Ultrastructure and receptor cell responses of the antennal grooved peg sensilla of Triatoma infestans (Hemiptera: Reduviidae)

Abstract

Ultrastructural examination of grooved-peg (GP) sensilla on the antenna of fifth instar Triatoma infestans nymphs by scanning electron microscopy and transmission electron microscopy reveal that they are 8-18 mm long with a diameter of about 2 -2.8 mm at the nonarticulated base. Some pegs have a terminal pore. These double-walled wall-pore (dw-wp) sensilla have an outer cuticular wall with 13 -18 longitudinal grooves at the distal part of the peg. Groove channels are present at the bottom of the grooves from which radial spoke channels lead into the inner sensillum-lymph cavity. A dendrite sheath connects the tip of the thecogen cell to the inner cuticular wall thus forming separated outer and inner sensillum-lymph cavities. Four or five bipolar receptor cells are ensheathed successively within the GP sensilla by the thecogen cell, trichogen and tormogen cells. The inner dendritic segments of each sensory cell give rise at the ciliary constriction to an unbranched outer dendritic segment which can reach the tip of the sensillum.Electrophysiological recordings from the GP sensilla indicate that they house NH 3 , short-chain carboxylic acid and short-chain aliphatic amine receptor cells and can be divided into three functional sub-types (GP 1 -3). All GP sensilla carry a receptor cell excited by aliphatic amines, such as isobutylamine, a compound associated with vertebrate odour. GP type 1 and 2 sensilla house, in addition, an NH 3 -excited cell whereas the type 2 sensilla also contains a short-chain carboxylic acid receptor. No cell particularly sensitive to either NH 3 or carboxylic acids was found in the grooved-peg type 3 sensilla. GP types 1, 2 and 3 represent ca. 36, 10 and 43% of the GP sensilla, respectively, whereas the remaining 11% contain receptor cells that manifest normal spontaneous activity but do not respond to any of the afore mentioned stimuli.

Olfaction in vector-host interactions

“…36% of the total GP population, whereas GP2 and GP3 represent 10 and 43% respectively. The receptor cells of 11% of the GPs did not respond to any of the olfactory stimuli tested so far (Diehl et al 2003). At least one of the unresponsive receptor cell types is a hygroreceptor cell (Bernard 1974).…”

“…All segments bear sensilla, hair-like cuticular structures that house sensory cells of different modalities, including chemoreception, mechanoreception, thermoreception and hygroreception (Barth 1952, Catalá 1994, Catalá and Schofield 1994, Diehl et al 2003, Guerenstein and Guerin 2001, Mayer 1968, Bernard 1974, Insausti et al 1999, Justo and Tramezzani 1977, McIver and Siemicki 1984, Lazzari and Wicklein 1994, Wigglesworth and Gillet 1934.…”

“…To date, the structure of three morphologically distinguishable types of olfactory sensilla has been described on the antennae of triatomines. Those sensilla are the grooved pegs (GP; Diehl et al 2003, Guerenstein and Guerin 2001, named basiconica in Catalá 1997, the basiconic sensilla (BS, Guerenstein and Guerin 2001, named trichoid thin-walled in Catalá 1997) and the trichoid sensilla (Guerenstein 1999, named trichoid thick-walled in Catalá 1997). The latter are 35 µm long single (thick)-walled wall-pore sensilla (Bernard 1974).…”

“…The morphology and ultrastructure of the GPs of 5 th instar nymphs of T. infestans have been studied in detail (Diehl et al 2003). They are non-articulated, double-walled wall-pore sensilla, 8-18 µm long, with a diameter of about 2.5 µm at their base.…”

“…To date, the proportion of GPs with a terminal pore has not been established and, moreover, it is unknown if this pore has any functional role. Fifth instar nymphs posses 80-110 of such sensilla per antenna (Diehl et al 2003).…”