MOSQUITO ANTENNAL OLFACTORY SENSILLA : ULTRASTRUCTURE AND DISTRIBUTION (DIPTERA : CULICIDAE)

Abstract

A classification system for mosquito antennal olfactory sensilla was developed based on key external and internal ultrastructures studied in both sexes of eight species of mosquitoes (Aedes aegypti, Aedes albopictus, Aedes togoi, Culex guinguefasciatus, Toxorhynchites splendens, Anopheles maculatus, Anopheles sundaicus and Anopheles vagus). The key ultrastructures were: i. external morphology of sensilla shaft, ii. nature of multiporous sensilla shaft, iii. nature of outer dendrites, iv. relative volume of receptor lymph cavity, and v. relative number and nature of trichogen cell lamellae. Five classes of olfactory sensilla were identified and termed POT, TAT, NTT, SB and SC. The first four classes of sensilla were present in all three mosquito subfamilies while SC was restricted to the Anophelinae. Ultrastructurally, POT, TAT and NTT had sensilla shafts that were single-walled while those of SB and SC were double walled. The latter two sensilla possessed spoke canal stimulus conduction system and two ( inner and outer) receptor lymph cavities. The three classes of trichodea (POT, TAT and NTT) had receptor lymph cavities with relative sizes that were proportional to the relative porosity of their sensilla shafts. Intraspecifically, sensilla shafts increased in thickness in the order of NTT, TAT, and POT. Females of seven species had larger pores on POT, except Cx. quinguefasciatus, but pore density was however very low (3-6 pores ?m-2). TAT and NTT had pore densities in the range of 9-23 pores and 19-75 pores ?m-2 respectively. Females also had more and larger pores on SB and SC than males except for the three aedines and An. vagus where pore densities were similar between the sexes. In the anophelines, SC was also more porous than SB. All the trichodea sensilla were innervated by two bipolar neurons; their outer dendrites were branched in TAT but not in POT and NTT. SB and SC had 2-5 and 3-5 bipolar neurons respectively: outer dendrites always unbranched. Trichogen cell lamellae were adpressed in POT but 'free' in the other sensilla classes. Sensilla distribution patterns were genus-specific; an attribute usable as a taxonomic character in the separation of the mosquito genera studied. The number of olfactory sensilla was a species-specific measure independent of antennal size. All eight male mosquitoes possessed more sensilla on the last flagellomere than on the penultimate flagellomere. POT in both sexes was predominantly situated medially while TAT was significantly peripherally located. Significantly, more SB was located peripherally in all eight females and males of the three anophelines. NTT was either exclusively or predominantly located on the peripheral antennal surface in both sexes of all eight species except in male anophelines. Female aedines also had NTT oriented in a linear manner from one flagellomere to the next. SC was exclusively peripheral in females and medial in males of the three Anopheles. This unique peripherally oriented NTT and SC may provide information on the direction of the chemical source via temporal differences in afferent inputs from the two groups of peripherally displaced sensilla on the two antennae.