Part 34

[1457] It may not be unprofitable here to mention those works which the Entomologist may find it most useful to consult in various departments of the science. For descriptions of the _Genera_ and _Species_ of insects in general, he must have recourse to the _Entomologia Systematica emendata et aucta_ of Fabricius, and its _Supplement_; to the volumes he subsequently published under the titles _Systema Eleutheratorum_, _Rhyngotorum_, _Glossatorum_, _Piezatorum_, and _Antliatorum_; to the _Genera Crustaceorum et Insectorum_ of Latreille; to the same department of the _Règne Animal_ of Cuvier; and to the _Animaux sans Vertèbres_ of Lamarck. He will find the genera of Linné and Fabricius illustrated by _figures_, in Rœmer's _Genera_; and many of the species described by the latter in Coquebert's _Illustratio Iconographica_. In our countryman Drury's beautiful _Illustrations of Natural History_, a large number of new and rare insects are depicted; and in Mr. Donovan's _Insects of China, India, and New Holland_, some of the most brilliant and interesting that have been imported from those countries. Panzer's _Faunæ Insectorum Germanicæ Initia_ has little short of 3000 figures of insects of every Order (a considerable number of which are found to inhabit Britain), by the celebrated Sturm; and the latter, in his _Deutschlands Fauna_, has illustrated many Coleopterous genera analytically (as has also M. Clairville the weevils and Predaceous beetles of Switzerland in his _Entomologie Helvétique_) by his admirable pencil. Beetles in general are well figured and described in Olivier's splendid _Entomologie_; as are those of Europe in a beautiful work now in course of publication, under the title of _Coleoptères d'Europe_, by MM. Latreille and Dejean. The latter author has also begun a work on this Order under the title of _Species général des Coléoptères de la Collection de M. Le Comte Dejean_; two volumes of which have appeared, containing part of the _Carabici_ Latr. but I fear it has stopped for want of encouragement. Had the descriptions been less verbose it would have had a better chance of success. For the _Orthoptera_ and _Hemiptera_, the student must have recourse to Stoll's _Spectres_, _Mantes_, _Sauterelles_, _Grillons_, _Blattes_, _Cigales_, and _Punaises_. To a knowledge of the species of _Lepidoptera_, the admirable figures of Cramer (_Papillons Exotiques de trois Parties_ _du Monde_), Esper (_Schmetterlinge_, _Tagschmetterlinge_), Hübner (_Schmetterlinge_, &c.), and Ochsenheimer's valuable _Schmetterlinge von Europa_, with the continuation by Treitschke, will afford a useful avenue. Meigen also, author of a most valuable work on the Europæan _Diptera_, is publishing at this time a work on _Lepidoptera_ under the title of _Europäische Schmetterlinge_. To the _Hymenoptera_ Jurine and Christian are the best guides, and to the _Diptera_ Meigen.

With regard to works in British Entomology in general--Donovan's _Natural History of British Insects_, and Samouelle's _Entomologist's Useful Compendium_, will be found very excellent helps to the student. For the British Genera, the most important work that has yet appeared is Mr. John Curtis's _British Entomology_, in which not only are the insects admirably represented, but their trophi correctly delineated, accompanied by able descriptions. For the _Coleoptera_ of our country, Mr. Marsham's _Entomologia Britannica_ should be consulted: for the _Lepidoptera_, the _Butterflies_ of Lewin, Mr. Haworth's useful _Lepidoptera Britannica_, and Miss Jermyn's _Butterfly-Collector's Vade Mecum_; and for the English species of Linné's genus _Apis_, the _Monographia Apum Angliæ_. A British _Fauna Insectorum_, under the title of _Illustrations of British Entomology_, has at length been happily begun by a gentleman (J. F. Stephens, Esq.) who both by his accurate knowledge of the subject, and the extent of his collection of British Insects, is best qualified to undertake it. As far as it has proceeded, it is ably executed, and possesses this advantage, (an advantage seldom to be obtained in works published periodically,) that it finishes, as far as possible, as it goes.

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.

