Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/153

141 INSECTS rriHE INSECTA, or INSECTS, form the largest class of that JL division of the animal kingdom formerly called Articu- lata, but for which the more expressive term Arthropoda (joint-footed) is now more generally employed. This term includes, besides Insecta, the classes Crustacea, Arachnida, and Myriopoda. The chief diagnostic characters of an Insect, as com- binedly distinguishing it from a Crustacean, an Arachnid, or a Myriopod, are as follows : Legs usually (never more than) six in number ; two antennye ; ordinarily two pairs of more or less membranous wings ; head, thorax, and abdomen distinctly separated ; respiration effected by means of internal tracheae, which communicate with the air by lateral openings termed spiracles or stigmata, or by external plates or filaments (these ordinarily only in the preparatory conditions of aquatic forms), which absorb air and convey it to the trachea?. A reference to the articles on the other classes of Arthropoda will indicate in what way these diagnostic points are modified in them. As in all organized beings, the limits of the class are not strongly defined, for, although it is not difficult to indicate an insect, speaking broadly, there are certain small groups that do not satisfactorily fall into the class as limited by strongly marked lines of demarcation. These will be especially alluded to hereafter. Number of Species of Insects. At the head of this article it is stated that the Insecta form the largest group of the Arthropoda ; it might probably be said with justice that they far outnumber all the other members of the animal kingdom combined. It is certain that at the present time 80,000 presumably distinct species of beetles have been described, and it is safe to assume that the number of known species of other orders is greater, thus giving a total of about 200,000. And yet we are only on the threshold of a knowledge of the forms that actually exist in nature, many enormous groups of minute forms being still only very partially studied. In fact, it may be confidently anticipated that some day the number of known forms will not fall far short of 1,000,000. Antiquity of Insects. Fossil indications have been dis covered in the Devonian series, and in the Carboniferous they become rather more numerous ; but, with few exceptions, these all belong to those orders in which the metamorphoses are incomplete, and there is no evidence that any antho- philous insects (such as Lepidoptera or Hymenoptera) were then in existence. Ascending the geological scale to the Mesozoic age, the representatives of the older groups become very numerous, and often of gigantic size. Coleoptera are fairly well indicated; and the flower-loving Lepidoptera and Hymenoptera make their appearance, but in very small numbers. In the Tertiary rocks remains become sometimes very abundant, and of all orders ; and in the post-Tertiary or Quaternary period these remains consist largely of those of species now existing. One of the most interesting features in fossil entomology is the well-known occurrence of myriads of insects entombed in the fossil resin known as amber, preserved in the most beautiful manner, and belonging for the most part to genera now existing, but differing specifically. In alluding to this it is well to mention that the insects found in gum copal and other recent resins are, on the contrary, of existing species. As in other animals, and also plants, the fossil remains prove that the distribution of heat and cold on the earth was once very different from what it now is : a fossil beetle of rather large size was discovered by our last Arctic Expedition almost at the highest northern point attained. Geographical Distribution. It may be asserted that no part of the earth s surface is without insects. They have been discovered in the Arctic and Antarctic regions at the highest point reached, and even showy butterflies of several species enliven the dreary solitudes of almost everlasting ice, as was abundantly proved by the naturalists of the "Alert" and "Discovery," who found them almost up to 83 N. lat. But, as a rule, the larger and more brilliant forms occur within the tropics. Yet it must not be assumed, as is sometimes erroneously done, that the majority of tropical insects are large and brilliant, and the smaller and more obscure forms comparatively less numerous. Recent investigations by competent observers show that the latter are at least as abundant in the tropics as in temperate regions, and that it is the wealth of large forms that has caused the others to be overlooked. The attempts at subdivision of the globe into zoological regions, so successful with regard to mammals, and in a smaller degree with birds, have not been so entirely satis factory with regard to insects, more especially as concerns the separation of the Pakearctic and Nearctic regions (see DISTRIBUTION) ; still there is often a very marked local ization in particular groups, which divide themselves specifically to an infinite extent within very circumscribed areas, and are found nowhere else. The results obtained from minute investigation of insular faunoe have derived much of their value from insects, and have occasioned much valuable philosophical speculation on the origin both of the islands themselves and of their faunae and flora?. Space will not permit of detailed allusion to the apparent affinity shown by the insect inhabitants of regions now very widely separated, such, for instance, as that of Western Europe with Western (rather than Eastern) North America, of Australia and New Zealand with Chili, of Chili and the southern extremity of South America with the Palaearctic region, &c. As special points of distribution may be mentioned the occurrence of insects in hot springs, in brine, in the deepest caves (these are usually blind), below low-water mark, and even on the surface of the ocean (the genus Halobates in the Hemiptera) very far from land. The power of many insects to acclimatize themselves rapidly when accidentally introduced into new regions is very marked, and adds to the difficulty often experienced in considering what species are really endemic and what introduced, especially in islands. Some of the common and noxious British species thrive enormously when introduced into Australia and New Zealand; and there is every reason to believe that the grape-vine pest (Phylloxera) was origin ally an importation from America. Duration of Life. The maximum duration of the life of a perfect insect is probably attained in bees and ants, the females or queens of which are known to live at least seven years; the minimum is found in some species of May-flies (Ephemeridx), in which twenty-four hours is perhaps the limit. But the length of life of a perfect insect is sometimes in direct opposition to that of the same insect in its preparatory stages, and some of the Ephcmeridx that live at most but a few days in their aerial form have taken three years to complete their growth in their sub- aquatic stages. Temperature also has a marked effect on some species. The common house-fly, for instance, will complete its whole life cycle from embryo to fly in a few days in the heat of summer, but requires very much longer in cold weather. Economic Entomology. Within the limits of an encyclo-