—centipedes, spiders, mites, etc.—which further study has shown to present essential differences of structure, and in popular language any fairly minute animal is still an insect, just as any insect is popularly a “fly”—or, in the United States, a “bug.” Scientifically the use of the term Insect is now much restricted, though still extensive enough in all conscience, since it includes many more than a quarter of a million known species. Zoologists recognise a large group of animals characterised by having no internal skeleton but a more or less firm external coating of a peculiar substance called _chitin_, often strengthened by calcareous deposits, which necessitates the presence of joints in their bodies, and especially in their limbs if they are to move freely, just as medieval suits of armour required to be jointed. These are the Arthropoda. One subdivision of this group consists of aquatic animals, breathing by gills, and known as Crustacea. Crabs, lobsters, shrimps and “water-fleas” are familiar examples, and with the exception of the so-called land-crabs the only Crustaceans habitually found on land are wood-lice.