Carnivorous and phytiphagous insects may be further subdivided according to the _state_ in which they take their _food_,--whether they attack it while _living_, or not till after it is _dead_. To adopt Mr. W. S. MacLeay's phraseology, the former may be denominated _thalerophagous_, and the latter _saprophagous_. The British saprophagous _carnivorous_ insects, compared with those that are thalerophagous, are about as 1:6; while the phytiphagous ones are as 1:9. The _thalerophaga_ in both tribes may be further subdivided as they take their food by _suction_ or _mastication_: in the _carnivorous_ ones, the suckers to the masticators in Britain are nearly as 1:6; but with respect to the _phytiphagous_ tribe you must take into consideration that some insects imbibing their food by _suction_ in their _perfect_ state (as the great body of the _Lepidoptera_), _masticate_ it when they are _larvæ_: deducting therefore from both sides the insects thus circumstanced, the masticators will form about _three fourths_ of the remaining British thalerophagous insects. Another circumstance belonging to this head must not be passed without notice:--there are certain insects feeding upon liquid food that do not _suck_, but _lap_ it. This is the case with the _Hymenoptera_, who, though they are mandibulate, generally lap their food (the nectar of flowers) with their tongue, and may be called _lambent_ insects: nor is this practice confined to that order, but all the mandibulate insects that feed on that substance merit the same appellation. The absorption of this nectar is so important a point in the economy of nature, that a very large proportion of the insect population of the globe in their perfect state, are devoted to it. Considerably more than half the species indigenous to Britain fulfill this function, and probably in tropical countries the proportion may be still larger.

To push this analysis still further--Amongst our carnivorous thalerophaga, _aphidivorous_ insects are about as 1:14; and amongst the phytiphagous, the _fungivorous_ ones form about a _twentieth_; and the _granivorous_ about a _twenty-fifth_ part of the whole. Again: in the _saprophaga_ the _lignivorous_ tribes form more than _half_, and the _coprophagous_ ones more than a _third_.

If you wish to know further the relative proportions of the different _Orders_ to each other--The _Coleoptera_ may be stated as forming at least 1:2 of our intire insect population; the _Orthoptera_ and _Dermaptera_ as about 1:160; the _Hemiptera_ as 1:15; the _Lepidoptera_ as more than 1:4; the _Neuroptera_ with the _Trichoptera_ as 1:29; the _Hymenoptera_ as about 1:4; the _Diptera_ as not 1:7; and the _Aptera_ and _Arachnida_ as perhaps amounting to 1:19[1470].

To extend this inquiry to _exotic_ and more particularly to _extra-European_ insects, in the present state of our knowledge, would lead to no very satisfactory results. The lists we have are so imperfect, that those which tell most in this country,--I mean the more minute insects and the _Brachyptera_--have hitherto formed a very small, if any part, of the collections made _out_ of Europe. Mr. W. S. MacLeay however, who, besides his father's (particularly rich in _Petalocera_), has had an opportunity of examining the Parisian and other cabinets, finds that the species of _coprophagous_ insects _within_ the tropics, to those _without_, are nearly in the proportion of 4:3; and that the coprophagous _Petalocera_, to the remainder of the saprophagous ones, may be represented by 3:2[1471]. It may be inferred, from the superabundance of plants and animals in equinoctial countries, that the number of species of insects in general is greater within than without the tropics: the additional momentum produced by the vast size of many of the tropical species must also be taken into consideration.

II. There are three principal points that call for attention under the _second_ branch of our present subject--the _topographical_ distribution of insects; namely, their _Climates_, their _Range_, and their _Representation_.

