Odonatologica38(4): 329-340 December I, 2009 A comparative investigation of the antennal sensilla in adult Anisoptera M. Rebora*, S. Piersanti and E. Gaino Dipartimentodi BiologiaCellulare e Ambientale,Università di Perugia, Via Elce di Sotto, I-06123 Perugia, Italy Received May 4, 2009 / Revised and Accepted July 9, 2009 A fine structural overview of the flagellar sensilla of Onychogomphus forcipatus (Gomphidae), Aeshna cyanea (Aeshnidae),Somatochlora metallica (Cordulidae)and boltonii Cordulegaster (Cordulegastridae) revealed the presence of pits containing sensilla located the latero-ventral typically on side of the first flagellar segments in all four species. These sensilla are represented by coeloconic single-walled olfactory sensilla and sunken sensilla deeply styloconica (type-1 and type-2) sharing common features typicalof thermo-hygroreceptors. Sensilla styloconica arelocated inside deep convoluted cavities. It is suggested that olfactory and thermo-hygrosensory sensilla the main are sensilla on the antennae of all anisopteran families. The attribution of the coeloconic sensilla of dragonfliesto single-walledolfactory sensilla (confirmed by the finding of pore tubules in O. forcipatus), together with their common occurrence in the suborder Anisoptera, are relevant for phylogenetic studies. INTRODUCTION “The face of is a dragonfly practically nothing but eyes!”, as a Japanese haiku states. Indeed, dragonflies are remarkable in their powers of vision (CORBET, this the 1999). In regard, sensilla located on the antennae of Odonatahave been An overview the antennal poorly investigated. early on flagellum of some Ani- and & soptera Zygoptera species (SLIFER SEKHON, 1972) reports the pres- ence of coeloconic sensilla located in simple and compound cavities. Some behav- ioural studies on Orthetrum cancellatum (GEWECKE et ah, 1974; GEWECKE & consider the and sites for ODENDAHL, 2004) scape pedicel as important air speed perception and flight control. Recently, a finestructural investigation of the adults of the dragonfly Libellula * Corresponding Author: e-mail [email protected]; - tel. ++390755855729; - fax ++390755855733 330 M. Rebora, S. Piersanti & E. Gaino depressa (Libellulidae) performed by REBORA et al. revealed the (2008) pres- of sensilla located in ence pits on the latero-ventralside of the antennalflagellum. sensilla These are represented by sensilla coeloconica and by deeply sunken sen- silla styloconica. The structure of the sensilla coeloconica is coherent with that for insect reported single-walled chemoreceptors. The deeply sunken sensilla are kinds of sensilla represented by two styloconica sharing common features typi- cal of thermo-hygroreceptors. The aims present paper to investigate whether the putative olfactory sensilla and the two kinds of putative thermo-hygrosensory sensilla described in L. de- in pressa are widespread Anisoptera. Thus, the sensilla located on the antennal flagellum of adults belonging to Onychogomphus forcipatus (Gomphidae), Aesh- na cyanea (Aeshnidae), Somatochlora metallica(Cordulidae) and Cordulegaster boltonii (Cordulegastridae) are analyzed using scanning electron microscopy (SEM), These four species, together with L. depressa (Libellulidae) (REBORA et al., 2008), represent the five Anisoptera familiesfound in Italy. In order add information to on the internal structure of the above reported sensilla, already described in L. depressa (REBORA et al., 2008), transmission electron microscopy (TEM) has been carried out on the antennae of O. forcipa- tus, a common species in southern Europe (DIJKSTRA & LEW1NGTON, 2006) belonging to the superfamily Aeshnoidea. The results here of presented are part a programme on the fine structure of antennal sensilla in well Studies the zygopterans as as anisopterans. on sensory of in consideration biology Odonata, of their belonging to Paleoptera, are also useful for phylogenetic studies. MATERIAL AND METHODS Adult and Aeshna Onychogomphusforcipatus cyanea were collected in Umbria (central Italy) dur- 2007. Somatochlora metallica ing summer Dry and Cordulegasterboltonii specimens were investigated after a procedure of rehydration (ALBA-TERCEDOR, 1987). In the laboratory, antennal flagella were dissected from anaesthetised specimens and fixed for 12 hours in 2.5% glutaraldehydein cacodylate buffer, pH 7,2. For scanning electron microscopy (SEM) analysis, the fixed material was rinsed for three times in the same buffer, dehydratedusing a graded ethanol series and critical-point dryed in a CPD 030 Bal- Tec critical-point dryer (Bal-Tec Union Ltd, Balzers, Liechtenstein). Specimens were mounted on stubs with silver with in Emitech K550X conducting paint, sputter-coated gold-palladium an sput- terer (Emitech, Ashford, England), and observed with a Philips XL30 SEM (Philips, Eindhoven, The of 18kV. Netherlands), at anaccelerating voltage For observations of the inner cuticular wall of the antenna, the flagella were sectioned longitudinally with a razor blade and cleaned with KOH-solu- tion. These then in flagellawere dehydrated a graded ethanol series, dried in an oven and gluedonto