CHAPTER IV.

ORGANS AND CHARACTERS OF VARIATION.

Having in the preceding chapter briefly alluded to some of the principal causes by which the outward aspect of the insect tribes would seem to be in a large measure (though within definite specific limits) regulated, it may perhaps be desirable to gather into a small compass, from those remarks, what the chief organs and characters are which appear to be more peculiarly beneath the control of the various influences which we have been just discussing. To imagine that when an insect has become much altered in its general contour, all the parts of which it is composed are equally affected, is contrary to experience; since observation warns us that there are but few actual _members_ which are capable of change,--whilst even the external features, or secondary diagnostics, are only interfered with according to a fixed law, the workings of which are necessarily modified, in proportion as the constitutions of the several animals are differently organized and acted upon.

As regards positive structure, indeed, we can have but few observations to communicate,--seeing that the limbs and appendages themselves are usually of so constant a nature, that disturbing agencies have little or no power to divert them from their typical states. Still, there are occasional facts on record, which would tend to prove that even these are not altogether exempt from the deranging force of certain contingencies from without: the number of the antennal joints, for instance, in the tribes where those organs are multiarticulate, is said to vary; but how far this may be dependent on physical influences, I am not in a position to decide. The connateness of the elytra, again, is a character which we may at any rate define as _sub_-structural; and this I have myself noticed, at times, to fluctuate, according to the circumstances and conditions of the respective localities in which the particular species obtain. Such is eminently the case with the universal _Harpalus_ (the _H. vividus_, Dej.) of the Madeiran Group. Speaking of this peculiarity, in my volume on the Coleoptera of those islands, I made the following remarks: "But perhaps its most singular character, and in which it differs from every other _Harpalus_ with which I am acquainted, consists in the tendency of its elytra to become united or soldered together. I say 'the tendency,' because it is not always the case that they are joined (which, since the law exists at all, is perhaps the more remarkable), although in most instances, especially in localities much exposed and but slightly elevated above the sea-shore, they are. I have examples, however, from the upper as well as the lower regions, in which both states are represented; and others again in which the elytra are only partially connected, being free at the apex though firmly attached towards the scutellum. In every instance, however, even where they are united throughout their entire length, a little force will succeed in separating them, showing their structure, as I have indicated in the diagnosis, to be _sub_-connate rather than connate. But that it does require force to effect the disjunction, when they are really in the condition described, is proved to a demonstration to any one who has seen the _remains_ of the insect beneath the slabs of stone on many of the small adjacent islands where it most abounds, or drifting about over the surface of the rocks,--under which circumstances I have observed them in immense numbers, apparently the accumulation of two or three generations, which the violence of the elements had not been able to sever. It is rare in the sylvan districts to find them joined; nevertheless such is sometimes the case,--thus proving that the peculiarity is not actually essential, but merely one which it is the tendency of the species to assume, and which is more developed in some specimens, and under certain conditions, than in others.[46]"

But by far the greatest amount of variability to which insect structure is liable, is presented by the _wings_,--especially the metathoracic ones. The wings, indeed, unless I am much mistaken, are essentially (as compared with other primary details) organs of variation, capable of being more or less developed, according as the several countries in which the creatures are placed may necessitate their action. I will not recapitulate the evidence which I have already adduced, proving that islands have an especial capability of their own, either for increasing or neutralizing, as it may happen, the powers of flight (in which _latter_ case, however, a compensation is usually made for the loss); but I will point to the data which are there brought together, in support of the hypothesis for which I am now pleading,--believing that they will be found sufficient, on inquiry, to establish the doctrine of alary mutability, so far at least as it is connected with isolation as an element of control. If, however (irrespectively of its cause), the thing itself be recognized, the _principle_ is at once established; and we may reason upon it as a matter of fact. So that, if we can ensure this concession or acknowledgment, the occasional _proneness_ to variation of these thoracic appendages is, as a law, admitted. The only questions which would then appear immediately to suggest themselves, are: Under what circumstances do they principally fluctuate? and why should it happen that organs which are apparently so necessary as a medium of subsistence, should be subject to inconstancy?

