The Change Of Diet Here May Have Had
Some Influence In Producing The Disease; For I Was Informed By Dr.
Welweitsch,
an able German naturalist, whom we found pursuing his arduous labors here,
and whose life we hope may be
Spared to give his researches to the world,
that, of fifty-eight kinds of grasses found at Loanda,
only three or four species exist here, and these of the most diminutive kinds.
The twenty-four different species of grass of Golungo Alto are
nearly all gigantic. Indeed, gigantic grasses, climbers, shrubs and trees,
with but few plants, constitute the vegetation of this region.
NOVEMBER 20TH. An eclipse of the sun, which I had anxiously
hoped to observe with a view of determining the longitude,
happened this morning, and, as often took place in this cloudy climate,
the sun was covered four minutes before it began. When it shone forth
the eclipse was in progress, and a few minutes before it should
(according to my calculations) have ended the sun was again
completely obscured. The greatest patience and perseverance are required,
if one wishes to ascertain his position when it is the rainy season.
Before leaving, I had an opportunity of observing a curious insect,
which inhabits trees of the fig family (`Ficus'), upward of twenty species
of which are found here. Seven or eight of them cluster round a spot
on one of the smaller branches, and there keep up a constant distillation
of a clear fluid, which, dropping to the ground, forms a little puddle below.
If a vessel is placed under them in the evening, it contains
three or four pints of fluid in the morning. The natives say that,
if a drop falls into the eyes, it causes inflammation of these organs.
To the question whence is this fluid derived, the people reply
that the insects suck it out of the tree, and our own naturalists
give the same answer. I have never seen an orifice,
and it is scarcely possible that the tree can yield so much.
A similar but much smaller homopterous insect, of the family `Cercopidae',
is known in England as the frog-hopper (`Aphrophora spumaria'),
when full grown and furnished with wings, but while still in the pupa state
it is called "Cuckoo-spit", from the mass of froth in which
it envelops itself. The circulation of sap in plants in our climate,
especially of the graminaceae, is not quick enough to yield much moisture.
The African species is five or six times the size of the English.
In the case of branches of the fig-tree, the point the insects congregate on
is soon marked by a number of incipient roots, such as are thrown out when
a cutting is inserted in the ground for the purpose of starting another tree.
I believe that both the English and African insects belong to the same family,
and differ only in size, and that the chief part of the moisture
is derived from the atmosphere. I leave it for naturalists to explain
how these little creatures distill both by night and day as much water
as they please, and are more independent than her majesty's steam-ships,
with their apparatus for condensing steam; for, without coal,
their abundant supplies of sea-water are of no avail. I tried
the following experiment: Finding a colony of these insects busily distilling
on a branch of the `Ricinus communis', or castor-oil plant,
I denuded about 20 inches of the bark on the tree side of the insects,
and scraped away the inner bark, so as to destroy all the ascending vessels.
I also cut a hole in the side of the branch, reaching to the middle,
and then cut out the pith and internal vessels. The distillation
was then going on at the rate of one drop each 67 seconds,
or about 2 ounces 5-1/2 drams in 24 hours. Next morning the distillation,
so far from being affected by the attempt to stop the supplies,
supposing they had come up through the branch from the tree,
was increased to a drop every 5 seconds, or 12 drops per minute,
making 1 pint (16 ounces) in every 24 hours. I then cut the branch
so much that, during the day, it broke; but they still went on
at the rate of a drop every 5 seconds, while another colony
on a branch of the same tree gave a drop every 17 seconds only,
or at the rate of about 10 ounces 4-4/5 drams in 24 hours.
I finally cut off the branch; but this was too much for their patience,
for they immediately decamped, as insects will do from either a dead branch
or a dead animal, which Indian hunters soon know, when they sit down
on a recently-killed bear. The presence of greater moisture in the air
increased the power of these distillers: the period of greatest activity
was in the morning, when the air and every thing else was charged with dew.
Having but one day left for experiment, I found again that another colony
on a branch denuded in the same way yielded a drop every 2 seconds,
or 4 pints 10 ounces in 24 hours, while a colony on a branch untouched
yielded a drop every 11 seconds, or 16 ounces 2-19/20 drams in 24 hours.
I regretted somewhat the want of time to institute another experiment,
namely, to cut a branch and place it in water, so as to keep it in life,
and then observe if there was any diminution of the quantity of water
in the vessel. This alone was wanting to make it certain
that they draw water from the atmosphere. I imagine that they have
some power of which we are not aware, besides that nervous influence
which causes constant motion to our own involuntary muscles,
the power of life-long action without fatigue. The reader will remember,
in connection with this insect, the case of the ants already mentioned.
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