The Naturalist in La Plata

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

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.