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Conditions governing the distribution of insects in the free atmosphere. II. Surface and upper winds



Conditions governing the distribution of insects in the free atmosphere. II. Surface and upper winds



Canadian Ent 77(2): 21-28



Making use of data presented in Part I of this series (see preceding abstract), Part II discusses surface and upper winds. On the basis of laboratory expts. designed to simulate horizontal and vertical air currents, it was concluded that it is incorrect to assume that the horizontal distributive power of the lower winds may be applied directly to the winds in the upper air. On the contrary, it is believed that the plankton zone of the air (60 m. and above) is populated by vertical currents, especially at levels where the temp. is below the min. flight temp. of winged insects. It was recorded (in Part I) that certain min. temps. caused winged insects to cease flight and fold their wings. Hence less area per unit wt. was presented to an air stream in the lower temps. To determine the min. horizontal air speed necessary to move an inactive insect, dead, dried insects, with wings folded, half-extended, or fully extended were placed in a horizontal glass tube open at both ends. An air stream sufficient to move the insect body was applied and measured with a windmill anemometer. The vertical air speed necessary to support an insect body was found by placing such insects on wire gauze at the bottom of a vertical glass tube, and applying and measuring the speed of the air stream. In both cases the effect of friction was checked by hanging the insect on a fine hair. The results were plotted in 2 curves, mass against vertical and horizontal speeds, respectively. The horizontal air velocity necessary to move an insect increased rapidly with mass up to 7 mg. and then became quite constant up to at least 25 mg. The vertical velocity necessary to support an insect increased steadily with the mass. Results were checked by determining the horizontal and vertical velocities required to move and suspend certain selected insects. From the previously established curves, the masses of these were then estimated; the estimated mass was found to agree well with the actual dry wt. With insects larger than the average culicine mosquito, with large wing areas partly or fully extended, the relations broke down, but with smaller insects the position of the wings made no appreciable difference. The vertical currents necessary to support the small insects which make up the bulk of the plankton are small, but are twice the magnitude of the horizontal currents required to move them. The data given by Glick (U. S. Dept. Agr. Tech. Bull. 673, 1939) are examined and found not to support his contention that horizontal currents are alone agents in the distribution of insects in the upper air. It is suggested that, because of the effect of temp. on insects, that the plankton zone of the air be divided into a lower, active plankton, above +7[degree] C, in which insects maintain themselves by wing movements, and an upper, inert plankton, below +7[degree] C, in which vertical currents are necessary to support inactivated insects.

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