LOGY.
The CRUSTACEA are generally arranged as an order of insects. Dumeril has, however, arranged them in a separate class, (Traite Elementaire D' Histoire Naturclle, torn. ii.) and we think with great propriety, as their in ternal structure is more complicated than that of insects. Their articulated members arc at least ten in number ; they have usually a tail, and their body is in general co vered with a crustaceous shell ; they have rudiments of an internal ear ; they have teeth within the stomach, a complete circulating system, and they respire by means of gills. They lay eggs, but their young do not under go any remarkable metamorphosis in their progress to maturity.
They are distinguished into two orders, in the first of which the body is covered with a calcareous shell ; the eyes are moveable, and the mouth is provided with man dibles, each surmounted by a pa/id or feeler. These are the Canceres. In the second order, the body is general ly protected by a horny substance ; the eyes are im moveable, and the mandibles have no pall2i. These have been called Entromostacea. See CRUSTACEA.
In describing the general structure of INsEcrs it is necessary to remark, that most of them appear under three very different forms. When a butterfly, for ex ample lays her eggs, these, when hatched, produce a caterpillar or larva, having a soft long body, provided with several short feet, and a head furnished with jaws, and with several small legs. After the animal has ex isted for some time in this state, it assumes a very dif ferent form, and is changed into an oblong body, with out any distinct feet, and living without any apparent motion. This is called a chrysalid or nympha. After a
certain time, the shell that formed the peripheral cover ing of this ehrysalid bursts, and gives exit to an animal of an elegant appearance, furnished with several long jointed legs, two antenna, and wings. This is the per fect insect.
Cuvier, in imitation of Fabricius, distributes insects under two principal subdivisions, the first having jaws, the second having no jaws. He divides the first subdi vision into five orders or families, according to the num ber or structure of their wings, viz, Gnathaptera, that have no wings; Neuro/itera, that have four reticulated wings ; Hymenoptera, with four veined wings ; Coleop tera, having four wings, of which the upper or periphe ral are hard, and the lower or central transversely fold ed ; Orthoptera, having four wings of which the periphe ral are hard, and the central folded longitudinally. He divides the insects without jaws into four families, viz. Hemiptera, having four wings that are often found rest ing across each other, and an articulated beak ; Lepi doptera, having four scaly wings, and a trunk spirally twisted ; Diptera, having two wings ; and ?Ifitera, alto gether wanting wings.
This arrangement differs from that of Linne chiefly in being more minute and explicit. The aptera of the Swedish naturalist includes the gnathaptera of Cuvier, and the coleoptera of the former comprehends the or thoptera of the latter.
The anatomy of insects will be more minutely con sidered in the subsequent pages, and the differences of their external forms will be described under ENTOMO