Short communication Antennal lobe organization in the slender pigeon louse, Columbicola columbae (Phthiraptera: Ischnocera) José G. Crespo * , Neil J. Vickers Department of Biology, University of Utah, Salt Lake City, UT 84112, USA article info Article history: Received 12 October 2011 Received in revised form 27 February 2012 Accepted 27 February 2012 Keywords: Antennal lobe Olfaction Louse Columba livia Rock pigeon Ectoparasite abstract This study reports on the structure of the antennal lobe of the pigeon louse, Columbicola columbae. Anterograde staining of antennal receptor neurons revealed an antennal lobe with a few diffuse compartments, an organization distinct from the typical spheroidal glomerular structure found in the olfactory bulb of vertebrates and the antennal lobe of many other insects. This anatomical arrangement of neuronal input is somewhat reminiscent of the aglomerular antennal lobe previously reported in psyllids and aphids. As in psyllids, reports on the odor-mediated behavior of C. columbae suggest that the olfactory sense is important in these animals and indicates that a glomerular organization of the antennal lobe may not be necessary to subtend odor-mediated behaviors in all insects. The diffuse or aglomerular antennal lobe organization found in these two Paraneopteran insect orders might represent an independently evolved reduction due to similar ecological constraints. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The primary olfactory brain centers of many vertebrates and insects exhibit a distinctive anatomy that is readily recognized by the organization of the neuropil into globular units called olfactory glomeruli (Hildebrand and Shepherd, 1997). In many insects, these glomeruli comprise the structural and functional units of the antennal lobes (ALs), the first-order olfactory brain areas, which receive receptor neuron input from peripheral sensory sensilla. The number, size, and spatial arrangement of AL glomeruli are species-specific and consistent among different individuals of the same species (Anton and Homberg, 1999). The number of glomeruli found in the ALs of insects ranges from 10 to 1000 (Rospars, 1988). Since olfactory sensory neurons expressing the same receptor protein converge on a single glomerulus (Gao et al., 2000), the number of glomeruli approximately reflects the spec- trum of expressed receptor genes. Furthermore, glomerular size appears to be correlated to the number of incoming afferents of a particular type (Anton and Homberg, 1999). This is evidenced in the sexually dimorphic ALs, associated with mate finding, that have been described in several Hymenopteran, Lepidopteran, and Dictyopteran species (Rospars, 1988). In these orders a macro- glomerular complex, i.e. a male specific glomerular aggregation that is involved in the processing of sex-pheromone input, has been reported and its units found to be larger than ordinary glomeruli (e.g. Vickers and Christensen, 2003). This glomerular characteristic stems from the large number of sex-pheromone olfactory receptor neurons (ORNs) on the antenna which confer a high sensitivity to the female produced sex pheromone. The functional significance of glomeruli is supported by a wide range of studies in a variety of insect species (e.g. Rodrigues, 1988; Hildebrand, 1996; Galizia et al., 1999). Since each physiological type of ORN projects into a specific glomerulus, they form the basis of a so-called chemotopic map in the AL (Vosshall et al., 2000), in which qualitative features of differing odor mixtures are represented by unique combinations of spatial activity. In this study, we investigated the AL morphology of the slender pigeon louse Columbicola columbae (Phthiraptera: Ischnocera), an ectoparasite of the Rock Pigeon, Columba livia. The antennae of this insect consists of five annuli (scape, pedicel, and three flagellomeres) but only the last two flagellomeres bear sensilla other than mecha- noreceptors (Smith, 2001). In spite of the fact that C. columbae harbors few sensilla on its antennae, behavioral reports have shown that this insect is attracted to the smell of its host (Rakshpal, 1959) and to that of the hippoboscid fly Pseudolynchia canariensis, involved in the phoretic behavior of this species of lice (Harbison et al., 2009; Harbison and Clayton, 2011). Our investigations of C. columbae ALs revealed a non-globular compartmentalization of the neuropil reminiscent of the aglomerular AL found in psyllids and aphids (Kristoffersen et al., 2008; Kollmann et al., 2011). The lack of defined * Corresponding author. Department of Biology, University of Utah, 257 S 1400 E., Rm. 201, Salt Lake City, UT 84112, USA. Tel.: þ1 801 581 4795; fax: þ1 801 581 2174. E-mail address: jose.crespo@utah.edu (J.G. Crespo). Contents lists available at SciVerse ScienceDirect Arthropod Structure & Development journal homepage: www.elsevier.com/locate/asd 1467-8039/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.asd.2012.02.008 Arthropod Structure & Development 41 (2012) 227e230