Aglomerular Hemipteran Antennal Lobes--Basic Neuroanatomy of a Small Nose

Abstract

We have compared the basic organization of the primary olfactory centre, the antennal lobe (AL), in 4 hemipteran species representing the 2 major lineages in this order. The Homoptera were represented by the psyllid Trioza apicalis and its aphid relatives the grain aphid Sitobion avenae Fabricius and the rose-grain aphid Metopolophium dirhodum Walker, whereas the Heteroptera were represented by the pentatomid stink bug Euschistus heros Fabricius. The olfactory systems of psyllids and aphids are generally very small, with low numbers of afferents in comparison to other insect groups, and the smallest described so far belongs to T. apicalis, comprising less than 50 olfactory receptor neurons (ORNs). Originally, we tried to estimate numbers of olfactory glomeruli in the AL of T. apicalis, which in insects generally correspond closely to the number of different types of ORNs. Neither immunocytochemical staining nor anterograde staining of ORNs revealed any glomerular structures in the ALs of T. apicalis or the 2 aphids that were included for comparison. In contrast, the ALs of the pentatomid stink bug E. heros displayed numerous distinct and well-delineated glomeruli, showing that aglomerular ALs are not typical of all insects within the order Hemiptera. Glomeruli are hallmark features of olfactory lobes in many different phyla, and the absence of glomerular structures in psyllids and aphids appears to be unique in insects that depend on olfactory orientation.

Revisiting the anatomy of the central nervous system of a hemimetabolous model insect species: the pea aphid Acyrthosiphon pisum

“…A reduction of the AL in aphids toward an aglomerular structure might originate in the small number of olfactory receptor neurons present on the antenna. It coincides with only a minor role of olfaction in general in these insects, in accordance with the poorly developed higher olfactory integration centers, viz., the MBs, as mentioned above (Kristoffersen et al 2008). In the pea aphid, however, 70 genes in the olfactory receptor family have been identified (The International Aphid Genomics Consortium 2010) and 15 genes coding for odorant-binding proteins (Zhou et al 2010).…”

“…Although glomerular structures can be discerned in certain preparations by performing antennal backfills (Fig. 4f), glomeruli are not well delineated, as described in the aphid Sitobion avenae and in related hemipteran insect species (Kristoffersen et al 2008;Barrozo et al 2009). Possibly, glomeruli are not well separated by glial sheaths in aphids, as in Diptera (Stocker et al 1990).…”

“…The two arms of the PB approach each other anteriorly. Bars 20 μm insects, clear differences have been found in comparison with sister groups, such as land-living Hemiptera, which have more developed MBs and a clearer glomerular organization of the ALs (Pflugfelder 1937;Pathak 1972;Kristoffersen et al 2008;Barrozo et al 2009). The highly fused nervous system of the pea aphid and the welldeveloped PC and CB are, on the other hand, common features of hemipteran brains (Pflugfelder 1937;Pathak 1972).…”

Chordotonal organs in hemipteran insects: unique peripheral structures but conserved central organization revealed by comparative neuroanatomy

“…9) take different but close trajectories. Presumed JO afferents give rise to axon terminals in the dorsal region of the antennal mechanosensory and motor center of the brain (Kristoffersen et al, 2008) and descend toward abdominal neuromeres by providing collaterals almost exclusively to the mVAC in each neuromere. Some overlap between axon terminals of presumed JO and those of the tymbal CO was evident in the brain (Fig.…”

Variations on a Theme: Antennal Lobe Architecture across Coleoptera

“…For instance, the ALs of the Odonata Libellula depressa consist of small, spherical knots [67], while previously the Odonata ALs (like the ALs of Ephemeroptera) had been described as a- or nonglomerular [21,67]. In Hemiptera, the ALs have also been described as aglomerular ( Trioza apicalis [68]) or as diminutive with only 13 glomeruli-like structures ( Scaphoideus titanus [69]). Also the ALs of the Phthiraptera Columbicola columbae show no clearly defined glomeruli or any other compartments [70].…”