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