APTERA, in the Linnwan system of zoology, the seventh and last order of In sects, the distinguishing characteristic of which is, that the insects comprehended in it have no wings. Of this order there are three divisions. In A. the insects are distinguished by having six legs; head dist;nct from the thorax : there are five genera ; Dir. the Lepisma, .Pulex, Pediculus, ,Termes.
Podura, In the division B, the insects have from 8 to 141egs ; head and thorax united : of these there are eight genera ; viz. the Acarus, Monoculus, Aranea, Oniscus, Cancer Phalangium, Hydrachna, Scorpio.
In the division C, the legs are nume rous ; head distinct from the thorax ; of which there are two species ; pi; the Joins, and Scolopendra.
This order comprehends all kinds of spiders, the lice of different animals, scor pions and crabs. Upon these ,we may make 'a few general observations. The nets spread out by spiders, to catch their prey, are coniposed of similar materials to the silk of the silk-worm, and are also spun from the animal's body. The cob webs of the gossamer are frequently seen floating in the air in a sunny day, snd are sometimes so abundant as to fall in show ers. Each of these has been compared to a balloon transporting the little aeronaut that formed it, by means of its specific lightness. This speciesof spider attach ing its first formed thread to the leaf' or branch of_a tree, by dropping to a certain distance lengthens it, then climbs up the thread, and dropping. again, draws out another and so on, till a sufficient quan tity of dlis silk is formed to buoy the spi der up in the air. He then separates the whole from the leaf, and running down to his seat at the bottom, trusts himself and his balloon to the mercy of the wind. Ma ny species of spiders effect the something by attaching themselves to an eminence by their claws, and after ejecting a suffi cient quantity of web, which is wafted on wards by the wind, they suddenly relax their hold and appear to spring into the air. By this method these animals are
transported from tree to tree, and from wood to wood in search of food. The cobwebs that are spread over the surface of the grass, and that offer so beautiful an object to the eye early in the summer's morning, through the brilliancy of the dew-drops formed and suspended on their silken threads, and the reflexion of the sun's rays from each, are the work of another species of spiders.
The different kinds of lice are exceed ingly numerous, almost everykind of ani mal having its particular sort of vermin. They are all carnivorous, or perhaps ra ther sanguivorous insects, living on the blood of other animals. Their eggs are all nits. The Scorpio-genus abound in hot climates, and are troublesome in ne glected places, and where cleanliness is not attended to. The crab tribes cast their shells every year, and are then in a soft, helpless state, unable to make resistance, and therefore at that time become the. prey of many kinds offish, when not pro vided with a guardian. It is a remarkable fact, that the edible crab of the United States, when in this state of imbecility, places itself, for security, under the pro tection of one ofits own species whose shell is hard and firm. This confidence is never misplaced ; his protector defends him against every assault at the risk of his own life or limbs ; and even when taken in the net, and thrown on shore, is still firmly held by his faithful companion. Their shells, but more especially those of the class testacea, afford a principal con stituent in the formation of chalk-beds, and beds of marl, which are formed at the bottom of the, sea. Specimens of entire shells are frequently met with in chalk pits which are now many miles inland ; and there is little doubt, that in a commi nuted state they form a principal ingredi ent in most calcareous earth. Under each genus will be found an account of a few of the more remarkable species. See