The third class of Arthropods (_Arachnida_) consists of the scorpions and spiders with their poor relations, the mites and tics, together with the very peculiarly-shaped _Pycnogonida_ (which present us with a good image of "no body"--being all legs and no body), and the singular worm-like parasite _Linguatula_. Lastly, we come to the most zoologically important and numerous of all the classes of Arthropods--namely, to the "class" of insects--_Insecta_. Therein we meet with the power of flight in its most perfect form--_i.e._, in the Dragon-flies--and most of the species are aërial in their adult (or _Imago_) condition. Some, however, are burrowers as, for example, the mole-cricket--an insect which presents some curious analogies in structure to the beast referred to in its name. Amongst insects may be mentioned the most familiar of all, the House-fly (which belongs to the order _Diptera_), and Beetles of all kinds (which constitute the order _Coleoptera_), some of which latter are luminous, as is the well-known glow-worm, and the exotic beetles _Pyrophorus_. Another order (_Orthoptera_) is made up of the earwigs, cockroaches, crickets, grass-hoppers, and their allies the locusts, with Bamboo-insects and the curious walking-leaf (so-called from their resemblance to a Bamboo twig and a foliage leaf respectively), the praying mantis, and other curious kinds. Bees and Ants, which belong to the order _Hymenoptera_, are, as every one knows, celebrated for their wonderfully complex instincts and community-life (which will occupy us later), and to the same order also belong the Ichneumon insects, which are provided with long appendages at the hinder ends of their bodies wherewith to pierce the bodies of animals in order to deposit their eggs within them, or to pierce the substance of plants, so producing "galls" which are structures of much interest from several points of view. Butterflies and Moths form another order of insects called _Lepidoptera_, amongst which may be mentioned as (having to be referred to hereafter) the true butterflies (_Papilio_), and the hawkmoths (some of which in their flight so much resemble Humming-birds), the clear-wing moths, and those moths the grubs of which are known as "silk-worms," and certain moths of the genera _Solenobia_ and _Psyche_. The numerous group of bugs is allied to the plant-lice (_Aphides_), which so often infest our Pelargoniums when kept in dwelling-rooms. Allied to them, again, are the small creatures the nature of which was so long disputed, though familiar to commerce as "Cochineal." Really, they are small, singularly inert, plant-lice, which adhere to the surface of certain "Cacti." The Dragon-flies, before referred to, are the types of the order _Neuroptere_.