BULLETlN DE L' INSTlTUT ROYAL DES SCIENCES NATURELLES DE BELGIQUE ENTOMOLOGlE, 77: 113-120, 2007 BULLETIN VAN HET KON LNKLLJK BELGJSCH INSTITUUT VOOR NATUURWETENSCHAPPEN ENTOMOLOGLE, 77: 11 3-1 20, 2007

Sensilla and cuticular structures on the larval caudal appendages of Erythromma lindenii (SELYS, 1840) (: Zygoptera: )

by Michel J. FAUCHEUX

Abstract Introduction

The cuticular slruclmes and the sensilla on the larval caudal Little is known about the strncture, diversity and function appendages (caudal lamellae) of E1ythromma lindenii have been of mechanoreceptors in dragonfly larvae (CORBET, studied using scanning electron microscope. Four types of sensilla are visible on the external face of the two lateral lamellae and 1999). The larval antennal of Zygoptera are only now both faces of median lamella. The aporous sensilla fi liformia are beginning to be studied. In Erythromma lindenii, they vibroreceptors which detect a predatory or hostile presence. The possess rnechanoreceptors of various types: sensilla aporous sensilla campaniforrnia are proprioceptors which may play filiformia, sensilla campaniformia, spatula-shaped the role of osmoreceptors here. An olfactive function is presumed sensilla chaetica and curved sensille chaetica (F AUCHEUX, fo r the multiporous sensilla coeloconica and a tactile function for the aporous spatula-shaped sensilla chaetica. The sensilla filiformia 2006; MEURGEY & F AUCHEUX, 2006). As regards the and the sensilla campaniformia have already been described on caudal appendages, seceral functions have attributed the caudal lamellae of Odonata. The sensilla coeloconica and the to them. They may be used for swimming, some of spatula-shaped sensil la chaetica are observed for the first time on them are important respirato1y organs. They also have these appendages. the facility to be autotomized, enhancing the survival Key words: Odonata, Erythromma, caudal appendage, sensilla, of larvae in the presence of predators or aggressive campanofonuium, filiformium, coeloconicum, spatula-shaped conspecifics. In certain species, the caudal appendages chaeticum. perform an adhesive function, or they can be used to detect vibrations (CORBET, 1999). Indeed, a recent study carried out on the Jamellous caudal appendages of Resume Lestes sponsa reveals the presence of aporous sensilla Les structure cuticulaires et les sensilles situees sur les appendi ces filiformia, acting as vibration receptors, and aporous caudaux, ou lamelles caudales, d' Erythromma linden ii sont etudiees sensilla campanifonnia which are proprioceptors with it !' aide de la microscopie electronique a balayage. Quatre types de a presumed osmoreceptive function (FAUCHEUX, 2005). sensilles sont presents a la face externe des lamelles laterales et The aim of the present study is to discover the existence Sur Jes deux faces de la lamelle mediane. Les sensilles filiformes sans pore sont des vibrorecepteurs qui detectent la presence d'un of the above mentioned sensilla in another zygopteran predateur ou d'un ennemi. Les sensilles campaniformes sont des odonatan, Erythromma lindenii (SEL vs, 1840) whose propriocepteurs qui peuventjouer ici le role d'osmorecepteurs. Une antenna! sensory equipment is known (FAUCHEUX, 2006; fonction olfactive est presumee pour les sensilles coeloconiques M.EURGEY & F AUCHEUX, 2006). multipores et une fonction tactile pour les sensilles chetifom1es sans pore en forme spatule. Les sensilles filiformes et campaniformes ont deja ele decrites sur Jes lamelles caudales des odonates. Les sensilles coeloconiques et les sensilles chetiformes en forme de spatule sont Material and methods observees pour la premiere fo is sur les appendices. The larvae of E. lindenii were taken from a pond at Mots-cles: Odonates, Erythromma, appendice caudal, sensilles, campanifonne, filifom1e, coeloconique, chetiforme en forme de "l'Ile Neuve, Le Cell ier, Loire-Atlantique (44), France" spatule. on 15-VI-2003. For the study with scanning electron microscope (S.E.M.), the caudal appendages of three larvae were separated from the abdomen, dehydrated 114 Michel J. FAUCHEUX in absolute ethanol, mounted both on the external and Aporous spatula-shaped sensilla chaetica internal face, on specimen holders and coated with a thin layer of gold and palladium in a JFC 1100 sputter The aporous spatula-shaped sensilla chaetica have the coater. Preparations were examined in a Jeol JSM 6400 shape of a hair 20µm long, cylindrical at the base, then SEM at different magnifications. The terminology of widening to form a hollowed spatula on the external SCHNEIDER (1964) and Z ACHARUK (1985) is used in face, terminating