By CHARLES DARWIN*
Excerpts from the recently released book “Darwin no Brasil”, organized by Pedro Alencastro
Clouds over Corcovado
Everyone has heard about the beauty of Botafogo’s landscape. The house where I lived was just below the famous Corcovado mountain. They rightly say that these abruptly conical hills are characteristic of the formations that Humboldt calls granite-gneiss. And nothing can be more impressive than the effect of these enormous round masses of bare rock emerging from luxuriant vegetation.
I used to watch the clouds that came from the sea and formed a blanket just below the top of Corcovado. This mountain, like so many others when partially covered, seemed much more imposing than its actual height of 700 meters. In his meteorological essays, John Daniell, a British chemist and meteorologist, observed that a cloud sometimes appears to settle on the top of a mountain, even though the wind continues to blow over it.
Here the same phenomenon happens, but with a slightly different appearance. In this case, the cloud clearly became entangled and passed quickly over the top, without decreasing or increasing in size. The sun was falling, and a gentle southerly breeze, hitting this side of the rock, mingled with the cooler air above. And so the steam condensed. But as the light wreaths of cloud passed over the slope, now under the influence of the warmer air on the northern slope, they immediately dissolved again.
Torrential rain and musical frogs
The weather in May, June and early winter was delightful. The average temperature, recorded at 9 am and evening, was only 22°C. It usually rained a lot, but the dry winds from the south soon made the walks pleasant. One morning, four centimeters of rain fell in six hours. As the storm broke in the forests surrounding Corcovado, the noise of the drops hitting the countless leaves was so loud that it could be heard 400 meters away, just like the roar of a large body of water.
At the end of hot days, it was pleasant to sit quietly in the garden and watch the afternoon fall. Nature in these climates chooses a chorus of singers more humble than those of Europe. A small frog sits on a blade of grass, a few inches above the water, and sings its happy croak. When several come together, they sing harmoniously in different notes. I had difficulty catching a specimen of this frog. The genus has small suction cups on the tips of its fingers and I discovered that this animal can climb vertically up a window.
Countless mosquitoes and crickets also produced, simultaneously, an uninterrupted shrill noise, which, softened by the distance, was not at all unpleasant. Every night, just after dark, this great concert began, which I sat down to listen to many times, until my attention was diverted by some curious insect passing by.
Phosphorescent insects
When it gets dark, fireflies come flying from one bush to another. On a dark night, their light can be seen from about 200 steps away. It is remarkable that in all the different types of luminous insects that I observed, bright elaterids and also marine animals (crustaceans, jellyfish, nereids and corallines), the light is always a very accentuated green. All the fireflies I collected here belong to the Lampyridae (a family that includes the English firefly), and most of the specimens are from Lampyris.
I discovered that this insect emits brighter flashes when it is irritated, and in between its abdominal rings darken. The flash was almost simultaneous in both rings, but at first noticeable only in the anterior ring. The luminous substance is fluid and quite sticky. Where the skin had been torn, small dots continued to glow with a slight flicker while the intact parts turned dark.
When decapitating the insect, the rings remained uninterruptedly bright, but not as bright as before, and local stimulation with a needle always increased the intensity of the light. The rings, on one occasion, maintained their luminous properties almost 24 hours after the insect's death. Given this, it seems likely that the animal can only hide or extinguish the light for short intervals and that, in other situations, the emission is involuntary.
On the muddy and damp gravel paths I found a large number of Lampyris larvae, which generally resemble female English fireflies. These larvae have weak light power. Unlike their parents, at the slightest touch they pretended to be dead and stopped glowing, and no stimulus could produce new light emissions.
I kept several of them alive for a while. Their tails are quite unique, as they function as suction cups or attachment organs, and also as reservoirs for saliva and other fluids. I repeatedly fed raw meat to the larvae, and invariably noticed that, from time to time, the end of the tail was directed towards the mouth, expelling a drop of fluid onto the meat as it was consumed. The tail, despite this assiduous movement, does not seem to find its way to the mouth, as the neck was always touched first, apparently as a guide.
*Charles Darwin (1809-1882) was a naturalist, geologist and biologist. Author, among other books, of The origin of species (Edipro).
Reference
Pedro Alencastro (org.). Darwin in Brazil. Charles Darwin's trip to Brazil and his contributions to the theory of evolution. Porto Alegre, Duas Aspas, 2023, 272 pages. [https://amzn.to/3VwL6F4]
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