Highly active insects and crabs depend on visual motion information for detecting and tracking mates, prey, or predators, for which they require directional control systems containing internal maps of visual space. A neural map formed by large, motion-sensitive neurons implicated in processing panoramic flow is known to exist in an optic ganglion of the fly. However, an equivalent map for processing spatial positions of single objects has not been hitherto identified in any arthropod. Crabs can escape directly away from a visual threat wherever the stimulus is located in the 360°field of view. When tested in a walking simulator, the crab Neohelice granulata immediately adjusts its running direction after changes in the position of the visual danger stimulus smaller than 1°. Combining mass and single-cell staining with in vivo intracellular recording, we show that a particular class of motion-sensitive neurons of the crab's lobula that project to the midbrain, the monostratified lobula giants type 1 (MLG1), form a system of 16 retinotopically organized elements that map the 360°a zimuthal space. The preference of these neurons for horizontally moving objects conforms the visual ecology of the crab's mudflat world. With a mean receptive field of 118°, MLG1s have a large superposition among neighboring elements. Our results suggest that the MLG1 system conveys information on object position as a population vector. Such computational code can enable the accurate directional control observed in the visually guided behaviors of crabs.

Crabs have panoramic compound eyes, which can show marked regional specializations of visual acuity. These specializations are thought to be related to the particular features of the animal's ecological environment. Modern knowledge on the neuroanatomy and neurophysiology of the crabs' visual system mainly derives from studies performed in the grapsid crab Neohelice granulata (=Chasmagnathus granulatus). However, the organization of the visual sampling elements across the eye surface of this animal had not yet been addressed. We analyzed the sampling resolution across the eye of Neohelice by measuring the pseudopupil displacement with a goniometer. In addition, we measured the facet sizes in the different regions of the eye. We found that Neohelice possesses an acute band of high vertical resolution around the eye equator and an increase in horizontal sampling resolution and lenses diameter towards the lateral side of the eye. Therefore, the analysis of the optical apparatus indicates that this crab possesses greater visual acuity around the equator and at the lateral side of the eye. These specializations are compared with those found in different species of crabs and are discussed in connection to the particular ecological features of Neohelice's habitat.