i. Entomologists, taking _heat_ for the principal regulator of the station of insects, have divided the globe into entomological _climates_. Fabricius considers it as divisible into _eight_ such climates, which he denominates the _Indian_, _Egyptian_, _Southern_, _Mediterranean_, _Northern_, _Oriental_, _Occidental_, and _Alpine_. The first containing the tropics; the second, the northern region immediately adjacent; the third, the southern; the fourth, the countries bordering on the Mediterranean sea, including also _Armenia_ and _Media_; the fifth, the northern part of Europe interjacent between Lapland and Paris; the sixth, the northern parts of Asia where the cold in winter is intense; the seventh, North America, Japan, and China; and the eighth, all those mountains whose summits are covered with eternal snow[1472]. M. Latreille objects to this division, as too vague and arbitrary and not sufficiently correct as to temperature; and observes, with great truth, that as places where the temperature is the same, have different animals, it is impossible, in the actual state of our knowledge, to fix these distinctions of climates upon a solid basis. The different elevations of the soil above the level of the sea, its mineralogical composition, the varying quantity of its waters, the modifications which the mountains, by their extent, their height, and their direction, produce upon its temperature; the forests, larger or smaller, with which it may be covered; the effects of neighbouring climates upon it,--are all elements that render calculations on this subject very complicated, and throw a great degree of uncertainty over them[1473]. This learned Entomologist would judiciously consider entomological climates under another view,--that which the genera of _Arachnida_ and insects _exclusively_ appropriated to determinate spots or regions would supply[1474]. Linné's _dictum_ with regard to genera will here also apply; "Let the insects point out the climate, and not the climate the insects." If you expect invariably to find the same insects within the same parallels of latitude, you will be sadly disappointed; for, as our author further observes, "The totality or a very large number of _Arachnida_ and insects, the temperature and soil of whose country are the same, but widely separated, is in general, even if the countries are in the same parallel, composed of different species[1475]." The natural limits of a country,--as mountainous ranges, rivers, vast deserts, &c.,--often also say to its insect population, "No further shall ye come;" interposing a barrier that it never passes[1476]. Humboldt observes, with respect to the _Simulia_ and _Culices_ of South America, that their geographical distribution does not appear to depend solely on the _heat_ of the climate, the excess of humidity, or the thickness of forests; but on local circumstances that are difficult to characterize[1477]: and Mr. W. S. MacLeay makes a similar observation upon that of _Gymnopleurus_[1478]. So that the real insect climates, or those in which certain groups or species appear, may be regarded as fixed by the will of the CREATOR, rather than as certainly regulated by any _isothermal_ lines. Still, however, under certain limitations, it must be admitted that the temperature has much to do with the station of insects. The increase of caloric is always attended with a proportional increase in the number and kind of the groups and species of these beings. If we begin within the polar regions of ice and snow, the list is very meager. As we descend towards the line, their numbers keep gradually increasing, till they absolutely _swarm_ within the tropics. Something like this takes place in miniature upon _mountains_. Tournefort long since observed at the summit of Mount Ararat the plants of Lapland; a little lower, those of Sweden; next, as he descended, those of Germany, France, and Italy; and at the foot of the mountain, such as were natural to the soil of Armenia. And the same has been observed of insects. Those that inhabit the _plains_ of _northern_ regions have been found on the _mountains_ of more _southern_ ones; as the beautiful and common Swedish butterfly _Parnassius Apollo_, on the mountains of France, and _Prionus depsarius_ on those of Switzerland[1479].

M. Latreille, having given a rapid survey of the peculiar insect-productions of different countries, next attempts a division of the globe into _climates_, which he thinks may be made to agree with the present state of our knowledge, and be even applicable to future discoveries. He proposes dividing it primarily into _Arctic_ and _Antarctic_ climates, according as they are situated _above_ or _below_ the equinoctial line; and taking twelve degrees of latitude for each climate, he subdivides the whole into _twelve_ climates. Beginning at 84° N. L. he has _seven Arctic_ ones, which he names _polar_, _subpolar_, _superior_, _intermediate_, _supratropical_, _tropical_, and _equatorial_: but his _antarctic_ climates, as no land has been discovered below 60° S. L., amount only to _five_, beginning with the _equatorial_ and terminating with the _superior_. He proposes further to divide his climates into _subclimates_, by means of certain _meridian lines_; separating thus the _old_ world from the _new_, and subdividing the _former_ into two great portions,--an _eastern_, beginning with _India_, and a _western_, terminating with _Persia_. He proposes further that each climate should be considered as having 24° of longitude, as well as 12° of latitude[1480]. In this chart of insect Geography he states that he has endeavoured to make his climates agree with the actual distribution of insects[1481]; and it should seem that in many cases such an agreement actually does take place: yet the division of the globe into climates by _equivalent_ parallels and meridians, wears the appearance of an artificial and arbitrary system, rather than of one according with nature.

He has also pointed out another index to insect climates, borrowed from the _Flora_ of a country. Southern forms in Entomology, he observes, _commence_ where the _vine_ begins to prosper by the sole influence of the mean temperature; that they are _dominant_ where the _olive_ is cultivated; that species still _more_ southern are compatriots of the _orange_ and _palmetto_; and that some _equatorial_ genera accompany the _date_, the _sugar-cane_, the _indigo_ and _banana_[1482]. The idea is very ingenious, and, under certain limitations, supplies a useful and certain criterion. For though none of these plants are _universal_ in isothermal parallels of latitude; yet, as plants are more conspicuous than insects, the Entomologist, furnished with an index of this kind, may by it be directed in his researches for them; and in all countries in which there is a material change of the climate, as in France, there will be a proportional change in the _vegetable_ accompanied by one in the _insect_ productions.