Aerial Oviposition Flight of Toxorhynchites amboinensis (Diptera: Culicidae)

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Abstract
The aerial oviposition flight of Toxorhynchites amboinensis Doleschall is described quantitatively from videotaped behavior. Typical of the genus, the females fly counterclockwise in a sequence of ellipses (6–43) obliquely oriented to the oviposition container, until an egg is ejected on the downward flight of the final ellipse. Throughout the sequence, complex but consistent changes occur in the ellipses. Major and minor axis lengths and calculations of circumference show that ellipses are initially small. They increase rapidly in size for about the first quarter of the sequence, and remain relatively large until the final quarter, when they rapidly become smaller until the final (egg) ellipse. Ellipses become progressively flatter through most of the sequence, but with some reversal of this trend just prior to and during the egg ellipse. The angle of the major axis to horizontal diminishes progressively, as does the average flight speed (excepting the egg ellipse) and distance from the center of the oviposition dish. Individual ellipses are composed of two stages, downflight (lower, and towards the dish center) and upflight. Flight speed in each stage passes through accelerative and decelerative phases. In the larger ellipses, acceleration, maximum speed, and deceleration are greater in upflight than in downflight. In the egg ellipse, acceleration in downflight considerably exceeds that in all preceding ellipses; also, the female's abdomen swings forward through an arc of ca. 80°, causing the abdominal tip to attain a maximum speed slightly in excess of 100 cm/s, before suddenly and very rapidly decelerating when full forward extension is reached. The egg is presumed to leave the abdomen at high velocity, owing to inertial forces occurring at the start of deceleration. In two experiments to measure the trajectory angle of ejected eggs, 88.7 and 97.7% of eggs, respectively, were thrown within a 24° arc 48–72° relative to vertical, which caused most eggs to be directed at the junction between the bottom of the dish and its vertical edge. The special problems posed by aerial oviposition, possible mechanisms of orientation during the ellipse sequence, and the significance of the trajectory arc are discussed.