When reading the Voyage, it is impossible to miss the observation that much of the time Darwin was engaged in adolescent boy behavior: Pulling the heads off insects, noting how long they would wiggle after cut in half, closely examining the ooze and guts, occupied much of his time. Obviously, careful observation and a strong stomach were not all that was required to think up Natural Selection and his other theories, or the Origin of Species would have been written dozens of times by dozens of grown up kids.
In the following passages, Darwin is still along the Atlantic Coast, in “The Brazils,” in the general vicinity of Rio de Janeiro, ant it early summer 1832. All of these passages illustrate why insects dreaded Darwin:
At … times … fireflies are seen flitting about from hedge to hedge. All that I caught belonged to the family of Lampyridæ…. I found that this insect emitted the most brilliant flashes when irritated … 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. … On the muddy and wet gravel-walks I found the larvæ of this lampyris in great numbers: they resembled in general form the female of the English glowworm. These larvæ 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 … 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 …. 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 elytra. The effort being suddenly relaxed, the head and thorax flew up, and, in consequence, the base of the elytra 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.
From these detailed observations came insight at many levels. For instance, in regards to illumination, Darwin makes the following observation:
On a dark night the light could be seen at about two hundred paces distant. It is remarkable that in all the glowworms, shining elaters, and various marine animals, which I have observed (such as the crustacea, medusæ, nereidæ, a coralline of the genus Clytia, and Pyrosoma), the light has been of a well-marked green colour.
Certainly, the more one pokes around the more one learns:
I was much surprised at the habits of Papilio feronia [a butterfly]. This butterfly is not uncommon, and generally frequents the orange-groves. Although a high flier, yet it very frequently alights on the trunks of trees. On these occasions its head is invariably placed downwards; and its wings are expanded in a horizontal plane, instead of being folded vertically, as is commonly the case. This is the only butterfly which I have ever seen that uses its legs for running. Not being aware of this fact, the insect, more than once, as I cautiously approached with my forceps, shuffled on one side just as the instrument was on the point of closing, and thus escaped. But a far more singular fact, is the power which this species possesses of making a noise….
An evolutionary understanding of life both helps to explain this kind of observation, but also can derive from it.
Similarly, this observation of butterflies and moths as well as beetles compares biomes and raises some interesting questions:
The large and brilliantly-coloured Lepidoptera bespeak the zone they inhabit, far more plainly than any other race of animals. I allude only to the butterflies; for the moths, contrary to what might have been expected from the rankness of the vegetation, certainly appeared in much fewer numbers than in our own temperate regions. ….
I was disappointed in the general aspect of the Coleoptera [Beetles]. The number of minute and obscurely-coloured beetles is exceedingly great. (I may mention, as a common instance of one day’s (June 23d) collecting, when I was not attending particularly to the Coleoptera, that I caught sixty-eight species of that order…. ) … It is sufficient to disturb the composure of an entomologist’s mind, to look forward to the future dimensions of a complete catalogue. The Carabidæ appear in extremely few numbers within the tropics. This is the more remarkable when compared to the opposed case of the carnivorous mammalia, an order which they certainly represent among insects. I was struck with this observation both on entering Brazil, and when I saw the elegant and active forms of the Harpalidæ reappearing on the temperate plains of La Plata. Do the very numerous Arachnidæ and rapacious Hymenoptera supply the place of these carnivorous beetles? The carrion-feeders and Brachelytra are very uncommon; on the other hand, the Rhyncophora and Chrysomelidæ, all of which depend on the vegetable world for subsistence, are present in astonishing numbers. I do not here refer to the number of different species, but to that of the individual insects ; for on this it is that the most striking character in the entomology of different countries depends. The orders Orthoptera and Hemiptera are particularly numerous; as likewise is the stinging division of the Hymenoptera; the bees, perhaps, being excepted. A person, on first entering a tropical forest, is astonished at the labours of the ants : well-beaten paths branch off in every direction, on which an army of never-failing foragers may be seen, some going forth, and others returning, burdened with pieces of green leaf, often larger than their own bodies.
In the end, one gets the impression reading the Voyage, that Darwin is (subconsciously?) laying out the research program for the life sciences, or at least, organismic biology, for the next century or two…
The other posts in this series can be found by clicking this link.