Both of these have, in reality, been already replied to in the preceding chapter. Nevertheless, we may briefly repeat, that, so far as the first is concerned, it is in islands that we detect the maximum of instability to which the wings of the Insecta are liable, and that it is in seasons of extraordinary heat that their development is everywhere inclined (if at all) to be especially stimulated: whilst, as regards the second, it will be sufficient to state, that in _continents_, when any decided alteration takes place in the organs of flight, it for the most part comes to pass that an _increased_ (rather than diminished) action is the result; whereas in _islands_, provided that the species are not absolutely dependent on aërial progression for their food (in which case, in order to prepare for the contingency of being blown out to sea, the capacity of the wings is commonly augmented), the _reverse_ is nearer the truth. So that the _second_ problem,--the _reason why_ appendages thus apparently essential should be subject to inconstancy,--is at once rendered intelligible from the consideration, that it is only under circumstances in which the indiscriminate employment of those organs would be apt to bring the creatures into trouble that (when not an actual _sine quâ non_ to their existence) they are liable to be taken away; whilst, even in that case, it generally happens that some partial equivalent for the privation incurred is granted, as a recompense.

Mr. Westwood, in his admirable _Introduction to the Modern Classification of Insects_, has recorded many instances of alary variation; which, however, as he does not appear to have noticed the peculiarity of island faunas, are principally in corroboration of what I have just insisted upon as the usual tendency in continents,--namely, an _enlargement_ of the erratic powers. Speaking of the _Aphelocheirus æstivalis_ (a member of the Hemiptera), he observes: "My British specimens have but short, rudimental, oval hemelytra, like those of the bed-bug; but I possess one of Bosc's original examples, described by Fabricius, not quite so large as the others, in which _the wings are fully developed_. I do not, however, on that account, regard the former either as pupæ or distinct species, but as undeveloped specimens in the imago state[47]." And whilst discussing the _Hydrometridæ_, he expresses himself thus: "It appears to me, that, from causes of which we are ignorant, numerous individuals of many of the species of these tribes are subjected to an inferior kind of development in the imago state, which does not allow the acquirement of wings,--which, however, in certain cases, _acquire their full size_. Hence, I consider that the apterous specimens of _Hydrometra stagnorum_, those with very short elytra, and those with the full-sized wings and wing-covers, are all in the imago state, although some are more perfect than others[48]." And, again, in his reflections on the Hemiptera, Mr. Westwood says (and most entomologists are aware of the fact): "The species of _Gerris_, _Hydrometra_, and _Velia_ are mostly found perfectly apterous, though _occasionally with full-sized wings_. _Chorosoma miriforme_, _Prostemma guttula_, _Pachymerus brevipennis_, &c., are generally found with very short wing-covers, but sometimes with full-sized wings[49]." In like manner, the _Cimex apterus_, Linn. (one of the _Lygæidæ_) "exhibits, in an eminent degree, the ordinary occurrence of an imperfect perfect-state; whilst individuals are occasionally found _with fully developed organs of flight_[50]". _Lyæus brevipennis_, Lat., also ordinarily occurs with abbreviated hemelytra; but it has been found with them perfect by Westwood, as well as with metathoracic wings.

