Neuroarchitecture of the Central Complex in the Madeira Cockroach Rhyparobia maderae: Tangential Neurons

ABSTRACT Navigating in diverse environments to find food, shelter, or mating partners is an important ability for nearly all animals. Insects have evolved diverse navigational strategies to survive in challenging and unknown environments. In the insect brain, the central complex (CX) plays an important role in spatial orientation and directed locomotion. It consists of the protocerebral bridge (PB), the central body with upper (CBU) and lower division (CBL), and the paired noduli (NO). As shown in various insect species, the CX integrates multisensory cues, including sky compass signals, wind direction, and ego‐motion to provide goal‐directed vector output used for steering locomotion and flight. While most of these data originate from studies on day‐active insects, less is known about night‐active species such as cockroaches. Following our analysis of columnar and pontine neurons, the present study complements our investigation of the cellular architecture of the CX of the Madeira cockroach by analyzing tangential neurons. Based on single‐cell tracer injections, we provide further details on the internal organization of the CX and distinguished 27 types of tangential neuron, including three types of neuron innervating the PB, six types of the CBL, and 18 types of the CBU. The anterior lip, a brain area unknown in flies and highly reduced in bees, and the crepine are strongly connected to the cockroach CBU in contrast to other insect species. One tangential neuron of the CBU revealed a direct connection between the mushroom bodies and the CBU. The central complex is a group of interconnected neuropils spanning the midline of the insect brain. In the Madeira cockroach, we identified 27 types of tangential neuron providing input from various brain areas to different compartments of the central complex. The study complements our analysis of neuronal cell types of the cockroach central complex and sheds further light on species‐specific modifications of circuitries underlying goal‐directed navigation.