ORDER I. _Pulmonariæ._ The Arachnides comprised in this division have air-sacks, which serve for lungs, a heart with distinct vessels, and from six to eight simple eyes. There are two distinct families: viz. _Araneides_, comprising all the spiders and spinners; and Pedipalpi, comprising the tarantula and scorpions. ORDER II. _Tracheariæ._ These Arachnides are distinguished by their respiratory organs, which consist of radiated or branched tracheæ, receiving air by two circular openings. Their eyes vary from two to four. The principal animals belonging to this division are the long-legged spiders (_Phalangium_), and the mites (_Acarus_), including the gardener’s pest, the little red spider (_Acarus telarius_), the cheese mite (_Acarus Siro_), and the harvest bug (_Acarus_ or _Leptus autumnalis_). IV. _The Insecta_ form the fourth and last class of articulated animals, and they derive their name from the Latin word _insectum_, which signifies “cut into,” in allusion to the distinct divisions of head, thorax, and abdomen in the true insects: and in contradistinction to the Annelides, the bodies of which present no such divisions. The true insects are defined as animals without vertebræ, possessing six feet, with a distinct head furnished with antennæ, and breathing through stigmatic openings, which lead to interior tracheæ. The Myriapoda have, however, more feet. The following are the twelve orders into which this class is divided. SECTION I. _Insects undergoing Metamorphosis._ 1. _Coleoptera_ (from two Greek words signifying sheathed wings). These are the beetles, which are all furnished with membranous wings, with which they fly, and which are protected by horny upper wings, or wing-cases, called elytra. They are all masticators, and are all provided with mandibles or projecting jaws, and maxillæ. 2. _Orthoptera_, or straight-winged insects. This order comprises the crickets, grasshoppers, locusts, and similar insects. They have their upper wings of the consistence of parchment, and have mandibles and maxillæ. 3. _Hemiptera_, or half-winged insects, have frequently half the upper wing membranous, like the under ones, while the other half is leathery. To this division belong the bugs, the water-scorpions, the cicadæ or froghoppers, and the aphides. These insects have neither mandibles nor maxillæ, but in their place have a sheath and sucker. 4. _Neuroptera_, or nerved-winged insects, such as the dragon-flies, have both pairs of wings membranous, naked, and finely reticulated. The mouth is adapted for mastication, and furnished with mandibles and maxillæ.