1. M. Latreille has observed, that where the empire of Flora ceases, there also terminates that of Zoology[1486]. Phytiphagous animals can only exist where there are plants; and those that are carnivorous and feed upon the former, must of necessity stop where they stop. Even the gnat, which extends its northern reign so high[1487], must cease at this limit; while, where vegetation is the richest and most abundant, there the animal productions, especially the insect, must be equally abundant. I call that, therefore, a predominant group, members of which are found in all the countries between these points, or from the limits of animal-depasturing vegetation in the polar regions to the line.
Generally speaking, the carnivorous insects, whether thalerophagous or saprophagous, are of this description. Calosoma, which devours Lepidopterous larvæ, though poor in species and individuals, is widely scattered. Captain Frankland found C. calidum in his Arctic journey; C. laterale and curvipes inhabit tropical America[1488]: C. Chinense, as its name indicates, is Chinese[1489]; Mr. MacLeay has an undescribed species from New Holland; and C. retusum was taken in Terra del Fuego. Another genus, equally universal and richer in numbers, is the lady-bird (Coccinella), which keeps within due limits the Aphides of every climate from pole to pole. The Libellulina pursue their prey both in Greenland and New Holland. The saprophagous carnivora are also similarly predominant;—the Silphidæ, the Dermestidæ, the Brachyptera, the Muscidæ, prey on carcases wherever the action of the solar beam causes them to become putrid. Many of the above insects have probably their capital station, or that where the species are most numerous, in or near the tropics; but the metropolis of the Brachyptera, at least as far as we can judge from our present catalogues, is within the temperate zone, particularly in Britain[1490]. The coprophagous Petalocera are most abundant in the hottest climates; but the Aphodiadæ form a predominant group: Professor Hooker took one species in Iceland[1491], and it probably ascends higher; others are found in India and China: but the metropolis of the group is within the temperate zone. Perhaps no genus is more completely universal than Bombus (Bremus Jur.), which, although its centre or metropolis is likewise in the northern temperate zone, extends from Melville Island to the line. It is remarkable that some of the tropical Bombi wear the external aspect of Xylocopæ, the kindred genus most prevalent in warm climates; and, vice versâ, some Xylocopæ resemble Bombi. I have a Brazilian undescribed species of the latter genus, whose black body and violet-coloured wings would almost cause it to be mistaken for a variety of X. violacea; and B. antiguensis and caffrus F., (though their aspect belies it,) which misled Fabricius, are true Xylocopæ. I shall mention only one other predominant group, but that one of no common celebrity, formed of the gnats, or genus Culex. These piping pests, with their quiver—"venenatis gravida sagittis"—annoy man almost from the pole to the line. What remarkably distinguishes them, (as was formerly observed[1492],) and also the Simulium or true mosquito,—they appear to prevail most in the coldest and the hottest climates, and the Laplander and the tropical American are equally their prey; while the inhabitants of the temperate zone, with some exceptions, suffer but little from them: so that they may be stated to have both an arctic and a tropical metropolis.
2. There are other groups which, though their empire extends to the tropics, fall short of the polar circles:—these I call dominant groups. Of this description are some of the Scarabæidæ. Onthophagus is found both in the old world and in the new, and in the temperate and torrid zones. Its principal seat appears to be within the tropics, but it may almost be said to have also a northern metropolis. More than one species have been taken in New Holland. In general, tropical insects exceed those of colder climates in size; but in the genus we are speaking of, the European species are usually larger than the Indian. Copris seems more abhorrent of cold than its near relation Onthophagus. C. lunaris, which ranges northward as far as Sweden, is the only recorded species found in Europe out of Spain. Latreille says, that all the large species of this genus are equinoctial: but C. Tmolus, described and figured by Fischer[1493], found in Asia near Orenburg, north of 50° N. L., is as big as C. Gigas or bucephalus. Another dominant group of Petalocera, remarkable for the bulk and arms of its tropical species, are the mighty Dynastidæ, the giants and princes of the insect race. Though their metropolis is strictly tropical, yet the scouts of their host have wandered even as far as the south of Sweden, where one of them, Oryctes nasicornis, is extremely common. O. Grypus[1494] and some other species are found in South Europe; but though in a torpid state they can endure unhurt the severity of a Scandinavian winter, they cannot when revived stand the cold that often pinches Britons in the midst of summer, and therefore are unknown in our islands[1495]. The Sphæridiadæ, whose metropolis is within the northern temperate zone, extend from thence beyond the line, since Dr. Horsfield found two species in Java[1496]. It is probable, indeed, that this group is predominant. Some dominant groups begin at a lower latitude. Of this description are the carpenter-bees (Xylocopa), whose larvæ are preyed upon by that of the Horiadæ[1497] under two forms, which extend from the tropics to about 50° N. L. Others are not common to both worlds. Thus, while Cantharis is the gift of Providence to America as well as the old world, Mylabris is confined to the latter, where its range is very extensive;—in Europe, from South Russia to Italy and Spain; in Asia, from Siberia to India; and in Africa, from the shores of the Mediterranean to the Cape of Good Hope; which last continent, to judge from our present lists, especially the vicinity of the Cape, may be called the metropolis of the group[1498]. On the other hand, the Rutelidæ and Chlamys, which have a range from Canada to the tropics, (within which is their metropolis,) are purely American groups. Many more might be named under this head, but these will suffice for examples.
