The sensory structures of the antennal flagellum in Hyalesthes obsoletus (Hemiptera: Fulgoromorpha: Cixiidae): A functional reduction? Roberto Romani a, * , Marco Valerio Rossi Stacconi a , Paola Riolo b , Nunzio Isidoro b a Dipartimento di Scienze Agrarie e Ambientali, Perugia University, 06121 Perugia, Italy b Dipartimento Scienze Ambientali e delle Produzioni Vegetali, Marche Polytechnic University, 60131 Ancona, Italy article info Article history: Received 3 April 2009 Accepted 6 August 2009 Keywords: Ultrastructure Sensilla Scolopidia Thermo-hygroreceptors CO 2 receptors Phytoplasma vectors FIB abstract Despite their relevance as harmful pests on plants of economic importance, Hemiptera Fulgoromorpha have been poorly studied as regards their antennal sensory structures. In particular, the flagellum has been neglected and, therefore, to date there are no data on its structural organization and sensory equipment. In order to fill this gap, we carried out a study on the sensillum types and distribution on the flagellum of the planthopper Hyalesthes obsoletus Signoret, an efficient vector of the stolbur phytoplasma, the cause of various crop diseases. In this cixiid species the antenna is composed of three segments, the scape, an enlarged pedicel and a long flagellum. This latter is made of a single segment and presents a basal, bulb-like enlargement from which two processes arise, a short spur and a long arista. Combining scanning electron microscopy, transmission electron microscopy and focused ion beam investigations, we discovered the presence of a total number of 6 sensilla, belonging to 4 different types: a single scolopidium extending from the bulb to the arista, three sensilla styloconica within the cuticular spur and two different sensilla coeloconica inside the bulb. As far as structural data can suggest, these sensilla might be involved in the perception of mechanical stimuli (possibly air-borne vibrations), temperature and humidity variations and CO 2 concentration. The strong reduction in sensillum number in this species is discussed as possible functional specialization of the flagellum itself. The ultrastructure of the sensilla in the flagellum of a species of Fulgoromorpha is here presented for the first time. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Cixiidae, one of 21 families of Hemiptera Fulgoromorpha, are composed mainly of phytophagous species feeding on plant phloem (Bourgoin et al., 2004). Comprising about 160 genera and 2000 species, the family is distributed in all zoogeographical regions (Holzinger et al., 2002), species richness being higher in the tropics. Some cixiid species are considered to be pests of economically important crops, acting as vectors of plant pathogens such as viruses, bacterium-like organisms and phytoplasma. Hyalesthes obsoletus Signoret (Homoptera, Fulgoromorpha, Cix- iidae) is a palaearctic species of economic importance, since it is reported to transmit a phytoplasma known as Bois Noir (BN) or Vergilbungskrankheit (VK) to grapevine (Vitis vinifera L.) (Maixner, 1994; Maixner et al., 1995). H. obsoletus can acquire the phyto- plasma during the post-embryonic development while feeding on the roots of several wild plants. Different preferred plant species have been recorded in different countries or geographic areas [Convolvulus arvensis L. and Urtica dioica L. in Germany (Langer and Maixner, 2004); Lavandula hybrida Reverchon and Lavandula angustifolia Miller in France (Sforza et al., 1999); Vitex agnus-castus L. in Israel (Sharon et al., 2005)]. The winged adults then move to other host plants and, occasionally, to grapevine. Here, through feeding punctures, adults of H. obsoletus transmit the disease, which causes discoloration and necrosis of leaf veins and leaf blades, downward curling of leaves, lack or incomplete lignification of shoots, stunting and necrosis of shoots, abortion of inflores- cences and shrivelling of berries. Although some aspects of the biology and behavior of this pest are known (Leclant, 1968; Alma et al., 1987; Sforza et al., 1998; Darimont and Maixner, 2001; Langer et al., 2003), there is still scant information on the mechanism that drive both host and mate selection. In particular, there are no data on the antennal sensory equipment of this insect, despite the crucial role that the antennae play in insect’s host location and recognition (Anderson et al., 2000; Isidoro et al., 2001; Kristoffersen et al., 2006), as well as mating behavior (Bartlet et al., 1994; Romani et al., 2008). Antennal sensilla have been proved to be involved in the perception of different kind of stimuli in different insect orders (Altner and Loftus, 1985; Keil, 1999; Kristoffersen * Corresponding author. Tel.: þ39 0755856032; fax: þ39 0755856039. E-mail addresses: rromani@unipg.it (R. Romani), elivale82@tiscali.it (M.V. Rossi Stacconi), p.riolo@univpm.it (P. Riolo), n.isidoro@univpm.it (N. Isidoro). Contents lists available at ScienceDirect Arthropod Structure & Development journal homepage: www.elsevier.com/locate/asd 1467-8039/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.asd.2009.08.002 Arthropod Structure & Development 38 (2009) 473–483