Lehr's fields of campaniform sensilla in beetles (Coleoptera): Functional morphology. I. General part and allometry Leonid Frantsevich a, * , Stanislav Gorb b, 1 , Vladimir Radchenko c , Dmytro Gladun c , Alexey Polilov d a Department of Ethology and Social Biology of Insects, Schmalhausen-Institute of Zoology, B. Chmielnicki Street 15, 01601 Kiev, Ukraine b Department of Functional Morphology and Biomechanics, Christian-Albrechts-University of Kiel, Am Botanischen Garten 1-9, D-24098 Kiel, Germany c Institute for Evolutionary Ecology, Acad. Lebedev Street 37, 03143 Kiev, Ukraine d Department of Entomology, Biological Faculty, Lomonosov Moscow State University,1-12 Leninskie Gory,119991 Moscow, Russia article info Article history: Received 14 February 2014 Received in revised form 28 August 2014 Accepted 6 September 2014 Available online 21 September 2014 Keywords: Insect flight Coleoptera Campaniform sensilla Articulation of the elytron Allometric relationships abstract In this first of three articles we show the construction of the articular part of the elytron, the root. The root bears a conspicuous field of campaniform sensilla. This field was studied using light and scanning electron microscopes. The diversity of shape of the field among beetles, types of orientation of elongated sensilla within the field, individual variability of their number among conspecifics are demonstrated. Elongated sensilla point to the junction of the elytron with the second axillary plate. Presumably, they monitor twist movement in this junction, which is possible if the elytron is open. The goal of the whole project is to reveal the effect of both structure and function of the hind wings and elytra on the morphology of this mechanosensory field. Our data on allometric relationships between the animal size and quantitative characteristics of the field in normally flying beetles provide an important background for further functional analysis of this sensory organ. We selected 14 series of several species belonging to the same taxon but differing in size from big to small. It is revealed that the area of the sensory field is directly proportional to the elytral area, whereas the number of sensilla is proportional to the square root of the elytral area. Despite the great range in the elytral area (1500 times) in series of selected species the area of an external pit or cap of a single sensillum varies only 25-fold. The density of sensilla per unit area of the sensory field increases with decrease of the elytral area. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction A campaniform sensillum (CFS) is an elementary mechano- sensory organ in the insect integument. It consists of a neuron and three accessory cells two of which penetrate the thick integument all the way through. The channel tapers toward the external face. The channel profile resembles a bell outline, the reason for the sensillum name. A process of one accessory cell, reaching the external surface, produces a flat cuticular cap. In turn, the external face of the integument around the cap is indented as a pit: round or elongated. The morphology of CFS in beetles was described by Lehr (1914) and by Pfau and Honomichl (1979). Deformation of the cap, caused by strain in the integument, is transmitted to the dendrite of the neuron. A sensillum with an elongated pit is selectively sensitive to strain directed transversely to the long axis of the pit (H€ oßl et al., 2007). CFS are solitary or gather in sensory fields on various body parts (Hochreuter, 1912; Gnatzy et al., 1987). Fields of CFS are present on wing veins in various studied insects, some fields account for hundreds of CFS (Pringle, 1957). Hind wings in beetles also bear such fields (Lehr, 1914; Pfau and Honomichl, 1979). The latter authors supposed that fields on veins follow the sequence of the wing base twist and monitor temporal points of strain during pronationesupination in the hind wing. A century ago, Richard Lehr (1914) described a field of CFS on the articular process of an elytron (the root) in the diving beetle, Dytiscus marginalis, and studied the histology of CFS. This field is referred to here as Lehr's field. Two years earlier, it was observed and depicted in a draft by Hochreuter (1912). Both articles contribute to treatises on anatomy and ontogenesis in D. marginalis * Corresponding author. Tel.: þ38 (044) 278 34 69. E-mail address: leopup@izan.kiev.ua (L. Frantsevich). 1 Tel.: þ49 (0)431 880 4513; fax: þ49 (0)431 880 1389. Contents lists available at ScienceDirect Arthropod Structure & Development journal homepage: www.elsevier.com/locate/asd http://dx.doi.org/10.1016/j.asd.2014.09.001 1467-8039/© 2014 Elsevier Ltd. All rights reserved. Arthropod Structure & Development 43 (2014) 523e535