the SEM specimen supports in order to allow the observation of the inner side of the antenna. For transmission electron microscopy (TEM), the fixed antennal flagella were rinsed three times in buffer cacodylate and post-fixed for 1 hour at 4°C in 1% osmium tetroxide in cacodylate buffer. Afterward the material was washed three times in the same buffer, dehydrated using a graded etha- nol series and embedded in an Epon-Aralditeresin mixture. Ultrathin sections, cut on a Leica EM Antennal sensilla in adult Anisoptera 331 UC6 ultracut (Leica Microsystem GmbH, Wien, Austria), were collected on formvar-coated cop- stained with uranyl acetate per grids, and lead citrate, and examined with a Philips EM 208 (Philips, Eindhoven,The Netherlands). RESULTS OBSERVATIONS UNDER SEM - The antenna consists of Onychogomphus forcipatus (Gomphidae). a scape, The a pedicel and a two-segmented flagellum. proximal flagellar segment is well terms of its in developed (in sensory structures) comparison with the distal one and bears sensilla located in numerous pits on its latero-ventral side (Fig. la). Most of these sensilla coeloconicawhile few pits (about 40) host a of them (about 2) are widerand represent the openings of large cavities inside the antenna (Figs The sensilla coeloconica visible the la-c). are on antennal surface as pegs locat- ed in pits (Figs la, c, d). The cuticle of the peg bears pore-like structures clearly evident under the SEM (Fig. The cavities each measure about 40 Id). large pm in width and about 20 in The is convoluted and hosts pm depth. cavity very nu- merous type-1 and type-2 deeply sunken sensilla styloconica (Fig. 1c), bothchar- wide acterized by a stylus bearing an apical cone (Fig. le). Type-1 sensilla have a cone with cuticular ‘fingers’ in its distal portion (Fig. le). Type-2 sensilla have with cuticular surface a cone a rugged (Fig. le). No pore is visible on the cuticle of either type-1 or type-2 sensilla (Fig. le). Aeshna — The consists of cyanea (Aeshnidae). antenna a scape, a pedicel and antennal an flagellum of four segments. The first (proximal) and the second flag- ellar show segments wide pits on their latero-ventral sides (Fig. If), with about 6 the first and on segment 3 on the second. Images of the inner cuticular wall re- veal that these wide the of pits are openings convoluted cavities, which measure about20 in width and 30-50 in pm pm depth (Fig. Ig). Longitudinally fractured cavities show coeloconic located the pegs on edge of the pits (Figs 1 h, i) and type- 1 and type-2 deeply sunken sensilla styloconica located inside the cavities (Fig. The sensilla coeloconica bear their Ij). pore-like structures on surface (Fig. li). Type-1 sensilla have a cone with cuticular ‘fingers’ in their distalportion (Fig. Ij). sensilla have with cuticular surface No is Type-2 a cone a rugged (Fig. Ij). pore visible the on cuticle of either type-1 or type-2 sensilla (Fig. Ij). Somatochlora metallica(Corduliidae). — The antenna consists of a scape, a and antennal of four pedicel an flagellum segments. The first (proximal) and the second their flagellar segments show, on latero-ventral sides, a number of pits with about 40 the first and (Fig. 2a), on segment 3 on the second. The pits are isolated or in groups of two or three (Figs 2a, b) and most of them host sensilla coeloconica (Figs 2a, b, d), while some represent the openings of deep cavities (Figs The sensilla coeloconica are evident on the antennal surface 2a-c). as pegs with pore-like structures on the cuticle (Fig. 2d). Each deep cavity is parallel to 332 M. Rebora, S. Piersanti & E. Gaino Fig. 1. Longitudinallyfractured antennal flagella of Onychogomphusforcipatus(a-e) and Aeshna cya- nea (f-j) under SEM; (a) outercuticular wall of the first flagellar segment showing pits, on its latero- ventral side, containingsensilla coeloconica; arrows point to the openings of large cavities bearing deeply sunken sensilla styloconica. Scale bar 50 pm; — (b) inner cuticular wall showing the shape of the cavities Scale bar — view of fractured large (arrows). 50 pm; (c) enlarged a cavity showing type- sensilla bar - detail of sensillum I and type-2 deeply sunken styloconica. Scale 10 pm; (d) a coelo- Antennal sensilla in adult Anisoptera 333 the longitudinal axis of the antenna and measures about 15 jam in width and 70 in It is |am depth. very convolutedand hosts numerous type-1 and type-2 deeply sunken sensilla sensilla have styloconica (Fig. 2c). Type-1 a cone with cuticular in its distal sensilla have with ‘fingers’ portion (Fig. 2e). Type-2 a cone a fairly cuticular surface No is visible the of rugged (Fig. 20. pore on cuticle either type- 1 or type-2 sensilla (Figs 2e, f). Cordulegaster boltonii - The (Cordulegastridae). antenna consists of a scape, and a pedicel an antennal flagellum of four segments. The first and the second flagellar segments show wide pits on their latero-ventral sides (Fig. 2g), with 9 and about on the first segment 3 on the second. Images of the inner cuticular wall reveal that the wide the pits are openings of convoluted cavities measuring about 20 in widthand 20-30 in pm pm depth (Fig.