in an excrescence (Fig. 6). The naming the types of sensilla. base with a diameter of 2.3 µm is located in a narrow internal cavity of 3 ~tm in diameter, surrounded by a semi-circular depression of 4.5 µm which limits the Results movement of the hair. No pore was observed at the apex or on the wall. All the sensilla are oriented towards the A - Gross morphology and cuticular structures of distal extremity of the lamellae. caudal appendages. Aporous sensilla .filiformia According to the classification of caudal appendages of larval Zygoptera in relation with shape and posture, The aporous sensi Ila filiformia are very long, slender and habitat (TILLYARD, 1917; DAVIES & TOBIN, 1984), sensilla, measuring from 45 µm to 90 µm in length the appendages of E. lindenii are of type D: lame11ate (Figs 7, 8) and retain a diameter of 1 µm over the and are held in the vertical plane (GARDNER, 1954). whole of their length apa1t from the apex and the base The shape and posture of caudal lamellae correlate (Fig. 9). Unlike the sensilla filiformja of terrestrial within the larval habitat which is lentic permanent in , they are not very rigid and adhere to the cuticle E. lindenii. of lamellae when out of water or when being prepared There are three caudal lamellae: two lateral and one for the study in SEM. Tbei r base is located in a narrow median. An axial trunk divides each lamella into an aperture of roughly the same diameter of the sensillum, upper and a lower one (Figs 1, 3, 4). When the lamellae pierced in a flat bottomed membrane of flexible aspect, are held vertically, it is possible to distinguish both a which is itself bordered by a circular fold, the whole dorsal and a ventral edge. Each lateral lamella possesses is surrounded by a thick cuticular ring (Fig. 9). The an external and internal face; the axial trunk resembles sensillum wall is without pores. a kind ofridge which is convex on the external face and flattened on the internal one (Figs 1, 4). Aporous sensilla campaniformia Strong spines, from 45 to 55 µm long, articulated on a cylindrical base of 30 µm, are located on the ventral The aporous sensilla campaniformia of lamellae are margin of each lamella, except for the distal third, and recognisable thanks to the star-shape of the cuticular totality of axial trunk (Fig. 1). They overlap and two reticulum around their dome (Fig. 7). This dome, 5 µm longitudinal rows of spines can be observed side by side in diameter, is slightly convex and delicately granulous (Figs 1, 2). In the median region of the lamellae, they (Fig. 10). It also possesses a central hollow closed by are more widely spaced (Fig. 3). The dorsal margin is a globulous papilla of 200 µm in diameter (Fig. 11). without spines (Fig. 4). The axial trunk is smooth on the The sensillum appears to be embedded in the cuticle internal face oflamellae (Fig. 5). The external surface of of the lamellae. The characteristic ring of the sensilla lamellae also possesses spines, but only in the proximal campaniformia has an outer diameter of 7 µm and is region; the spines are much thinner and of variable difficult ro distinguish from the smrnunding cuticular length, although most of them do not exceed that of half furrows (Fig. 10). of the previous spines (Fig. 1). Apart from the region with spines, the cuticle shows alveolar structures which Multiporous sensilla coeloconica are visible both in the lower part and in the upper part of lamellae (Figs 3, 5). When enlarged, the cuticle reveals The multiporous sensilla coeloconica can at first sight a reticulous pattern (Fig. 7). be confused with pores (Fig. 7), but they differ by their convex cupula shape, 1.8 µm in diameter, and B - The different types of sensilla their internal aperture measuring 0. 8 µm. The structure inside the cavity is difficult to identify. Their general Four types of sensilla are present on both faces of form makes us classify them with the coeloconicum lamellae type (ZACHARUK, 1985). Caudal sensilla of Erythromma 115

Figs L-6 - 1-6: structures and sensilla on the caudal appendages of E1ythromma lindenii. 1. base of lateral lamella showing noninnervated spines (s. aITows) on the external surface, the axial trunk (T) and the ventral margin. 2. detail of Fig. L. 3. median region oflamella seen on the external face with spines (S) on the margin and axial trunk, and the lower part without spines. 4. internal face of lamella in the median region showing the upper pa.it (U), the lower part (L) and the trunk without spines. 5. detail of Fig. 4 showing the alveola pattern of the upper part (arrow). 6. spatula-shaped sensillum chaeticum on the external face of lamella.

