5. That they do not respire air by the mouth, but that they inspire and exhale it by means of organs which are placed on the body. 6. That they move the jaws from right to left, not up and down. 7. That they have neither eye-lid nor pupil. To these we may also add, that the mechanism resulting from the LIFE of insects is not of so compound a nature as in animals of a larger size. They have less variety of organs, though some of them are more multiplied; and it is by the number and situation of these that their rank in the great scale of beings is to be determined. These characters are often united in the same insect; there are, however, some species in which one or two of them are wanting. The student in entomology, who wishes to attain a proper knowledge of the science, and indeed every microscopic observer, desirous of availing himself of the discoveries of others, and of communicating intelligibly his own, will find it necessary to make himself conversant with the various classes, genera, &c. into which insects have been divided by Linnæus. Every system has its defects, and probably some may be found in that of this truly celebrated naturalist, but the purpose of science is answered by using those discriminations which are generally adopted. The following general idea of the Linnæan classes may serve as a foundation for this knowledge: a more particular account may be obtained by consulting the under-mentioned works. Institutions of Entomology, a translation of Linnæus’s Ordines et Genera Insectorum, or systematic arrangement of insects, &c. by Thomas Pattinson Yeats. Fundamenta Entomologiæ, or an Introduction to the Knowledge of Insects, translated from Linnæus by W. Curtis, the ingenious author of Flora Londinensis, the Botanical Magazine, &c. The Genera Insectorum of Linnæus, exemplified by various Specimens of English Insects, drawn from Nature, by James Barbut.[62] [62] This work contains two excellent plates, illustrative of the Distinctions of the Ordines and Genera Insectorum, by their antennæ, tarsi of the feet, &c. EDIT. Class the first. COLEOPTERA. The insects of this class have four wings; the upper ones, called the elytra, are crustaceous, being of a hard horny substance; these, when shut, form a longitudinal suture down the back, as in the scarabæus, melolontha, or cockchaffer, &c. 2. HEMIPTERA. These have also four wings; but the elytra are different, being half crustaceous, half membranaceous: the wings do not form a longitudinal suture, but extend the one over the other, as in the gryllus, grasshopper, &c. 3. LEPIDOPTERA. Those which have four membranaceous wings covered with fine scales, appearing to the naked eye like powder or meal, as in the butterfly and moth. 4. NEUROPTERA. These have four membranaceous transparent wings, which are generally reticulated, the tail without a sting, as in the libellula, or dragon fly.