Fine Structure of the Sense Organs on the Labella and Labium of the Mosquito Aedes aegypti (L.)

ABSTRACT

Fine structure of the sense organs on the labella and labium of male and female mosquito Aedes aegypti is described. Labellar hair on the outside of the two labellar lobes are consisted of long mechanoreceptive hairs, medium-sized chemoreceptive hairs containing 3-5 dendrites, and short papillae which are probably olfactory receptors. Two apical hairs each containing five dendrites not reported before are found deeply embedded inside each labellum. They emerged between folds at the tip of the labellum. These and other sensory hairs on the outside of the labella are probably involved in finding a suitable place for feeding after a mosquito has landed on a host. Six anteriorly directed papillae each containing 3-5 dendrites are found on the oral surface of each labellar lobe with no evidence of mechanoreceptors associated with these papillae. These papilla are probably chemosensory and are involved in detecting the food entering the food canal when a mosquito feed on water and other liquid diet such as nectar. A chordotonal organ with two sensory cells is found inside each labellum, and this organ has not been described on mosquito mouthparts before. These chordotonal organs probably function to monitor the spreading and closing of the labellar lobes during feeding. Mosquito spread their labellar lobes when feeding on water and sugar but these lobes are firmly pressed against each other when they feed on blood. Ligular hairs are definitely not sensory because of a lack any dendrites inside these hairs. Labial hairs proximal to the labella are probably mechanoreceptors because only one nerve cell is associated with each hair with nerve terminating at the base of the hair. Based on results from behavioral and functional studies, the function of these sensilla during feeding is described.

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