None of the above examples however would appear to do more than refer to the alary instability of the Insecta, as a matter of fact; but this is all for which we are now contending,--the preceding chapter having been in part devoted to some of the presumptive _causes_ of it. Whether the specimens of _Oncocephalus griseus_, to which Spinola called attention, were insular ones, I cannot say; but he seems to have noted an example in which an _opposite_ phænomenon to those which Mr. Westwood has cited, was displayed, and moreover to have speculated on the conditions producing it, when he suggests: "L'influence du climat septentrional parait avoir arrêté le développement des organes du vol[51]." And, again, when commenting upon the other tendency in a representative of the _Reduviadæ_, he says ('Essai,' p. 96): "Je pense que la présence des ailes et leur développement dépendent du climat." Whilst treating of two British species of the same family, Mr. Westwood observes: "The _Prostemma guttula_, Fab., and _Coranus subapterus_, Curt., are interesting on account of their being generally found in an undeveloped imago state,--the latter being either entirely apterous or with the fore-wings rudimental, although occasionally to be met with having the fore-wings completely developed[52]." The common _Phosphuga atrata_ of our own country has the organs of flight very rudimentary, and much too small for use: yet the late Mr. Holme of Oxford has mentioned[53], that he has several times taken it on the wing, during the hot sunshine. And, concerning the _Olisthopus rotundatus_, he states[54] that every specimen which he captured in the Scilly Islands was subapterous.

But facts like those are, after all, nothing more than such as we may trace the counterpart of in higher animals than the Insecta. Mr. Gould informs me, that the Swallows of Malta, which have but a comparatively narrow space to cross over, to the African continent, constitute (although specifically identical with them) a distinct race from those of England,--all of which, he believes, winter in Morocco. But, what are the differences displayed? From amongst many minor ones, of a climatal or geographical nature, the most conspicuous is _the length of the wings_,--those which have annually a longer journey to perform having, through a course of ages, acquired, as a race, a superior capacity for flight. And, in answer to a late query on this subject, he adds that _all_ the sylvan birds in Malta, such as the Black-caps, Willow-wrens, &c., though unquestionably of the same species as those of Great Britain, exhibit small local characteristics by which they may be immediately distinguished,--such as the length of the wings, size of the bills, and tints of the plumage. So that the migratory birds generally, which pass to and fro between Europe and Africa in that particular latitude, would appear to form separate races from those which traverse the ocean to our own country; and to be, most of them, remarkable, _inter alia_, for a slight shortening of their organs of transit.

If, however, the members of the insect tribes are capable of but small variation in actual _structure_, with the exception, in certain instances, of the greater or less development of the wings; we shall find that their external characters are much more prone to instability. There is not an item indeed of all their secondary diagnostics which does not admit of a positive change; and, though it be only within fixed limits that the several modifications can occur, those boundaries are frequently far apart, and include at times numerous phases within their embrace which have been too often looked upon as specific. Thus, whether we regard their bulk, outline, colour, or sculpture, anything like absolute constancy, under all circumstances and conditions, does not so much as exist; and we are driven to admit, that the physical influences to which these various creatures are exposed have a very decided power over their general configuration and aspect. It would be needless, however, to attempt to discuss the above details of aberration separately; because, where any one of them is especially interfered with, it usually happens that the others are more or less involved with them: but we may offer a few desultory remarks, which will tend to show that disturbing agents are apt to mar them both individually and as a whole,--and not only so, but to affect them in a permanent manner (as indeed has been already intimated), according as similar combinations of them are, from local causes (as it were), _selected_, to be acted upon.

I have stated in the last section of the preceding chapter that insect stature is eminently beneath the control of contingences from without; adducing, amongst other examples, in support of this, the Madeiran _Ptinus albopictus_,--a species which, whilst it averages more than a line in length on the central island of the group, is reduced to _less than half_ that bulk on a small and weather-beaten rock (the Ilheo Chão) at a distance from it. Judging indeed from many hundred specimens of the _Ptini_ which I have submitted to a close comparison, "the most constant of their characters would seem to be outline and sculpture, whilst size and colour are apparently the least to be depended upon:--so that trifling differences may be of specific indication in the former case, where in the latter much larger ones are worthless[55]." I have in fact generally noticed, that size and colour are more peculiarly liable to be affected _together_. This, however, is nothing more than what we should anticipate, since the same causes which have stunted the dimensions, during a long series of ages, of any particular creature, will for the most part be found to have also impaired the brilliancy of its tints. Luxuriance of vegetation and sheltered districts are alike conducive, in the Annulosa, to the development both of the body and its adornment; or, in other words, where the vegetable creation attains its maximum (which it certainly does not do in situations which are exposed to the irritating consequences of a perpetually stormy atmosphere), there the animal world will be usually observed to thrive.

