Carbon-dioxide sensing structures in terrestrial arthropods

Sensory structures that detect atmospheric carbon dioxide have been identif ied and described to the subcellular level in adults of Lepidoptera, Dipter a, Hymenoptera, Isoptera, Chilopoda, and Ixodidae, as well as in lepidopter an larvae. The structures are usually composed of clusters of wall-pore typ e sensilla that may form distinct sensory organs, often recessed in pits or capsules. In insects, they are located on either the palps or the antennae , in chilopods on the head capsule, and in ixodids on the forelegs. In the two cases where the central projections have been examined (Lepidoptera and mosquitoes), the clustering is preserved to the level of second order neur ons, which are located in the deutocerebrum. Individual sensilla usually co ntain a single receptor neuron that is sensitive to CO2; it may be accompan ied by other neurons that respond to other olfactory qualities. The distal dendritic processes of CO2-sensitive neurons invariably show an increased s urface areal dividing into many cylindrical branches or into lamellar struc tures. Lamellar membranes are often closely linked to arrays of microtubule s. Fine pore canal tubules are usually associated with the cuticular pores. (C) 1999 Wiley-Liss, Inc.