There is not only one common house-fly in this country: there are three kinds, in addition to the blue-bottle or blow-fly, which is distinguished at once by its great size and blue colour, and lays its eggs in carrion. Late in the year you may often see what would pass for young or starveling house-flies going about among the others. This is a distinct species, the Homalomyia canicularis of entomologists. The third kind only to be distinguished by careful examination with the aid of a magnifying glass, is Anthomyia radicum. Both these are much less abundant than the common house-fly (Musca domestica), with which they almost always occur. Their breeding habits are similar to those of the common house-fly.
A fourth kind of fly is invariably mistaken for the common house-fly when it is noticed, as it sometimes is, in consequence of the sharp stab which it inflicts. As recently as the beginning of November last year I was “bitten” or pricked by one of this fourth kind in a London club. They are common enough on the sea shore in autumn, and may be a severe nuisance. People generally take them for common house-flies which have lost their temper in the hot weather and give way to the bad habit of “biting” out of sheer exasperation. Really, of course, a house-fly could not stab or prick with its broad-ended proboscis. The fly in question, which looks almost exactly like a well-grown house-fly, but possesses a sharp and business-like beak or proboscis, is known to scientific men as Stomoxys calcitrans. There are many kinds of Stomoxys scattered all over the world, and it is probable, though not actually proved, that they carry parasites such as the trypanosomes of horse and cattle diseases from one animal to another, as do the species of Glossina or tsetse-fly.
But we have yet to learn more about these flies and the parasites they transfer. In the case of the gnat, it has been discovered that the malaria parasite is swallowed by the gnat, and multiplies in it, producing thousands of spores in its blood, and it is these spores which the gnat hands or rather “mouths” on to man. No such multiplication of the trypanosome in the tsetse-fly (Glossina) is known. The tsetse-fly passes on the trypanosome as it received it, and yet it seems as though it is not any and every biting fly which can pass on the trypanosome of nagana, or of sleeping sickness, but only the particular species of tsetse-fly. Perhaps it is a case of greater abundance, the tsetse-flies being the obvious and dangerous carriers of trypanosome disease where they occur, on account of their abundance and the fierceness and celerity of their attack. It is almost certain that in India, Burma, and South America some other flies must transfer the trypanosomes from animal to animal, causing the diseases known as surra and mal de caderas, because no tsetse-flies—that is to say, no flies of the genus Glossina—occur in those countries, and no other mode of transference, except by some blood-sucking insect, seems probable.
Ants in Africa are carriers of infection, and possibly also in London kitchens, where a little red ant sometimes abounds. The black beetle or cockroach is a creature to be got rid of, as it is very probable that it spreads certain kinds of infection over food and dishes during the hours of “revelry by night” which kind-hearted people allow it to enjoy in their kitchens.
19. Cerebral Inhibition
The best golf-player does not think, as he plays his stroke, of the hundred-and-one muscular contractions which, accurately co-ordinated, result in his making a fine drive or a perfect approach; nor does the pianist examine the order of movement of his fingers. His “sub-liminal self,” his “unconscious cerebration,” attends to these details without his conscious intervention, and all the better for the absence of what the nerve-physiologists call “cerebral inhibition”—that is to say, the delay or arrest due to the sending round of the message or order to the muscles by way of the higher brain-centres, instead of letting it go directly from a lower centre without the intervention of the seats of attention and consciousness. The sneezing caused in most people by a pinch of ordinary snuff can be rendered impossible by “cerebral inhibition,” set up by a wager with the snuff-taking victim that he will fail to sneeze in three minutes, however much snuff he may take. His attention to the mechanism of the anticipated sneeze, and his desire for it, inhibit the whole apparatus. So long as you can make him anxious to sneeze and fix his attention on the effort to do so, by a judicious exhortation at intervals, he will not succeed in sneezing. When the three minutes are up, and you both have ceased to be interested in the matter, he will probably sneeze unexpectedly and sharply. I was set on to this train of thought by a recent visit to an exhibition of photographs.
There were many very interesting illustrations of the application of photography to scientific investigation. Among others I saw a fine enlarged photograph of the common millipede (Julus terrestris), and my desire was renewed to have a bioscopic film-series of the movements of this creature’s legs. Some years ago I attempted to analyse, and published an account of, the regular rhythmic movement of the legs of millipedes. I found that the “phases” of forward and backward swing are presented in groups of twelve pairs of legs, each pair of legs being in the same phase of movement as the twelfth pair beyond it. But instantaneous photography would give complete certainty about the movement in this case, and in the case of the even more beautiful “rippling” movement of the legs of some of the marine worms. Some kindly photographer might take up the investigation and prepare a series of films. The problem is raised and the effects of “cerebral inhibition” described in a little poem which I am told we owe to the author of “Lorna Doone.” As it is not widely known, I give it here as a record of “cerebral inhibition”:
“A centipede was happy ’til
One day a toad in fun
Said, ‘Pray which leg moves after which?’