According to Sambon, the explanation lies in the circumstance that the men, in their vocation as fishermen, often have occasion to visit the mainland and to pass the night there, and are consequently exposed to the bites of Simuliidæ and other midges; whereas the women, having no occasion to leave the island, escape infection.
Fig. 71.—Simulium larva.
(Drawn by W. McDonald.)
Simuliidæ are minute (1-4½ mm.), thick-set, dark-coloured flies, provided with moderately long legs often banded with white, and large, broad, transparent wings. They are known in America as "buffalo gnats." They have a peculiar dancing flight, and the females are vicious biters (the males are inoffensive). The eggs are laid in patches on stones or water-weeds, to which they become attached. The larvæ are unmistakable; they have an elongated, club-shaped body, of a greenish-brown colour, supplied with a large terminal sucker at the larger posterior extremity, by means of which they attach themselves to pebbles, water-weeds, and drift-wood. The larva spins a silken thread which offers a further means of attachment and serves to weave the characteristic pocket or slipper-like cocoon at the time of pupation. The cocoon is open at the larger anterior end, from which protrudes the anterior portion of the pupa with two lateral tufts of long feathery filaments. (Figs. 71, 72, and 73.)
Chironomidæ.—Of this large family only a few genera (Culicoides, Leptoconops, Ceratopogon) come under suspicion. They are exceedingly minute dusky flies, the females seldom more than 1½ or 2 mm. in length. They usually occur in swarms, and are chiefly confined to wooded and shady spots at the mouth of rivers and in the vicinity of the sea. They are among the most irritating and bloodthirsty of insects. The larvæ are worm-like and transparent. They are aquatic, and are found at the surface of stagnant water in ponds, pools, hollow tree-stumps, and epiphytic plants.
Goldberger, who failed to communicate the disease to any one of 16 men by the administration of the discharges, secretions, and tissues of no fewer than 67 pellagrins, rejecting the idea of a germ cause of