Distribution and numbers ofsensilla sensilla are distributed without clear pattern and always in limited numbers; for example, Fig. 7 presents only The four types of sensilla are present on the thr9e three sensilla which belong to different types. If the lamellae, they are slightly more numerous on the spatula-shaped sensilla chaetica are oriented towards the external face of lateral lamellae and exist in equivalent distal end of lamellae, the sensilla filiformia adopt very numbers on the two faces of median lamella. The varied orientations. Coun ting the sensilla on the external 116 Michel J. FAUCHEUX face of a lateral lamella produced the following results: E. lindenii whereas they are regularly laid out on the edge 3 spatula-shaped sensilla chaetica, 5 sensilla :filiformia, of horizontal lamellae as in L. sponsa (FAUCHEUX, 2005). 3 sensilla campaniformia and 4 sensilla coeloconica. According to VASSEROT (1957), mechanoreceptors on the antennae of Calopte1yx splendens detect vibrations Discussion in the water, but the sensilla were not :figured. Later, fine setae have been described on the dorsal surfaces of the The spines are too numerous, too regularly distributed antennae, tibiae and tarsi of Xanthocnemis zealandica and too close to one another to be confused with sensory (RowE, 1994); they are apparently mechanoreceptors organs. When the larva is placed on a pond bed or on which mediate prey detection within a space around a plant, the spines on the margin of lamella come into each of these appendages (CORBET, 1999). In E. lindenii, contact with the substrate to which they adhere; they the antenna! sensilla :filifonnia probably function during also thicken and rigidify the lamella, which presents prey detection (FAUCHEUX, 2006) whereas the sensilla a :fine, delicate surface, and protect it from all risk of :filiforrnia on the caudal appendages would appear to deterioration. The spines located on the external face warn larvae of the reaiward approach of a predator. of the axial trunk of the two lateral lamellae probably Indeed, it is the latter role which is carried out by the also have a protective function. It is significant that the caudal appendages of tetTestrial insects such as the cerci axial trunk jg spineless on its internal side, indeed, if of cockroaches (FOLLER & ERNST, 1977) or crickets this were not the case, two adjacent lamellae could be in (GNATZY & TAUTZ, 1980) in which the stimulation of danger of inte1iwining. cereal filifonn hairs provokes the behaviour of excape. One particular difference distinguishes the sensilla Spatula-shaped aporous sensilla chaetica :filiformia of antennae from the same sensilla on the caudal appendages of E. linden ii. On the antennae, they The spatula-shaped sensilla are present in great constitute three types (sh01t, median or long sensilla) numbers on both faces of the scape and pedicel of the and they should be capable of responding to a wide antenna of E. lindenii (Faucheux, 2006). According spectrum of frequencies as bas in paiiicular been shown to their morphological characteristics, they are in the cricket G1y llus bimaculatus (SHIMOZAWA & mechanoreceptors which are stimulated by direct KANou, 1984). The existence of a single type of sensi lla contact. Since they exist on the three caudal lamellae, filiformia on the caudal appendages of the odonatan the lamella's sensilla are stimulated by contact with larva is probably related to a situation of urgency (flight) a neighbouring lamella that happens to be too close. which appears not to require various types of sensilla. Thus, these sensilla seem to allow the larva to keep its The slightest unusual stimulation of sensilla :filiformia lamellae well separate, and therefore more efficient. In no doubt provokes the reflex of flight. the present case, the spatula-shaped sensilla appear to In the rare larvae of odonatans whose sensilla have possess a proprioceptive function. so far been studied, the sensilla filiformia exist both on the antennae and