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    Ch. 13: Nature's Night Lights

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    Chapter 14
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    _(Remarks about Fireflies and other matters.)_

    It was formerly supposed that the light of the firefly (in any family
    possessing the luminous power) was a safeguard against the attacks of
    other insects, rapacious and nocturnal in their habits. This was Kirby
    and Spence's notion, but it might just as well be Pliny's for all the
    attention it would receive from modern entomologists: just at present
    any observer who lived in the pre-Darwin days is regarded as one of the
    ancients. The reasons given for the notion or theory in the celebrated
    _Introduction to Entomology_ were not conclusive; nevertheless it was
    not an improbable supposition of the authors'; while the theory which
    has taken its place in recent zoological writings seems in every way
    even less satisfactory.

    Let us first examine the antiquated theory, as it must now be called. By
    bringing a raptorial insect and a firefly together, we find that the
    flashing light of the latter does actually scare away the former, and is
    therefore, for the moment, a protection as effectual as the camp-fire
    the traveller lights in a district abounding with beasts of prey.
    Notwithstanding this fact, and assuming that we have here the whole
    reason of the existence of the light-emitting power, a study of the
    firefly's habits compels us to believe that the insect would be just as
    well off without the power as with it. Probably it experiences some
    pleasure in emitting flashes of light during its evening pastimes, but
    this could scarcely be considered an advantage in its struggle for
    existence, and it certainly does not account for the possession of the
    faculty.

    About the habits of Pyrophorus, the large tropical firefly which has the
    seat of its luminosity on the upper surface of the thorax, nothing
    definite appears to be known; but it has been said that this instinct is
    altogether nocturnal. The Pyrophorus is only found in the sub-tropical
    portion of the Argentine country, and I have never met with it. With the
    widely-separated Cratomorphus, and the tortoise-shaped Aspisoma, which
    emit the light from the abdomen, I am familiar; one species of
    Cratomorphus--a long slender insect with yellow wing-cases marked with
    two parallel black lines--is "the firefly" known to every one and
    excessively abundant in the southern countries of La Plata. This insect
    is strictly diurnal in its habits--as much so, in fact, as diurnal
    butterflies. They are seen flying about, wooing their mates, and feeding
    on composite and umbelliferous flowers at all hours of the day, and are
    as active as wasps during the full glare of noon. Birds do not feed on
    them, owing to the disagreeable odour, resembling that of phosphorus,
    they emit, and probably because they are to be uneatable; but their
    insect enemies are not so squeamish, and devour them readily, just as
    they also do the blister-fly, which one would imagine a morsel fitted to
    disagree with any stomach. One of their enemies is the Monedula wasp;
    another, a fly, of the rapacious Asilidas family; and this fly is also a
    wasp in appearance, having a purple body and bright red wings, like a
    Pepris, and this mimetic resemblance doubtless serves it as a protection
    against birds. A majority of raptorial insects are, however, nocturnal,
    and from all these enemies that go about under cover of night, the
    firefly, as Kirby and Spence rightly conjectured, protects itself, or
    rather is involuntarily protected, by means of its frequent flashing
    light. We are thus forced to the conclusion that, while the common house
    fly and many other diurnal insects spend a considerable portion of the
    daylight in purely sportive exercises, the firefly, possessing in its
    light a protection from nocturnal enemies, puts off its pastimes until
    the evening; then, when its carnival of two or three hours' duration is
    over, retires also to rest, putting out its candle, and so exposing
    itself to the dangers which surround other diurnal species during the
    hours of darkness. I have spoken of the firefly's pastimes advisedly,
    for I have really never been able to detect it doing anything in the
    evening beyond flitting aimlessly about, like house flies in a room,
    hovering and revolving in company by the hour, apparently for amusement.
    Thus, the more closely we look at the facts, the more unsatisfactory
    does the explanation seem. That the firefly should have become possessed
    of so elaborate a machinery, producing incidentally such splendid
    results, merely as a protection against one set of enemies for a portion
    only of the period during which they are active, is altogether
    incredible.

