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.
The other Arthropoda are air-breathing, and since their characteristic breathing organs are branching tubes known as tracheae, the term Tracheata is sometimes used to include them all. They fall naturally into three divisions, the Myriapoda, the Insecta and the Arachnida, and it is in this last-named division that we shall find the spiders.
The Myriapoda are the centipedes and millipedes, and having said this we may dismiss them, for insects and arachnids are strictly limited as to legs; and no myriapod can ever be mistaken for a spider.
The Arachnida are so varied in structure that it is not easy to give characteristics common to them all, and to any general statement there are bound to be exceptions, but for practical purposes it may be said that while an insect, when mature, has only six legs, and a pair of feelers or antennae of quite different structure, Arachnids have normally eight legs, and their feeling organs are not antennae but leg-like “pedipalps.”