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Ch. 13: Nature's Night Lights

_(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

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

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.

W. H. Hudson