3. I call those subdominant groups, which either never enter the tropics, or those tropical ones whose range does not exceed 50° of N. L. in the old world, or 43° in the new. I make this difference because, as M. Latreille observes, the southern insects which in Europe begin between 48° and 49° N. L., in America do not reach 43°.[1499] But though the winters in Canada, within the same parallel as France, are longer and more severe than those even of Great Britain or of Germany, yet the summers are intensely hot; so that though tropical species do not range so high, those of a tropical structure, as Mr. W. S. MacLeay has intimated[1500], may be found at a higher latitude in the new world than in Europe.
The genus Melöe affords an instance of a subdominant group of the first description. It ranges from Sweden to Spain and the shores of the Mediterranean, and seems a tribe almost confined to Europe, where it is not very unequally distributed. Of registered species Britain possesses the largest proportion; but Mr. W. S. MacLeay is of opinion that Spain is its true metropolis[1501]. I have a species of this genus, taken in North America by Professor Peck. The splendid genus Carabus ranges still further north than Melöe[1502]. A very fine species (C. cribellatus) inhabits the polar regions of Siberia[1503]; but the metropolis of the group appears to be the temperate zone: some, however, have been found in northern Africa; and Sir Joseph Banks captured one in Terra del Fuego. Of those whose range is between the tropics and 50° N. L. we may begin with Cicada. One species, indeed, has been found by Mr. Bydder and others, a little higher, near the New Forest, Hampshire. We may take Scolia for an example of a subdominant group beginning more southward. Its species first appear about 43° N. L., and abound in warm climates. In general most of those insects which M. Latreille denominates meridional,—such as Scarabæus, Onitis, Brentus, Scarites, Mantis, Fulgora, Termes, Scorpio, &c.—come under the present head, and in fact all tropical forms that wander to any distance within the above limits from their metropolis.
4. By quiescent groups I mean those that have none, or no high range as to latitude, from their centre or metropolis. I say as to latitude, because these groups have often an extensive one as to longitude. Thus, Mr. W. S. MacLeay has remarked to me, that Goliathus appears to belt the globe, but not under one form. The types of the genus are the vast African Goliaths (G. giganteus, &c.), which, as well as G. Polyphemus, and another brought from Java by Dr. Horsfield, have, like Cetonia[1504], the scapulars interposed between the posterior angles of the prothorax and the shoulders of the elytra[1505]: while the South American species (G. micans, &c.) have not this projection of the scapulars; in this resembling Trichius. Mr. MacLeay further observes, that the female of the Javanese Goliathus is exactly a Cetonia, while that of the Brazilian is a Trichius. But quiescent groups have not generally this ample longitudinal range. Thus, Euglossa, in both its types,—one represented by Eu. cordata, and the other by Eu. surinamensis,—is confined to the tropical regions of America. Doryphora, likewise American, seems equally confined. Asida, though a southern genus, is not found to enter the tropics; and Manticora and Pneumora are in nearly the same predicament.