the caudal appendages: E. lindenii Aporous sensilla filij'ormia (F AUCHEUX, 2006; this paper), Chalcolestes viridis (FAUCHEUX & MEuRGEY, 2006; personal observations). The sensilla :filiformia in teITestrial insects are known Their presence is fully justified only in larvae living to be vibroreceptive, they are deflected by faint in a calm aquatic environment in which the sl ightest air currents and low-frequency sounds or medium perturbation will be detected; this is the case of the two vibrations (GNATZY & TAUTZ, 1980). They occur on the zygopteran species previously mentioned. To confirm cerci of cockroaches (GNATZY, 1976), crickets (GNATZY this hypothesis, we propose to enquire subsequently & SCHMIDT, 1971), earwigs (FAUCHEUX, 1999a) and into their possible presence in the anisozygopteran embiids (FAuc1-rEux, 2002), and the antennae of bugs , Epiophlebia superstes, which lives in streams (MclvER & STEMlCKI, 1984). The have recently been in Japan. discovered in aquatic insects, on the larval caudal appendages of Lestes sponsa (FAUCHEUX , 2005), the Aporous sensilla campaniformia larval antennae of Chalcolestes viridis (FAUCHEUX & MEuRG EY, 2006) and of E. lindenii (FAUCHE UX, 2006) The sensilla campaniformia on the caudal appendages Their distribution on the surface of caudal lamellae of E. lindenii are morphologically different both from varies according to species, they are located on the those located on the antennae of the same species external face of lamellae held ve1tically, like those of (F AUCHEUX, 2006), and from those situated on the Caudal sensilla of Erythromma 117

Figs 7-12 - Sensi lla on the caudal appendages of Erythromma lindenii, located on the external face. 7. reticulated aspect of cuticle with sensnla campaniformium (C)., coeloconicum (CO), filiformium (F, arrow). 8. sensillum filiformium. 9. base of sensillum filiformium. 10. sensillum campaniformium. 11. globulous papilla closing the molting channel of sensillum campaniformium. 12. sensillum coeloconicum. caudal appendages of L. sponsa (FAUCHEUX, 2005). cuticle, called the "ma lting channel" (KE1L, 1997). The In these t\:vo examples, they are typical and possess closing malting channel usually is visible in the center a concentric broad cuticular bulge. Now, this annular of the cap of sensilla. It so happens that the malting structure is barely visible (Fig. 10) no doubt because of channel is absent in the caudal sensilla of L. sponsa the reti culated and wrinkled aspect of the integument and the antenna! sensilla of E. lindenii. It is visible, of the caudal lamellae. Furthermore, in hemimetabolan in the form of a cavity, on the pedicellar sensillum insects, such as the Odonata, the dendrite sheath of of adult odonat Brachythemis leucosticta (Faucheux, sensilla campaniformia is in serted in the cuticle of the unpublished). The presence of a globulous papilla dome, often running through a special channel in the closing the malting channel of sensilla campaniforrnia 118 Michel J. FAUCHEUX on the caudal lamellae of E. lindenii has so far not been to two different subtypes and do not have the same reported. function. On the antenna} pedicel, as on that of most As concerns their function, the sensilla adult insects, the single sensillum campaniformium is campanifonnia are proprioceptors which are found a proprioceptor which assures the correct disposition whenever mechanical deformations of the cuticle occur of the flagellum as regards the pedicel. On the caudal (GNATZY et al., 1987; GR ONERT & GNATZY, 1987). The lamellae, the sensilla campanifonnia perform rather the register all degrees of compression or extension which function of osmoreceptors. The function of the spatula­ occur on the body's surface. The caudal appendages, shaped sensilla chaetica is probably proprioceptive in which represent a relatively large lamellar surface, the two types of appendages. However, two sensillum are exposed to the osmotic pressure of the aquatic types are specific to one or other of the