[LETTER XLIX].
GEOGRAPHICAL DISTRIBUTION OF INSECTS; THEIR STATIONS AND HAUNTS; SEASONS; TIMES OF ACTION AND REPOSE.
Though no subject is more worthy of the attention of the Entomologist than the Geographical Distribution of insects, yet perhaps there is none connected with the science, for the elucidation of which he is furnished with fewer materials. The geographer of these animals sitting by his fireside, even supposing his museum as amply stored as that of Mr. MacLeay, and the habitats of its contents as accurately indicated, still labours under difficulties that are almost insuperable; so that it is next to impossible, with our present knowledge of the subject, to give satisfactory information upon every point which it includes. Had he the talents and opportunities of a Humboldt, and could, like him, traverse a large portion of the globe, he would endeavour to note the elevation, the soil and aspect, the latitude and longitude, the mean temperature and meteorological phænomena, the season of the year, the kind of country, and other localities connected with the insects he captured, and so might build his superstructure upon a sure basis. But these are things seldom registered by travellers that take the trouble to collect insects; who, if they specify generally the country in which any individual was found, think they have done enough. But to say that an insect was taken in India, China, New Holland, and North or South America,—when we consider the vast extent of those regions,—is saying little of what one wishes to know even with respect to its habitat. You must regard therefore, after all, what I have been able to collect,—and for which I am greatly indebted to the labours of my few but able precursors in this walk,—as merely approximations to an outline, rather than as a correct map of insect Geography.
Amongst the numerous obligations that he conferred upon Natural History, Linné was the first Naturalist who turned his attention to the Geographical Distribution of its objects, especially that of the Vegetable Kingdom[1458]: and the accomplished traveller Baron Humboldt, by the observations he made on this subject in the course of his peregrinations in tropical America, has furnished the Botanist with a clue which, duly followed, will enable him to perfect that part of his science; an end to which the learned observations of Messrs. R. Brown and Decandolle have greatly contributed[1459]. With regard to animals, Mr. White, so long ago as 1773, had observed that they, as well as plants, might with propriety be arranged geographically[1460]: and in 1778 Fabricius in his Philosophia Entomologica applied the principle to insects[1461]. Nearly forty years elapsed before any improvement or enlargement of this last department was attempted; when in 1815 M. Latreille, stimulated by what had been effected in Botany, in a learned and admirable memoir[1462] endeavoured to place Entomology in this respect by the side of her more fortunate sister: and subsequently Mr. W. S. MacLeay, in the memorable work so often quoted in our correspondence, has viewed the subject in another light, and added some important information to what had been before collected[1463].
The point now under consideration naturally divides itself into two principal branches;—the numerical distribution of insects, and the topographical.
I. By the numerical distribution of insects I mean not only the number which Providence has employed to carry on its great plan on this terraqueous globe, or any given portion of it; or of the species of which each group or genus may be supposed to consist; or of the comparative number of individuals furnished by each species,—points of no easy solution: but more particularly their distribution according to their functions, whether they prey upon animal or vegetable matter, and in its living or decaying state.
We have no data enabling us to ascertain with any degree of accuracy the actual number of species of insects and Arachnida distributed over the surface of the globe; but it is doubtless regulated in a great degree by that of plants. We should first then endeavour to gain some just though general notion on that head. Now Decandolle conjectures that the number of the species of plants, 60,000 being already known, may be somewhere between 110,000 and 120,000[1464]. If we consider with reference to this calculation, that though the great body of the mosses, lichens, and sea-weeds are exempt from the attack of insects, yet as a vast number of phanerogamous plants and fungi are inhabited by several species, we may form some idea how immense must be the number of existing insects; and how beggarly does Ray's conjecture of 20,000 species[1465], which in his time was reckoned a magnificent idea, appear in comparison! Perhaps we may obtain some approximation by comparing the number of the species of insects already discovered in Britain with that of its phanerogamous plants. The latter,—and it is not to be expected that any large number of species have escaped the researches of our numerous Botanists,—may be stated in round numbers at 1500, while the British insects, (and thousands it is probable remain still undiscovered,) amount to 10,000; which is more than six insects to one plant. Now though this proportion, it is probable, does not hold universally; yet if it be considered how much more prolific in species tropical regions are than our chilly climate, it may perhaps be regarded as not very wide of a fair medium. If then we reckon the phanerogamous vegetables of the globe in round numbers at 100,000 species, the number of insects would amount to 600,000. If we say 400,000, we shall perhaps not be very wide of the truth. When we reflect how much greater attention has been paid to the collection of plants than to that of insects, and that 100,000 species of the latter may be supposed already to have a place in our cabinets[1466], we may very reasonably infer that at least three fourths of the existing species remain undiscovered.
Certain groups and genera are found to contain many more species than others: for instance, the Coleoptera and Lepidoptera Orders than the Orthoptera and Neuroptera; the Rhincophora than the Xylophagi: the Dytiscidæ than the Gyrinidæ; Aphodius than Geotrupes; Carabus than Calosoma. Again, some insects are much more prolific than others. Thus the Diptera Order, though not half so numerous with respect to species as the Coleoptera, exceeds it greatly in the number of individuals, filling the air in every place and almost at every season with its dancing myriads. We rarely meet with a single individual of the most common species of Calosoma or Buprestis; whilst the formicary, the termitary, the vespiary, and the bee-hive send forth their thousands and tens of thousands; and whole countries are covered and devastated by the Aphides and the Locusts. An all-wise Providence has proportioned the numbers of each group and species to the work assigned to them. And this is the view in which the numerical distribution of insects is most interesting and important: and we are indebted to Mr. W. S. MacLeay for calling the attention of Entomologists more particularly to this part of our present subject.
With regard to their functions, insects may be primarily divided into those that feed upon animal matter and those that feed upon vegetable. At first you would be inclined to suppose that the latter must greatly exceed the former in number: but when you reflect that not only a very large proportion of Vertebrate animals, and even some Mollusca[1467], have more than one species that preys upon them, but that probably the majority of insects, particularly the almost innumerable species of Lepidoptera, are infested by parasites of their own class, sometimes having a different one appropriated to them in each of their preparatory states[1468], and moreover that a large number of beetles and other insects devour both living and dead animals,—you will begin to suspect that these two tribes may be more near a counterpoise than at first seemed probable. In fact, out of a list of more than 8000 British insects and Arachnida taken several years ago, and furnished chiefly by Mr. Stephens, I found that 3894 might be called carnivorous, and 3724 phytiphagous[1469]; so that, speaking roundly, they might be denominated equiponderant.