The Antennal Lobe of Orthoptera – Anatomy and Evolution

Abstract

The first odor-processing neuropils of insects comprise glomeruli, islets of neuropil, that are supplied by olfactory receptor neurons and give rise to efferent axons to higher brain centers. Glomeruli size and organization varies in a taxon-specific manner across the Insecta, suggesting possible correlates between their organization and chemosensory behaviors in different insect groups. Comparative studies of antennal lobe glomeruli within the Orthoptera have been used to infer how the various taxon-specific arrangements of odorant-processing structures (glomeruli) might have evolved. The cellular arrangements in glomeruli have been surveyed using anterograde filling and Golgi impregnation of antennal receptor neurons projecting to the antennal lobe in Stenopelmatidae, Tettigoniidae, Gryllidae, Tetrigidae and Acrididae. These taxa, which represent the two sub-orders of Orthoptera, reveal a high correlation between the neural architecture of the glomeruli and structures within the glomeruli. Using a recent molecular phylogeny of the Orthoptera we have mapped the occurrence of glomerular characteristics to infer the evolution of antennal lobe structures in orthopterans. The functional implications of these results are discussed.

The anatomical pathways for antennal sensory information in the central nervous system of the cricket, Gryllus bimaculatus

“…This division is suggested to respond to functional or odotypic grouping in honeybee (Müller et al, 2002;Kirchner et al, 2006). In the most dorsal part of the AL, where T4 terminate, the glomeruli are separated from others, as previously reported (Ignell et al, 2001). This characteristic of T4 is similar to that observed in honeybee (Suzuki, 1975;Galizia et al, 1999).…”

“…Almost all of the AL-PNs that project through IACT show uniglomerular arborization in the AL (Schildberger, 1984;Ignell et al, 2001) as they do in other insects (cockroach: Malun et al, 1993;Strausfeld and Li, 1999;honeybee: Abel et al, 2001;Müller et al, 2002;moth: Homberg et al, 1989;Kanzaki et al, 1989;fly: Stocker et al, 1990;Marin et 27 al., 2002;mosquito: Ignell et al, 2005). In the cockroach, AL-PNs project via ACT2 and there they also exhibit uniglomerular arborization (Malun et al, 1993).…”

“…Rospars and Hildebrand, 1992;Berg et al, 2002;Sadek et al, 2002) and 92-104 in Hymenopteran species (Smid et al, 2003). The majority of cricket glomeruli contained inner small spheral substructures called 'microglomerular structure' (Ignell et al, 2001). Anatomical analysis in this study demonstrates no sexual dimorphism in the spatial arrangement of the cricket AL, which confirms the previous study by Ignell et al, (2001).…”

Insect olfaction from model systems to disease control

“…3). ORNs expressing the same Or converge on a single glomerulus in Drosophila (16)(17)(18)60), as in mammals (61), although at least in locusts, the pattern seems to be more complex (62). The organization of the larval olfactory circuit of Drosophila is similar to that of the adult, but because each Or is expressed in only one ORN, there is no convergence (63).…”

Mechanisms of Development and Evolution of the Insect Olfactory System

“…Termed as macroglomerular complexes (MGC), this phenomena is thought to be involved in signal processing of female sex pheromones. An even more dramatic change in antennal lobe structure can be found in the order Orthoptera (grasshoppers and crickets), in which some species have evolved an antennal lobe consisting of thousands of microglomeruli innervated by highly branched ORNs and PNs, the purpose of which is as yet unknown [30,31].…”