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Each maxilla of fourth instar Toxorhynchites brevipalpis bears nine sensilla: Four are located at the tip of the maxillary palp and five on the maxillary body. At the palp tip are three tapered pegs on bulbous bases (MS1, MS2, MS6) that are innervated by four, two, and two neurons, respectively, and probably function in chemoreception. Also at the palp tip is a sturdy, cuticular rod with a lumen (MS5) that opens distally to the exterior. The proximal end of the rod is closed by a cuticular base to which a single unbranched dendrite containing only a few microtubules is attached. The function of MS5 is enigmatic; possibilities include mechanoreception and detection of infrared radiation. On the maxillary body are two tapered pegs on a common bulbous base (GS1, GS2) that are each innervated by three neurons, and probably are chemosensory. Three setae also occur on the maxillary body. They arise from prominent sockets and are each innervated by a neuron terminating at the hair base as a tubular body, a characteristic of cuticular mechanosensilla. The maxillary sensilla are innervated by a total of 18 neurons: 14 are probably chemosensory, three mechanosensory, and one is of unknown function. These results, combined with those from a previous study on antennal sensilla (Jez and McIver, '80), indicate that the mechanosensitive neurons of the antennae and maxillae are a relatively small percentage of the total mechanosensilla on the entire larva. In contrast the chemosensitive neurons of the antennae and maxillae provide most of the information about the chemical environment of the larva. T. brevipalpis has three less than the maximum of seven maxillary palpal sensilla found in larval mosquitoes so far studied. This difference may reflect a lesser need for sensory information about the acceptability of potential food in predators compared to browsers and filter-feeders. |
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