Fine structure of antennal sensilla of larval Toxorhynchites brevipalpis theobald (Diptera : Culicidae)

Abstract

The present investigation of the fine structure of antennal sensilla of fourth instar Toxorhynchites brevipalpis was undertaken to aid in our understanding of the sensory basis of behavior of predacious mosquito larvae and make contributions to the limited knowledge of the sensilla of immature, aquatic insects. Each antenna has 26 neurons which innervate 5 types of sensilla. At the antennal tip are a cone sensillum with a protuberance, a peg sensillum, and an unbranched hair sensillum. Two other unbranched hair sensilla, a branched hair sensillum, and a campaniform sensillum occur in the midregion. Another campaniform sensillum is at the base. Each cone sensillum is innervated by 11 neurons which occur in 6 distinct groups called neuronal units, each consisting of 1–3 neurons with inner and outer sheath cells. Medially on the cone is a small protuberance with a neuronal unit containing 2 neurons. The dendrite of one neuron extends into the protuberance where distally it branches somewhat, while that of the second terminates below the base of the cone and divides into lamellae. Each peg sensillum is innervated by 4 neurons. The dendrites of 3 extend to the peg tip, while that of the fourth ends subterminally. A role in chemoreception is suggested for all the neurons associated with the cone, protuberance, and peg, except for the lamellated dendrite, whose function is obscure. Each branched hair and campaniform sensillum is innervated by one neuron which terminates as a tubular body. Two neurons are associated with each unbranched hair sensillum. The dendrite of one terminates at the hair base as a tubular body, while that of the second extends into the hair shaft. In the proximal region of the hair the latter dendrite contains many microtubules in an electron-dense material, a feature reminiscent of tubular bodies. All neurons associated with the hair and campaniform sensilla probably function in mechanoreception. These results are generally similar to those reported for Aedes aegypti larvae, which are primarily filter-feeders. Apparently any differences in sensory requirements between predatory and filter-feeding larvae are not reflected in the number or structure of antennal sensilla.