    The current theory, which we owe to Belt, is a prettier one. Certain
    insects (also certain Batrachians, reptiles, &c.) are unpalatable to the
    rapacious kinds; it is therefore a direct advantage to these unpalatable
    species to be distinguishable from all the persecuted, and the more
    conspicuous and well-known they are, the less likely are they to be
    mistaken by birds, insectivorous mammals, &c., for eatable kinds and
    caught or injured. Hence we find that many such species have acquired
    for their protection very brilliant or strongly-contrasted
    colours--warning colours--which insect-eaters come to know.

    The firefly, a soft-bodied, slow-flying insect, is easily caught and
    injured, but it is not fit for food, and, therefore, says the theory,
    lest it should be injured or killed by mistake, it has a fiery spark to
    warn enemies---birds, bats, and rapacious insects--that it is uneatable.

    The theory of warning colours is an excellent one, but it has been
    pushed too far. We have seen that one of the most common fireflies is
    diurnal in habits, or, at any rate, that it performs all the important
    business of its life by day, when it has neither bright colour nor light
    to warn its bird enemies; and out of every hundred species of
    insect-eating birds at least ninety-nine are diurnal. Raptorial insects,
    as I have said, feed freely on fireflies, so that the supposed warning
    is not for them, and it would be hard to believe that the magnificent
    display made by luminous insects is useful only in preventing accidental
    injuries to them from a few crepuscular bats and goatsuckers. And to
    believe even this we should first have to assume that bats and
    goatsuckers are differently constituted from all other creatures; for in
    other animals--insects, birds, and mammalians--the appearance of fire by
    night seems to confuse and frighten, but it certainly cannot be said to
    _warn,_ in the sense in which that word is used when we speak of the
    brilliant colours of some butterflies, or even of the gestures of some
    venomous snakes, and of the sounds they emit.

    Thus we can see that, while the old theory of Kirby and Spence had some
    facts to support it, the one now in vogue is purely fanciful. Until some
    better suggestion is made, it would perhaps be as well to consider the
    luminous organ as having "no very close and direct relation to present
    habits of life." About their present habits, however, especially their
    crepuscular habits, there is yet much to learn. One thing I have
    observed in them has always seemed very strange to me. Occasionally an
    individual insect is seen shining with a very large and steady light, or
    with a light which very gradually decreases and increases in power, and
    at such times it is less active than at others, remaining for long
    intervals motionless on the leaves, or moving with a very slow flight.
    In South America a firefly displaying this abnormal splendour is said to
    be dying, and it is easy to imagine how such a notion originated. The
    belief is, however, erroneous, for sometimes, on very rare occasions,
    all the insects in one place are simultaneously affected in the same
    way, and at such times they mass themselves together in myriads, as if
    for migration, or for some other great purpose. Mr. Bigg-Wither, in
    South Brazil, and D'Albertis, in New Guinea, noticed these firefly
    gatherings; I also once had the rare good fortune to witness a
    phenomenon of the kind on a very grand scale. Riding on the pampas one
    dark evening an hour after sunset, and passing from high ground
    overgrown with giant thistles to a low plain covered with long grass,
    bordering a stream of water, I found it all ablaze with myriads of
    fireflies. I noticed that all the insects gave out an exceptionally
    large, brilliant light, which shone almost steadily. The long grass was
    thickly studded with them, while they literally swarmed in the air, all
    moving up the valley with a singularly slow and languid flight. When I
    galloped down into this river of phosphorescent fire, my horse plunged
    and snorted with alarm. I succeeded at length in quieting him, and then
    rode slowly through, compelled to keep my mouth and eyes closed, so
    thickly did the insects rain on to my face. The air was laden with the
    sickening phosphorous smell they emit, but when I had once got free of
    the broad fiery zone, stretching away on either hand for miles along the
    moist valley, I stood still and gazed back for some time on a scene the
    most wonderful and enchanting I have ever witnessed.

    The fascinating and confusing effect which the appearance of fire at
    night has on animals is a most interesting subject; and although it is
    not probable that anything very fresh remains to be said about it, I am
    tempted to add here the results of my own experience.

