The destruction of larvæ, and of their hiding places, is commenced by the clearing of grass and bushes, which are cut down with the machete, a short cutlass with which the Panamanian is very expert, machete work being, indeed, the principal rôle in which the Panamanian is employed by the Commission. Also ditches are cleaned out, and heavy oil poured upon the water in trenches and pools, and land-crabs are caught and the holes in which they dwell are filled in or oiled. Finally the soil is drained, which is the only means of making the ground permanently unfit for mosquito breeding. Subsoil draining is the best, a tile drain being put in; for, even in concreted gutters, pools will form, owing to accidental obstruction, and remain sufficiently long for the deposition and hatching of the larvæ. Such is the work of the anopheles brigade, and the stegomyia brigade carry out similar operations, in the neighbourhood more particularly of Panama.
With regard to the effect of these operations upon the numbers of the mosquitoes I may narrate my own experience. I arrived at Colon first in January, 1907, and spent one or two nights on board my ship. This was two years and a half after the commencement of the mosquito campaign, and the officers of the ship congratulated themselves upon the absence of the swarms of mosquitoes which formerly attacked them at night on their vessel. I found, however, that although there was no swarm of mosquitoes, such as I have seen, e.g., when on board ship in the harbour of Colombo, Ceylon, yet that the individuals who remained certainly caused me discomfort, and I think some subsequent indisposition. In April, 1908, however, during two days at Colon, I did not so much as see a single mosquito.
LOOKING NORTH FROM RAILWAY BRIDGE AT PARAISO.
ABANDONED FRENCH MACHINERY.
At Panama, in January, 1907, my wife and I stayed in the Commission's screened hotel on Ancon Hill, not caring to face the dirt and squalor of the old city. In April, 1908, finding the city properly paved, drained, and plumbed, we took up our quarters at the Hotel Central in the town, where we spent a fortnight in perfect health; and although this building, not being under the Commission, is unscreened, I was only bitten by mosquitoes, to my knowledge, twice during that time, and this without subsequent ill effect. I may add that the picturesque surroundings, not unlike those of some city on the Mediterranean, greatly enhance the pleasure of a stay on the Isthmus, now that they can be enjoyed without squalid accompaniments. I did not, except on one or two nights, even draw the mosquito curtains. Out of doors, in the city of Panama, I was not bitten once, though I was attacked once or twice by solitary mosquitoes when walking on roads or paths with shrub or jungle adjoining. This was near the end of the dry season. When the rains commence a greater number of mosquitoes must be expected.
Natives of the Isthmus and the West Indies are not immune from malaria, and in 1904-5 about one-half of the inhabitants who were examined proved to have the parasite of malaria in their blood. As the anopheles becomes infectious through biting a malarious man, it is evident that such a dissemination of the parasite throughout the blood of the human population renders mosquitoes especially dangerous. In the same proportion as the population becomes less malarious, so the mosquitoes become less dangerous, and theoretically a millennium is possible in which man and anopheles, mutually purged of the malarial organism, may live happily together. Unfortunately, a malarious man it is believed remains infectious to anopheles for no less than three years, instead of the three days' limit of yellow fever, and this greatly increases the difficulty of exterminating malaria.