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 aerial 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 qua 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 aestivalis_ (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 pupae or distinct species,
but as undeveloped specimens in the imago state[47]." And whilst
discussing the _Hydrometridae_, 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
_Lygaeidae_) "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]". _Lyaeus
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_ phaenomenon 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 arrete le developpement des organes
du vol[51]." And, again, when commenting upon the other tendency in a
representative of the _Reduviadae_, he says ('Essai,' p. 96): "Je pense
que la presence des ailes et leur developpement dependent 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
Chao) 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 _a 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
_a 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 minutiae 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
striae 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 _Pupae_ 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.