Palpal Sensilla of Selected Anopheline Mosquitoes

Abstract

Palps of both sexes of Anopheles stephensi, A. albimanus, A. quadrimaculatus, and female A. gambiae were examined and found to be equipped to perceive both mechanical and olfactory stimuli. Present on the palps are sensilla chaetica, tactile and/or air current receptors, campaniform sensilla, proprioceptors, and thin-walled capitate pegs, olfactory receptors probably sensitive to carbon dioxide. The fine structure of each type of sensillum is described. Mosquitoes have various types of antennal and palpal sensilla which are responsible for the reception of external stimuli, including those involved in host finding and selection. Several investigators have reported on the detailed structure (Elizarov and Chaika, 1972; McIver, 1971, 1972; McIver and Charlton, 1970; McIver and Hudson, 1972) and function (Bassler, 1958; Kellogg, 1970) of palpal sensilla of culicine mosquitoes, but information about those on anopheline mosquitoes has been lacking. This work was conducted to (a) determine the types of palpal sensilla on both sexes of Anopheles stephensi Liston, Anopheles albimanus Wiedemann, and Anopheles quadrimaculatus Say, and female Anopheles gambiae Giles and (b) describe the fine structure of the sensilla on female A. stephensi. MATERIALS AND METHODS Specimens of A. stephensi, A. gambiae, A. albimanus, and A. quadrimaculatus were obtained from laboratory colonies maintained, respectively, at the Department of Parasitology, Department of Botany, University of Toronto, Central American Malaria Research Station, San Salvador, and the Center for Disease Control, Atlanta, Georgia. For transmission electron microscopy, palps were removed from mosquitoes immersed in Karnovsky's fixative (Karnovsky, 1965) at pH 7 and 4 C and left in the fixative for 18 hr. After rinsing in 0.05 M Sorensen's 10% sucrose buffer, pH 7, the tissue was postfixed in 1% Os04 in veronal acetate buffer at pH 5 and at room temperature for 2 hr. Dehydration through ethanol was followed by embedding in Spurr's low viscosity epoxy medium (Spurr, 1969). For scanning electron microscopy specimens were fixed in 5% formalin and dehydrated through a graded series of ethanols to xylene (Slifer, 1972), attached to the stubs with silver conductive paint, Received for publication 9 December 1974. coated with gold during spin rotation, and examined in a Cambridge Stereoscan microscope. For counting the sensilla, double coverslip mounts of heads were prepared as described by McIver (1969). All numerical results are means of at least 10 counts or measurements. Statistical significance was determined using Student's t test at the 5% level and adjustment for multiple comparison effect was made. Permeable areas in the cuticle were demonstrated by staining whole specimens with crystal violet (Slifer, 1960) from 30 min to 2 hr.