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The fine structure of the thin-walled pegs known to respond to carbon dioxide was studied in Aedes aegypti (L.), Culex pipiens L., and Culex territans Walker. Each peg is composed of an external cuticular structure with numerous pores which occur in groups, three bipolar neurons, and sheath cells. In A. aegypti two morphologically different types of neurons were found innervating the same sensillum: two electron-dense neurons and one electron-lucid neuron per peg. In addition to distinctive cell bodies and nuclei, the two types of neurons have different organization patterns of their dendritic processes within the lumen. Culex pipiens is shown to have several times more area available for perception of carbon dioxide than A. aegypti.

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In this study, the spatial and temporal patterns of emergence of mosquitoes in a temporary autumnal pool were related to parasitism by larvae of the mites Arrenurus kenki Marshall and Arrenurus angustilimbatus Mullen. There was differential mite parasitism among the sympatric mosquito species; both mite species had an apparent host preference for Aedes cinereus Meigen and Aedes excrucians (Walker). Only A. cinereus, Aedes diantaeus Howard, Dyar and Knab, and A. excrucians had a significant temporal overlap with the parasitic phase of the two Arrenurus species. Therefore, host availability was seasonally limited. Although the abundance of parasitic mites was greater on female than on male. A. excrucians over the total emergence period, there was no difference when the sexes were compared on each sample date. This was due to close temporal association of mite abundance and female emergence, rather than to the mites discriminating between the sexes. The spatial distribution of the mites within the pool was not closely related to that of any mosquito species.