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Opportunities for Breeding of Mosquitoes at Rebecca Shoal

water, period, eggs, tanks and larva

OPPORTUNITIES FOR BREEDING OF MOSQUITOES AT REBECCA SHOAL Obviously, in order to draw correct conclusions concerning the occurrence of mosquitoes and flies at an isolated point, it is essential that breeding-places there be under control. It was found that the only fresh water available to mosquitoes at the Rebecca Shoal light station was contained in three sorts of receptacles; first, the four tanks on the lower floor for the storage of water obtained by draining through iron spouts the rain-water falling on the slate roof ; second, the small depressions formed in the angles of the iron frame understructure; and third, the bilge-water contained in the two boats suspended from davits.

Concerning the small pockets and crevices in the steel frame of the structure, it is only necessary to state that all water which lodged therein was entirely evaporated by the sun in less time than would be required merely for the hatching of mosquito eggs. Since rains were infrequent in the period during which these observations were made, the possibility of a replenishment was precluded. It is conceivable that in a period of daily rainfall eggs deposited by mosquitoes in these depressions might develop and by good fortune yield a few adults. Still the rapid overflow during the characteristically heavy showers would doubtless wash out most, if not all, eggs and larvae had they been present.

The storage tanks were accessible to mosquitoes since their covers did not fit closely; therefore it was necessary to be certain that no larva were present. Careful examination of the surface of the water

and of the contents on June 27 failed to reveal a single trace of larva, pupa, molted skins, or adults which might have been drowned in the act of depositing eggs. Indeed, the lighthouse keepers, Mr. Lopez and his assistants, all of whom were familiar with the larva, agreed that they had never noted in water drawn from the tanks a single "wiggle-tail." Had larvae been present they would probably have been detected, for the water is all drawn from the bottom of the reservoirs.

Owing to the ease and thoroughness with which frequent examina tions of these tanks could be made, it was deemed unnecessary to treat the water surfaces with kerosene. Vigilance and daily examinations throughout the period of observations disclosed no sign of mosquitoes at any stage of their life-history in the reservoirs.

The third possible breeding-place (the boats), is also an extremely unfavorable one. In the case of the power launch a heavy film of lubricating oil is always present, while the sea-water in the bottom of the small dory, besides being too saline owing to evaporation, is also easily examined. No mosquitoes bred there.

From the above statement of conditions and the past experience of the keepers it is clear that mosquitoes rarely, if ever, breed at Rebecca Shoal light-station. Certainly none attempted to do so during the period covered by these investigations.