    When travelling by night, I have frequently been struck with the
    behaviour of my horse at the sight of natural fire, or appearance of
    fire, always so different from that caused by the sight of fire
    artificially created. The steady gleam from the open window or door of a
    distant house, or even the unsteady wind-tossed flame of some lonely
    camp-fire, has only served to rouse a fresh spirit in him and the desire
    to reach it; whereas those infrequent displays of fire which nature
    exhibits, such as lightning, or the ignis fatuus, or even a cloud of
    fireflies, has always produced a disquieting effect. Experience has
    evidently taught the domestic horse to distinguish a light kindled by
    man from all others; and, knowing its character, he is just as well able
    as his rider to go towards it without experiencing that confusion of
    mind caused by a glare in the darkness, the origin and nature of which
    is a mystery. The artificially-lighted fire is to the horse only the
    possible goal of the journey, and is associated with the thought of rest
    and food. Wild animals, as a rule, at any rate in thinly-settled
    districts, do not know the meaning of any fire; it only excites
    curiosity and fear in them; and they are most disturbed at the sight of
    fires made by man, which are brighter and steadier than most natural
    fires. We can understand this sensation in animals, since we ourselves
    experience a similar one (although in a less degree and not associated
    with fear) in the effect which mere brightness has on us, both by day
    and night.

    On riding across the monotonous grey Patagonian uplands, where often for
    hours one sees not the faintest tinge of bright colour, the intense
    glowing crimson of a cactus-fruit, or the broad shining white bosom of
    the Patagonian eagle-buzzard (Buteo erythronotus), perched on the summit
    of a distant bush, has had a strangely fascinating effect on me, so that
    I have been unable to take my eyes off it as long as it continued before
    me. Or in passing through extensive desolate marshes, the dazzling white
    plumage of a stationary egret has exercised the same attraction. At
    night we experience the sensation in a greater degree, when the silver
    sheen of the moon makes a broad path on the water; or when a meteor
    leaves a glowing track across the sky; while a still more familiar
    instance is seen in the powerful attraction on the sight of glowing
    embers in a darkened room. The mere brightness, or vividness of the
    contrast, fascinates the mind; but the effect on man is comparatively
    weak, owing to his fiery education and to his familiarity with brilliant
    dyes artificially obtained from nature. How strong this attraction of
    mere brightness, even where there is no mystery about it, is to wild
    animals is shown by birds of prey almost invariably singling out white
    or bright-plumaged birds for attack where bright and sober-coloured
    kinds are mingled together. By night the attraction is immeasurably
    greater than by day, and the light of a fire steadily gazed at quickly
    confuses the mind. The fires which, travellers make for their protection
    actually serve to attract the beasts of prey, but the confusion and fear
    caused by the bright glare makes it safe for the traveller to lie down
    and sleep in the light. Mammals do not lose their heads altogether,
    because they are walking on firm ground where muscular exertion and an
    exercise of judgment are necessary at every step; whereas birds floating
    buoyantly and with little effort through the air are quickly bewildered.
    Incredible numbers of migratory birds kill them-selves by dashing
    against the windows of lighthouses; on bright moonlight nights the
    voyagers are comparatively safe; but during dark cloudy weather the
    slaughter is very great; over six hundred birds were killed by striking
    a lighthouse in Central America in a single night. On insects the effect
    is the same as on the higher animals: on the ground they are attracted
    by the light, but keep, like wolves and tigers, at a safe distance from
    it; when rushing through the air and unable to keep their eyes from it
    they fly into it, or else revolve about it, until, coming too close,
    their wings are singed.

    I find that when I am on horseback, going at a swinging gallop, a bright
    light affects me far more powerfully than when I am trudging along on
    foot. A person mounted on a bicycle and speeding over a level plain on a
    dark night, with nothing to guide him except the idea of the direction
    in his mind, would be to some extent in the position of the migratory
    bird. An exceptionally brilliant ignis fatuus flying before him would
    affect him as the gleam of a lamp placed high above the surface affects
    the migrants: he would not be able to keep his eyes from it, but would
    quickly lose the sense of direction, and probably end his career much as
    the bird does, by breaking his machine and perhaps his bones against
    some unseen obstruction in the way.
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