Evolutionarily conserved anatomical and physiological properties of olfactory pathway through fourth-order neurons in a species of grasshopper (Hieroglyphus banian)

Gross anatomy of the visual processing centers of Hieroglyphus banian

Hieroglyphus banian (H. banian) is a grasshopper species, endemic to South Asia. The optic lobe in Acrididae has been characterized to a great extent in orthoptera, predominantly using Locust species like Schistocerca gregaria, Schistocerca americana, and Locusta migratoria, which are closely related to each other. In this work, we characterize the anatomical features of the optic lobe and associated pathway in the grasshopper species H. banian using tract tracing, immunohistochemistry, and intracellular fills. All the areas of the visual pathways that have been reported in the other orthoptera species could be identified in H. banian. Visual pathways exhibited similar structure and connectivity as shown in immunohistochemistry and tract-tracing results supporting the conservation of these features across species within Acrididae. Two new structures in the posterior protocerebrum, PS1 and PS2 with prominent innervations from the optic lobe were identified. Novel structure PS1 is innervated from medulla via PS1T and, PS2 from aMe via PS2T, both new tracts we have identified.

Insect lifestyle and evolution of brain morphology

Insect lifestyles are extremely diversified and have important consequences for brain function. Lifestyle determines the resources and information that brains might access and also those that are required to produce adaptive behaviors. Most of the observed adaptations in brain morphology to variation in lifestyle are related to the first stages of sensory information processing (e.g. adaptations to diel habits). However, morphological signatures of lifestyles related to higher order processing of information are more difficult to demonstrate. Co-option of existing neural structures for new behaviors might hinder the detection of morphological changes at a large scale. Current methodological advances will make it possible to investigate finer structural changes (e.g. variation in the connectivity between neurons) and might shed light on whether or not some lifestyles (e.g. eusociality) require morphological adaptations.

Neuronal architecture of the second-order CO2 pathway in the brain of a noctuid moth

Many insects possess the ability to detect fine fluctuations in the environmental CO₂ concentration. In herbivorous species, plant-emitted CO₂, in combination with other sensory cues, affect many behaviors including foraging and oviposition. In contrast to the comprehensive knowledge obtained on the insect olfactory pathway in recent years, we still know little about the central CO₂ system. By utilizing intracellular labeling and mass staining, we report the neuroanatomy of projection neurons connected with the CO₂ sensitive antennal-lobe glomerulus, the labial pit organ glomerulus (LPOG), in the noctuid moth, Helicoverpa armigera. We identified 15 individual LPOG projection neurons passing along different tracts. Most of these uniglomerular neurons terminated in the lateral horn, a previously well-described target area of plant-odor projection neurons originating from the numerous ordinary antennal-lobe glomeruli. The other higher-order processing area for odor information, the calyces, on the other hand, was weakly innervated by the LPOG neurons. The overlapping LPOG terminals in the lateral horn, which is considered important for innate behavior in insects, suggests the biological importance of integrating the CO₂ input with plant odor information while the weak innervation of the calyces indicates the insignificance of this ubiquitous cue for learning mechanisms.