Matched function of the neuropil processing optic flow in flies and crabs: the lobula plate mediates optomotor responses in Neohelice granulata

When an animal rotates (whether it is an arthropod, a fish, a bird or a human) a drift of the visual panorama occurs over its retina, termed optic flow. The image is stabilized by compensatory behaviours (driven by the movement of the eyes, head or the whole body depending on the animal) collectively termed optomotor responses. The dipteran lobula plate has been consistently linked with optic flow processing and the control of optomotor responses. Crabs have a neuropil similarly located and interconnected in the optic lobes, therefore referred to as a lobula plate too. Here we show that the crabs' lobula plate is required for normal optomotor responses since the response was lost or severely impaired in animals whose lobula plate had been lesioned. The effect was behaviour-specific, since avoidance responses to approaching visual stimuli were not affected. Crabs require simpler optic flow processing than flies (because they move slower and in two-dimensional instead of three-dimensional space), consequently their lobula plates are relatively smaller. Nonetheless, they perform the same essential role in the visual control of behaviour. Our findings add a fundamental piece to the current debate on the evolutionary relationship between the lobula plates of insects and crustaceans.

Unraveling the functional organization of lobula complex in the mantis brain by identification of visual interneurons

The praying mantis shows broad repertories of visually guided behaviors such as prey recognition and defense against collision. It is likely that neurons in the lobula complex (LOX), the third visual neuropil in the optic lobe, play significant roles in these behaviors. The LOX in the mantis brain consists of five neuropils: outer lobes 1 and 2 (OLO1 and OLO2); anterior lobe (ALO); dorsal lobe (DLO); and stalk lobe (SLO), and ALO comprise ventral and dorsal subunits, ALO-V and ALO-D. To understand the functional organization of LOX, intracellular electrodes were used for recording from and staining neurons in these neuropils of the mantis (Tenodera aridifolia). The neurons belonged to three categories based on their response properties and morphologies. First, tangential ALO-V neurons projecting to ventromedial neuropils (VMNP) (TAproM1 and 2), tangential DLO (or ALO-D) neurons projecting to VMNP (TDproM1 and 2), and tangential ALO-V centrifugal neurons (TAcen) all showed directional sensitivity and sustained excitation to gratings drifting in preferred direction (outward-downward, inward-upward, outward-upward, inward-downward, and inward, respectively). Second, tangential OLO neurons projecting to VMNP or ventrolateral neuropils (VLNP) (TOproM or TOproL), columnar OLO commissural neurons (COcom), and SLO commissural neurons (Scom) all showed strong excitation to 2°-8° moving squares but little excitations to drifting gratings. COcom and SLO neurons ramified in both left and right LOX. Last, the class of tangential ALO-V neurons projecting to VLNP (TAproL1, 2, and 3) responded best to looming circles and showed little excitation to receding, darkening, and lightening circles.

Anatomy of the lobula complex in the brain of the praying mantis compared to the lobula complexes of the locust and cockroach

The praying mantis is an insect which relies on vision for capturing prey, avoiding being eaten and for spatial orientation. It is well known for its ability to use stereopsis for estimating the distance of objects. The neuronal substrate mediating visually driven behaviors, however, is not very well investigated. To provide a basis for future functional studies, we analyzed the anatomical organization of visual neuropils in the brain of the praying mantis Hierodula membranacea and provide supporting evidence from a second species, Rhombodera basalis, with particular focus on the lobula complex (LOX). Neuropils were three‐dimensionally reconstructed from synapsin‐immunostained whole mount brains. The neuropil organization and the pattern of γ‐aminobutyric acid immunostaining of the medulla and LOX were compared between the praying mantis and two related polyneopteran species, the Madeira cockroach and the desert locust. The investigated visual neuropils of the praying mantis are highly structured. Unlike in most insects the LOX of the praying mantis consists of five nested neuropils with at least one neuropil not present in the cockroach or locust. Overall, the mantis LOX is more similar to the LOX of the locust than the more closely related cockroach suggesting that the sensory ecology plays a stronger role than the phylogenetic distance of the three species in structuring this center of visual information processing.