On A Dark Night The Light Can Be Seen At
About Two Hundred Paces Distant.
It is remarkable that in
all the different kinds of glowworms, shining elaters, and
various marine animals (such as the crustacea, medusae,
nereidae, a coralline of the genus Clytia, and Pyrosma),
which I have observed, the light has been of a well-marked
green colour.
All the fireflies, which I caught here, belonged
to the Lampyridae (in which family the English glowworm
is included), and the greater number of specimens were of
Lampyris occidentalis. [4] I found that this insect emitted
the most brilliant flashes when irritated: in the intervals,
the abdominal rings were obscured. The flash was almost
co-instantaneous in the two rings, but it was just perceptible
first in the anterior one. The shining matter was fluid and
very adhesive: little spots, where the skin had been torn,
continued bright with a slight scintillation, whilst the
uninjured parts were obscured. When the insect was decapitated
the rings remained uninterruptedly bright, but not so brilliant
as before: local irritation with a needle always increased
the vividness of the light. The rings in one instance retained
their luminous property nearly twenty-four hours after the
death of the insect. From these facts it would appear probable,
that the animal has only the power of concealing or
extinguishing the light for short intervals, and that at other
times the display is involuntary. On the muddy and wet
gravel-walks I found the larvae of this lampyris in great
numbers: they resembled in general form the female of the
English glowworm. These larvae possessed but feeble luminous
powers; very differently from their parents, on the
slightest touch they feigned death and ceased to shine; nor
did irritation excite any fresh display. I kept several of
them alive for some time: their tails are very singular organs,
for they act, by a well-fitted contrivance, as suckers or organs
of attachment, and likewise as reservoirs for saliva, or some
such fluid. I repeatedly fed them on raw meat; and I invariably
observed, that every now and then the extremity
of the tail was applied to the mouth, and a drop of fluid
exuded on the meat, which was then in the act of being consumed.
The tail, notwithstanding so much practice, does not
seem to be able to find its way to the mouth; at least the neck
was always touched first, and apparently as a guide.
When we were at Bahia, an elater or beetle (Pyrophorus
luminosus, Illig.) seemed the most common luminous insect.
The light in this case was also rendered more brilliant by
irritation. I amused myself one day by observing the springing
powers of this insect, which have not, as it appears to
me, been properly described. [5] The elater, when placed on
its back and preparing to spring, moved its head and thorax
backwards, so that the pectoral spine was drawn out, and
rested on the edge of its sheath. The same backward movement
being continued, the spine, by the full action of the
muscles, was bent like a spring; and the insect at this moment
rested on the extremity of its head and wing-cases.
The effort being suddenly relaxed, the head and thorax flew
up, and in consequence, the base of the wing-cases struck
the supporting surface with such force, that the insect by
the reaction was jerked upwards to the height of one or
two inches. The projecting points of the thorax, and the
sheath of the spine, served to steady the whole body during
the spring. In the descriptions which I have read, sufficient
stress does not appear to have been laid on the elasticity of
the spine: so sudden a spring could not be the result of simple
muscular contraction, without the aid of some mechanical
contrivance.
On several occasions I enjoyed some short but most pleasant
excursions in the neighbouring country. One day I went
to the Botanic Garden, where many plants, well known for
their great utility, might be seen growing. The leaves of the
camphor, pepper, cinnamon, and clove trees were delightfully
aromatic; and the bread-fruit, the jaca, and the mango,
vied with each other in the magnificence of their foliage.
The landscape in the neighbourhood of Bahia almost takes
its character from the two latter trees. Before seeing them,
I had no idea that any trees could cast so black a shade on
the ground. Both of them bear to the evergreen vegetation
of these climates the same kind of relation which laurels
and hollies in England do to the lighter green of the deciduous
trees. It may be observed, that the houses within the
tropics are surrounded by the most beautiful forms of
vegetation, because many of them are at the same time most
useful to man. Who can doubt that these qualities are united
in the banana, the cocoa-nut, the many kinds of palm, the
orange, and the bread-fruit tree?
During this day I was particularly struck with a remark
of Humboldt's, who often alludes to "the thin vapour which,
without changing the transparency of the air, renders its
tints more harmonious, and softens its effects." This is an
appearance which I have never observed in the temperate
zones. The atmosphere, seen through a short space of half
or three-quarters of a mile, was perfectly lucid, but at a
greater distance all colours were blended into a most beautiful
haze, of a pale French grey, mingled with a little blue.
The condition of the atmosphere between the morning and
about noon, when the effect was most evident, had undergone
little change, excepting in its dryness. In the interval,
the difference between the dew point and temperature had
increased from 7.5 to 17 degs.
On another occasion I started early and walked to the
Gavia, or topsail mountain. The air was delightfully cool
and fragrant; and the drops of dew still glittered on the
leaves of the large liliaceous plants, which shaded the
streamlets of clear water.
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