There are many insects which appear to have _two distinct states_, both in magnitude and hue, which we are seldom (in some instances, I believe, never) able to unite by intermediate links, or grades; and yet which are universally admitted, although found in actually the self-same spots (a fact which prevents their being looked upon as separate, local modifications of a common type), to be mere varieties of each other. They are, however, exceptions to the general rule; and, although infringing on the strict definition of a "variety," as given at a preceding page[56], we nevertheless feel an _à priori_ conviction that they are by no means specifically dissimilar _inter se_. Such phases, as regards _stature_, are presented by the _Brachinus crepitans_ and _Lamprias chlorocephalus_ of our own country; whilst, as regards _colour_, the _Philhydrus melanocephalus_, _Aphodius plagiatus_, and the _Psylliodes erythrocephala_ (constituting in its paler garb the _P. nigricollis_, Mshm) may be quoted, as cases in point. Thus, also, in Madeira, the _Mycetoporus pronus_, Erich., has a large and small form, living in communion,--which I have never been able to connect, and yet which are unquestionably identical (differing in no respect except in size): and so have the _Stenus Heeri_, Woll., and the _Saprinus nitidulus_, Fab.[57]

As regards the instability displayed by _colour_, in the insect tribes, when subjected to the action of certain conditions and influences from without, so much has been said in the fourth section of the preceding chapter, that it is unnecessary to repeat it here. True it is that it was then my sole province to discuss the _causes_ which would appear to regulate, in a large measure, the external aspect of the Annulosa; yet the _existence_ of inconstancy, in the several organs and characters involved (with which alone we are now concerned), was, by the nature of the case, implied: so that if the _disturbing element_ was demonstrated, the mere fact that the thing (whatsoever it may have been) _was interfered with_, was surely proved _à fortiori_. I there pointed out the great proneness to a change in hue which divers circumstances are apt to induce; and I particularly instanced proximity to the sea-shore, and other saline spots, as well as an attachment to calcareous districts, as amongst the most powerful of the deranging contingences. In case, however, that any further evidence should be looked for, on this immediate subject, I will quote the following,--relating to the _Bembidium Atlanticum_ of the Madeira islands, which was but just touched upon in that chapter,--as a concluding example of the general effect of physical agents on the colour of these lower creatures. "Throughout all the Madeiran Coleoptera there is perhaps no insect which displays such an extraordinary range of colouring as the present one does; and although it is true that the section of _Bembidium_ to which it belongs is essentially a variable one, yet I am not acquainted with any _Peryphus_ in which the paler patches of the elytra are so remarkably unstable, or which appear to be so completely under the control of external circumstances, as are those of the _B. Atlanticum_: and indeed unless viewed in the mass, we should scarcely be inclined to recognize the same species in the many aspects which it puts on between its extremes. The examination, however, of a very large number of examples, and a careful consideration of the several localities and altitudes in which they were taken, has convinced me that there is unquestionably but a single type of form amongst my entire series, since the whole are so intimately connected, by successive gradations both of outline and colour, that it is perfectly impossible to isolate even a single specimen, or to draw a line of specific demarcation between any two consecutive members of the chain. It will be perceived, by a reference to the diagnosis, that the insect in question passes imperceptibly from nearly a pure green, through a well-defined spotted state, into one which has the elytra almost testaceous,--the paler portions being at last so largely developed as to become confluent, and almost to cover the entire surface. In Madeira proper the darker varieties would seem to be typical; whereas in Porto Santo the brightly coloured ones preponderate, and in fact are all but universal. Both extremes do nevertheless occur in both islands, the tendency being merely, in either case, to assume the particular modification characteristic of the spot[58]".

