Throughout the work of the Army Commission and down to the present time many fruitless efforts have been made to discover the specific organism of yellow fever. It was clearly established that the claims of Sanarelli for Bacillus icteroides were without foundation. It was found, too, that whatever the infective agent might be it was capable of passing through a Berkefeld filter and thus belongs to the puzzling group of "filterable viruses." It was further found that the virus was destroyed by heating up to 55° C for ten minutes. It is generally believed that the organism is a Protozoan.
The question of the hereditary transmission of the yellow fever organism within the mosquito was left unsettled by the Army Commission, though, as we have seen, it was raised by Finlay. Marchoux and Simond, of the French Commission devoted much attention to this phase of the problem and basing their conclusions on one apparently positive case, they decided that the disease could be transmitted through the egg of an infected Aëdes calopus to the second generation and thence to man. The conclusion, which is of very great importance in the control of yellow fever, has not been verified by other workers.
Once clearly established that yellow fever was transmitted solely by mosquitoes, the question of the characteristics, habits, and geographical distribution of the insect carrier became of vital importance.
Aëdes calopus, more commonly known as Stegomyia fasciata or Stegomyia calopus ([fig. 134]) is a moderate sized, rather strikingly marked mosquito. The general color is dark-brown or reddish-brown, but the thorax has a conspicuous broad, silvery-white curved line on each side, with two parallel median silvery lines. Between the latter there is a slender, broken line. The whole gives a lyre-shaped pattern to the thorax. The abdomen is dark with silvery-white basal bands and silvery white spots on each side of the abdominal segments. Legs black with rings of pure white at the base of the segments.
Size of the female 3.3 to 5 mm.; male 3 to 4.5 mm.
It is preeminently a domesticated species, being found almost exclusively about the habitation of man. "Its long association with man is shown by many of its habits. It approaches stealthily from behind. It retreats upon the slightest alarm. The ankles and, when one is sitting at a table or desk, the underside of the hands and wrists are favorable points of attack. It attacks silently, whereas other mosquitoes have a piping or humming note. The warning sound has doubtless been suppressed in the evolutionary process of its adaptation to man. It is extremely wary. It hides whenever it can, concealing itself in garments, working into the pockets, and under the lapels of coats, and crawling up under the clothes to bite the legs. In houses, it will hide in dark corners, under picture moldings and behind the heads of old-fashioned bedsteads. It will enter closets and hide in the folds of garments."—Howard.
It was claimed by the French Commission, and subsequently often stated in discussions of the relation of the mosquito to yellow fever that the mature Aëdes calopus will bite only at night. If this were true it would be of the greatest importance in measures to avoid the disease. Unfortunately, the claim was illy founded and numerous workers have clearly established that the exact converse is more nearly true, this mosquito being pre-eminently a day species, feeding most actively in early morning, about sunrise, and late in the afternoon. On cloudy days it attacks at any time during the day. Thus there is peril in the doctrine that infected regions may be visited with perfect safety during the daytime and that measures to avoid the mosquito attack need be taken only at night.
