REGULAR ARTICLE Species and sexual differences in antennal lobe architecture and glomerular organization in two parasitoids with different degree of host specificity, Microplitis croceipes and Cotesia marginiventris Prithwiraj Das & Henry Y. Fadamiro Received: 10 July 2012 / Accepted: 14 January 2013 / Published online: 19 February 2013 # Springer-Verlag Berlin Heidelberg 2013 Abstract The endoparasitic wasps (Hymenoptera: Braconidae), Microplitis croceipes (specialist) and Cotesia marginiventris (generalist), are parasitoids of lepidopteran larvae and differ in their degree of host specificity. Recent studies have reported key differences between the two spe- cies in the abundance of antennal olfactory sensilla and their response to host-related volatiles. Here, we have compared antennal lobe architecture and glomerular organization in the two parasitoid species by using a combination of axonal tract tracing techniques and confocal microscopy. In M. croceipes, the medial half of the antennal lobe is larger with a greater number of glomeruli compared with the lateral half, whereas in C. marginiventris, the lateral half is larger than the median half. The volume of the antennal lobe is approximately 2.5 times greater in M. croceipes than in C. marginiventris. However, the number of glomeruli per an- tennal lobe is only slightly higher in M. croceipes (females: 219–222; males: 220–224) than in C. marginiventris (females: 192–194; males: 193–196). A comparison of males and females within each species demonstrated a strik- ing sexual difference in terms of an enlarged glomerulus (macroglomerulus or MG) at the entrance of the antennal nerve and of a complex of 3–4 MG (CMG) in the posterior region of the antennal lobe of males of both species. Being specific to males, both the MG and CMG might be involved in the detection of female-related odor. Keywords Parasitoid . Specialist . Generalist . Antennal lobe . Glomeruli . Confocal microscopy . Macroglomerulus . Microplitis croceipes, Cotesia marginiventris (Insecta) Introduction The antennal lobe is the primary olfactory center in the insect brain. Because the insect antennal lobe is remarkably similar to the vertebrate olfactory bulb (OB), insects are attractive model organisms for investigating neuronal archi- tecture and mechanisms of olfaction in general (Hildebrand and Shepherd 1997; Aungst and Spehr 2005). Both the antennal lobe and OB consist of distinct morphological units called glomeruli, each of which receives input from olfac- tory receptor neurons (ORNs) expressing the same receptor type (Mombaerts et al. 1996; Vosshall et al. 2000). Briefly, the antennal lobe receives inputs from the ORNs housed within olfactory sensilla located on the antenna, the primary olfactory organ in insects. The ORNs from olfactory sensilla terminate in the glomeruli, in which they make synaptic contact with local interneurons, which interconnect subsets of glomeruli, and with projection neurons, which project to the higher brain centers. The morphology of the antennal lobe and the glomerular organization has been described in some insect taxa includ- ing moths (Lepidoptera; Rospars 1988; Homberg et al. 1989; Boeckh and Tolbert 1993; Hildebrand and Shepherd 1997; Anton and Homberg 1999; Hansson and Anton 2000), ants (Hymenoptera; Zube et al. 2008; Kelber et al. 2009), bees (Hymenoptera; Flanagan and Mercer 1989; Galizia et al. 1999), and flies (Diptera; Vosshall et al. 2000; Wong et al. 2002). Moreover, Smid et al. (2003) have described the architecture of the antennal lobe in two species This research was funded by a National Science Foundation (NSF) Grant (award number: 0641621) to H.Y.F. P. Das : H. Y. Fadamiro (*) Department of Entomology and Plant Pathology, Auburn University, 301 Funchess Hall, Auburn, AL 36849, USA e-mail: fadamhy@auburn.edu Cell Tissue Res (2013) 352:227–235 DOI 10.1007/s00441-013-1568-z