appendages. The environment, probably to a greater degree than the rest antennal curved sensilla chaetica act as proprioceptors of the body. When this pressure varies, the larvae are for positioning the flagellomeres in relation to one informed ofthe changes via their sensilla campaniformia another (FAUCHEUX, 2006). The sensilla coeloconica of which perform the function of osmoreceptors and, as a caudal Jamellae seem to possess an olfactive function. result, they can adapt their behaviour to these variations in the environment. This possible fucntion of sensilla campaniformia is impo1iant for an aquatic Conclusion and justified their permanent presence in the studied species (L. sponsa and E. lindenii). If the sensilla A study in transmission electron microscopy is campaniformia of the caudal appendages in zygopteran necessary to examine more closely and confirm the larvae are really osmoreceptive, it is only to be expected results obtained with the scanning electron microscope. that their number should differ between species living As it is, two particular types of mechanoreceptors are in streams with different levels of osmotic pressure. common to the small number of species described, these are the sensilla filiformia and campaniformia. The Multiporous sensilla coeloconica presence of sensilla coeloconica in E. linden ii supposes a possible olfactive function carried out by the caudal The sensilla coeloconica are observed for the first time appendages. on the caudal lamellae of larval Odonata. They do not resemble typical sensilla coeloconica in insects, for their internal cuticular structure is not very charactetistic. Acknowledgements In this sense, they are close to the cupuliform organs present on the antennae of the sunflower moth We wish to acknowledge the courtesy of Franc;:ois Meurgey for (FAUCHEUX, 1992), neopseustid moths (FAUCHEUX et providing the specimens of Erythromma lindenii. We are indebted to Pierre Watelet, Director of Natural History Museum of Nantes al., 2006) and of a carabid beetle, Ceroglossus buqueti for faciliting use of S.E.M., A. Barreau and S. Grolleau (Centre of (JAFFREZIC & FAUCHEUX, 2006). The sensilla coeloconica S.E.M., University of Nantes) and Vittorio Ballardini for technical of insects are usually olfactive chemoreceptors assistance. (ZACHARUK, 1985). To attribute an olfactive function to caudal sensilla References coeloconica of E. lindenii is curious but not surprising. Indeed, several cases of olfactive or gustative sensilla have recently been described on the ovipositors of B ANGA, N., ALBERT, P.J., KAPOOR, N.N. & McNEIL, J.N., lepidopteran insects (FAUCHEUX, 1988, 1999b; BANGA 2003. Structure, distribution, and innervation of sensill a et al., 2003). on the ovipositor of the spruce budworm, Choristoneura fumiferana, and evidence of a gustatory function for type II sensilla. Canadian Journal ofZoology, 81: 2032-2037. Comparison with the antenna! sensilla of E. lindenii CORBET, P.S., 1999. Dragonflies: Behaviour and Ecology of Odonata. Harley Books (B.H. Harley Ltd.), Matiins, Great The antennae and the caudal Jamellae of E. lindenii Horkesley, England, 829 pp. possess in common the sensilla filiformia, sensilla D AVIES, D.A.L. & TOBIN, P. , 1984. The dragonflies of the campaniforrnia and spatula-shaped sensilla chaetica. On world: a systematic list ofthe extant species ofOdonata. Vol. the two types of appendages, the sensilla filiformia are 1: Zygoptera, Anisozygoptera. RCSlO, Suppl. 3, 127 pp. vibration receptors. The sensilla campaniformia belong FAUCHEUX, M.J., 1988. Multiporous sensil la on the ovipositor Caudal sensilla of Erythromma 119 of Monopis crocicapftella Clem. (Lepidoptera: Tuneidae). GNATZY, W., 1976. The ultrastructure of te thread hairs on International Journal ofInsect Morphology and Embryology, the cerci of the cockroach Periplaneta americana L.: the 17: 473-475. intermoult phase. Journal of Ultrastructure Research, 54: FAUCHEUX, M.J., 1992. 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