Under the present head we may consider what may perhaps be denominated without much impropriety endemial groups; by which I mean those groups that are regulated, as to their limits, not so much by the temperature, or the northing and southing of the latitude, as by the general aspect and circumstances of the country. Thus, the vast and nearly insular continent of Africa, almost as wide as it is long, and situated in or near the tropics, instead of inland seas or sea-like rivers, is intersected by parched sandy deserts, extending far and wide; circumstances which, though in the vicinity of its streams it is humid, impart an unusual degree of aridity as well as heat to its general atmosphere; so that it well merits the poet's epithet, Leonum arida nutrix; and is also peculiarly fitted for all such animals, especially insects, as delight in a dry, sandy, hot country, particularly such as are predaceous in their habits. America, on the other hand, exhibits quite an opposite character. It is long, and comparatively narrow; surrounded, and almost divided into two continents, by immense circumfluent oceans; watered every where by rivers and lakes that emulate seas: in some parts covered by interminable forests; in others, intersected by ridges of the loftiest mountains. These circumstances, except in its Llanos, Pampas, or table-land, give a general character of humidity to its atmosphere, and fit it particularly for the production of a vast variety of peculiar plants, and for the residence of numerous and peculiar phytiphagous insects and other animals[1506]. Midway between these two continents lies a third (for so the vast island of New Holland may be denominated), which presents new features in its general aspect, and consequently new forms both in its Flora and Fauna, mixed with many old ones parallel to those both of the new world and the old. Perhaps Europe and Asia, with several that are peculiar, agree more in their animal productions than the continents just described.
Let us next particularize a few of the peculiar types that distinguish particular continents and countries. The genera Manticora, Graphipterus, Glaphyrus, Eurychora, Pneumora, Masaris, and many others, are peculiar to Africa. In Asia alone we find Mimela[1507], Euchlora[1508], Colliuris, Catascopus[1509], Apogonia[1510], a peculiar type of Horia, &c. In America, Agra, Galerita, Nilio, another type of Horia, Tetraonyx, Rutela, Doryphora, Alurnus, Erotylus, Scotinus[1511], Cupes, Corydalis, Labidus, Heliconius, Castnia, &c. And in New Holland, Helluo, Elephastomus, Anoplognathus, Diphucephala[1512], Cerapterus, Heleus, Adelium, Notoclea, Achilus, Thynnus, Nycterobius, &c.
The countries bordering upon the Mediterranean, the Black, and the Caspian seas, agree in producing similar insects. These countries, and the Cape of Good Hope though so distant from them, appear to be the principal seat of Heteromerous Coleoptera, of the genera Lixus and Brachycerus, and of the conical Buprestes[1513]. But the insects of Guiana, on one side the Cordilleras, differ from those of New Granada and Peru on the other; and similar differences are observed in other neighbouring countries separated by natural boundaries.
iii. Another head connected with the topographical distribution of insects relates to their representation of each other. Here we may observe, that some insects represent each other only in their form; others also in their function; and others in both. I shall give some instances of each. In Brazil there is a group of petalocerous beetles (Chasmodia), one of the Rutelidæ, which in New Holland has a representative, as to form, in one of the Cetoniadæ (Schizorhina[1514]), which, having soft mandibles, must have a different function:—it is to be observed, however, that these insects appear to approach each other in the series of affinities. Again, the Carabidæ may in the same country be said to have a representative in the remarkable heteromerous genus Adelium[1515], which is altogether an analogy. Others are representative only in their function. The general function of insects is to remove nuisances and to check redundances,—the saprophagous tribes do the one, and the thalerophagous the other. In going from the poles to the line,—in proportion as the heat increases, the quantum of work of both kinds increases; and new forms are either added to the old ones, so as to increase their momentum; or new ones, more powerfully talented, replace the old ones, and act in their stead: thus we see a gradual and interesting change take place in proportion as we approach the maximum of heat and of insect population. At the Cape, the universal Cicindelæ are aided by Manticora; in North America, the Silphidæ by a new group, the type of which is Silpha Americana (Necrophila, K.MS.); in South America, Copris by Phanæus. Again: Colliuris and Drypta of the old world, in the new give place to Ctenostoma and Agra. The honey and wax of Europe, Asia, and Africa, is prepared by bees congenerous with our common hive-bee (Apis); while in America this genus is not found as a native, but is replaced by Melipona and Trigona[1516]; and in New Holland by a still different but undescribed type. The Melolonthidæ and Rutelidæ of the old and new world appear to have their work done in that country by the brilliant and numerous Anoplognathidæ. The Rhipicera of Brazil is of a different type from that of New Holland. The singular genus Cremastocheilus of North America has its representative in Africa in Genuchus[1517]. The Lucani of the rest of the world give place in New Holland to Lamprima and Ryssonotus.—I could produce a much greater number of examples, but these are sufficient to explain my meaning.