Olfactory receptors in basiconic and grooved-peg sensilla on the antenna of fifth-instar Triatoma infestans nymphs respond to host odours. Gas chromatography analyses of host odour extracts coupled to electrophysiological recordings from basiconic sensillum

bugs, have served as models for studies on insect physiology for well over half a century (Wigglesworth and Gillett, 1934). Triatoma infestans (Klug) the most studied species, is one of the main vectors of American trypanosomiasis or Chagas’ disease in South America. Adults and larvae of this insect occupy predominantly domestic habitats, although they are also associated with peridomestic chicken coops, guinea-pig runs and goat and sheep corrals (Schofield, 1994; Gürtler et al., 1996). They feed at night when their hosts are asleep and take refuge in crevices of dwellings during daylight. Triatomines use different sensory cues to locate hosts, one of which is host odour. Host volatiles attracted triatomine bugs on a servosphere when delivered in complete darkness in an air-stream under conditions of constant temperature and humidity, suggesting that host finding can be achieved by the use of olfactory cues alone (Taneja and Guerin, 1995). CO2 has been shown to play an important role in host location by triatomines (Taneja and Guerin, 1995), as is the case for other haematophagous arthropods. However, other components of vertebrate odour were implicated in host-finding by triatomines since the attraction to host odour was stronger than to CO2 alone (Taneja and Guerin, 1995). The odours of stale host urine and one of its components, ammonia, were found to attract T. infestans (Taneja and Guerin, 1997). Furthermore, a complex blend of volatiles, including CO2, produced by the aerobic growth of baker’s yeast, successfully lured triatomines to traps in the laboratory (Guerenstein et al., 1995) and under natural conditions (Lorenzo et al., 1998). To date, the function of two morphologically distinguishable types of olfactory sensilla (Fig. 1) have been studied on the antenna of triatomines. Electron microscopy indicates that the single-walled wall-pore basiconic sensilla are non-articulated and 30 μm long, with a high density of wall pores linked to pore tubules (Bernard, 1974). These sensilla are multiinnervated (21–41 receptor cells) with extensive dendritic branching in the sensillar lumen (M. Vlimant, P. A. Diehl, R. A. Steinbrecht, P. G. Guerenstein and P. M. Guerin, unpublished data). Receptors within this sensillum type on adults were reported to respond to human breath, pyridine and furan (Mayer, 1968). The other morphological type, i.e. the double-walled wall-pore grooved-peg (GP) sensillum, is a nonarticulated longitudinally grooved 8–18 μm long peg, with wall pores that communicate via spoke channels crossing the double cuticular wall to the central lumen of the sensillum (Bernard, 1974). In these sensilla, there are 4–5 receptor cells without 585 The Journal of Experimental Biology 204, 585–597 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 JEB3108