And so it is with the outline and sculpture (no less than with bulk and hue): they also are equally liable to disturbance from physical causes, as indeed has been already insisted upon. Like most of the minutiæ of variation, however, to which we have called attention, it is more particularly on islands that this is to be observed,--isolation, during an interval sufficiently long, appearing to possess some especial control over the external contour and surface of the insect races. Thus, in the Madeiras, for instance, the _Caulotropis lucifugus_ has its prothorax more distinctly punctured, and its elytra more perceptibly striated, in the principal island, than on any of the smaller members of the group; in Porto Santo, indeed, it is almost free from sculpture of any kind; whilst its ally, the _C. conicollis_, apart from being somewhat larger, is, on the contrary, both more punctured and striated on the Dezerta Grande than it is in Madeira proper. The _Omias Waterhousei_, again (in addition to its slightly increased bulk and less shining envelope, in that locality), is more lightly impressed on the Dezerta than it is in Madeira and, not to mention other differences, the _Ellipsodes glabratus_ is densely beset with most minute granules on that same rock--whereas on the mountain slopes of the central mass, it is highly polished and glabrous. The _Helops confertus_, we have intimated at a previous page, is less coarsely sculptured in the lofty regions of Madeira, than in the lower ones: and the _H. futilis_ has its elytral tubercles apparent in Madeira proper, but evanescent on the Dezerta Grande. The _Eurygnathus Latreillei_ assumes a permanent variety on the Dezerta, the insect having become modified through a long isolation on those weather-beaten heights,--here it not only attains a more gigantic stature than in Porto Santo, but is invariably also more parallel and opake, has the sides of its prothorax more recurved, with the punctures towards the lateral angles almost obsolete, and the striæ of its elytra somewhat more evidently punctate[59].

Such examples, however, might be multiplied _ad infinitum_; and I will not therefore devote further space to the bringing together of facts which it is hardly possible will be disputed,--especially as it has been my wish, in the present chapter, merely to _enumerate_ what the organs and characters principally are which are more peculiarly sensitive to change, throughout the Annulose tribes. This I may venture to hope, though briefly, I have in part done; and I will consequently pass on to other considerations, which, even if somewhat alien to the immediate question of insect instability, should scarcely be altogether omitted in a treatise like this.

FOOTNOTES:

[46] Insecta Maderensia, pp. 56, 57.

[47] Introduction to the Modern Classification of Insects, ii. p. 466.

[48] _Id._ ii. p. 469.

[49] _Id._ ii. p. 454.

[50] Introduction to the Modern Classification of Insects, ii. p. 480.

[51] Essai, p. 103.

[52] Introduction to the Modern Classification of Insects, ii. p. 473.

[53] Trans. of the Ent. Soc. of London, ii. p. 60.

[54] _Id._ ii. p. 59.

[55] Insecta Maderensia, pp. 260, 261.

[56] Vide _supra_, p. 5.

[57] Although, in our ignorance of their real nature, we cannot cite them as actually analogous to these separate phases in certain members of the Insecta, yet we are forcibly reminded by the latter of the distinct states which many of the Terrestrial Mollusca present (frequently in equal proportions) in the same localities. Thus, most of the _Pupæ_ have at least two abruptly-marked forms,--a larger and smaller one. Many of the _Helices_ also exhibit this tendency in an eminent degree: I have indeed been shown specimens by Sir Charles Lyell of the _Helix hirsuta_, Say, from North America, one state of which is considerably more than double the dimensions of the other; and I believe it is a well-known fact that intermediate links _have_ not yet been observed to connect the extremes. May not therefore the gigantic _H. Lowei_ and _Bowdichiana_, which are now extinct in the Madeira Islands, have been but forms of the _H. Portosanctana_ and _punctulata_, respectively,--co-existent with them, though more sensitive to the great diminutions of altitude and area which were consequent on the breaking-up of a once continuous land? If such be the case, however, it is certain that they were far commoner at an early period than their smaller colleagues (which, now, in their proper districts, absolutely teem),--seeing that the _latter_ are extremely rare in the fossil deposits, whilst they themselves literally abound.

[58] Insecta Maderensia, p. 78.

[59] Insecta Maderensia, pp. 21, 22.