Central Complex

The central complex (CX) is a highly conserved, crystalline array of four interconnected midline neuropils situated in the center of the supraesophageal brain (protocerebrum) of insects and homologous to the central body of crustaceans and the arcuate body of chelicerates (Arthropoda; Entomology). Whereas the paired antennal lobes and mushroom bodies (corpora pedunculata) are duplicated symmetrically in the left and right cerebral hemispheres to process olfactory representations and associative memory, the central complex spans the interhemispheric midline as a unified spatial and premotor processor (Pfeiffer & Homberg, 2014; Honkanen et al., 2019). Comprising the protocerebral bridge (PB), the fan-shaped body (FB) (or upper division of the central body, CBU), the ellipsoid body (EB) (or lower division of the central body, CBL), and the paired ventral noduli (NO), the central complex contains a regular lattice of 16 to 18 vertical columns intersected orthogonally by horizontal tangential layers. Operating through a recurrent ring-attractor network of compass neurons, the central complex integrates celestial polarized-light cues from dorsal-rim retinula cells, solar azimuth, optic flow, wind mechanoreception from antennal sensilla, and rotational velocity to maintain an internal allocentric head-direction compass, perform trigonometric vector addition during path integration (homing), guide transcontinental migrations in locusts and monarch butterflies, and transmit steering commands via the lateral accessory lobes and subesophageal ganglion (Seelig & Jayaraman, 2015; Hulse et al., 2021).

Across all winged and apterygote hexapods, the central complex is flanked laterally by two accessory premotor neuropils—the bulb (BU) (formerly the lateral triangle) and the lateral accessory lobe (LAL)—and consists of four anatomically stereotyped midline structures (Ito et al., 2014; Pfeiffer & Homberg, 2014):

The Three Major Cell Classes: Tangential, Columnar, and Pontine Neurons

The wiring diagram of the central complex—reconstructed to single-synapse completeness in the Drosophila hemibrain connectome (~2,800 CX neurons; Hulse et al., 2021)—is built from three geometric neuron classes:

1. Tangential Neurons (ER / Ring Neurons and FB Tangential Neurons): Input neurons whose dendrites sample sensory or state hubs outside the CX (such as the bulb or superior protocerebrum) and whose axons sweep horizontally across all columns of a single layer in the ellipsoid body or fan-shaped body.
2. Columnar Neurons (E-PG, P-EG, P-EN, P-FN, hΔ, vΔ): Projection neurons that link specific glomeruli in the protocerebral bridge to restricted vertical columns in the ellipsoid body or fan-shaped body, often with a precise one-column anatomical phase shift (~45° angular offset) that implements mathematical vector rotation and addition.
3. Intrinsic Pontine and Δ7 Interneurons: Local inhibitory (GABAergic or glutamatergic via GluClα channels) interneurons within the PB or FB that connect columns separated by exactly 180° (half the array width), enforcing sinusoidal competitive inhibition across the compass ring.

Topographic 360° E-Vector Map in the Protocerebral Bridge

In pioneering intracellular recordings in the desert locust Schistocerca gregaria (Orthoptera) and the monarch butterfly Danaus plexippus (Lepidoptera), Uwe Homberg and Stanley Heinze discovered that polarized-light signals transduced by orthogonal UV-sensitive retinula cells in the dorsal rim area (DRA) of the ommatidia travel via the anterior optic tubercle and bulb to establish a linear topographic map of celestial e-vector orientation across the 16 columns of the protocerebral bridge (Heinze & Homberg, 2007). Moving step-by-step from the leftmost column (L8) to the rightmost column (R8) of the protocerebral bridge, the preferred e-vector polarization angle of the columnar neurons rotates systematically by ~22.5° per column across a complete 180°/360° cycle.

Live Calcium Imaging of the Ring Attractor in Drosophila

In 2015, Johannes Seelig and Vivek Jayaraman used two-photon Ca²⁺ imaging in tethered walking and flying Drosophila melanogaster to demonstrate that the population of E-PG ("compass") columnar neurons arborizing in the ellipsoid body (EB) and protocerebral bridge (PB) operates as a physiological ring attractor (Seelig & Jayaraman, 2015; Green et al., 2017):

• At any instant, recurrent excitation combined with global GABAergic/glutamatergic inhibition (mediated by Δ7 neurons in the PB and ring neurons in the EB) restricts neural activity to a single localized "bump" of activity (~60°–90° full-width at half-maximum) within the toroidal ellipsoid body, accompanied by two synchronized bumps in the left and right halves of the protocerebral bridge.
• As the insect turns clockwise or counterclockwise in the presence of visual landmarks—or in complete darkness using proprioceptive and efference-copy self-motion cues—P-EN ("angular velocity") neurons innervating the left versus right noduli become asymmetrically activated. Because left P-EN axons project to EB tiles shifted slightly counterclockwise relative to their PB glomerulus of origin (whereas right P-EN axons shift clockwise), turning the body asymmetric-ally excites one set of P-ENs and smoothly slides the E-PG compass bump around the ellipsoid body ring at the exact angular velocity of the insect's turn.
• Furthermore, anti-Hebbian synaptic plasticity at the all-to-all synapses between visual ER ring neurons and E-PG compass neurons allows the insect to tether its internal compass to novel visual scenes within seconds.

In central-place foragers such as desert ants (Cataglyphis fortis), sweat bees (Megalopta genalis), and honeybees (Apis mellifera), as well as seasonal migrants such as the Bogong moth (Agrotis infusa) and monarch butterfly, the fan-shaped body (FB) transforms the head-direction signal into a cartesian/polar vector calculator (Stone et al., 2017; Lyu et al., 2022; Mussells Pires et al., 2024):

1. Basis-Vector Coordinate Transformation (P-FN Neurons): Left and right P-FN neurons inherit the insect's current heading angle from the protocerebral bridge while receiving translational forward/backward optic-flow speed inputs via the noduli at ±45° orthogonal anatomical offsets. By summing sinusoidal activity patterns whose amplitudes scale with walking or insect flight speed along orthogonal axes (phasor representation), the fan-shaped body converts egocentric body motion into a world-centered (allocentric) travel vector and continuously integrates it inside a memory loop (CPU4 / hΔ neurons) to store the distance and direction back to the nest.
2. Goal-Directed Steering Comparison (PFL3 and PFL2 Output Neurons): Columnar output neurons (PFL3 / CPU1) compare the sinusoidal phase of the current heading bump against a stored goal-heading bump in the fan-shaped body. Because left and right PFL3 dendrites are shifted by ±90° in the PB relative to the FB, any deviation between the insect's current heading and its desired goal angle generates an immediate left–right firing-rate asymmetry in the lateral accessory lobes (LAL), recruiting descending neurons that traverse the subesophageal ganglion to steer the thoracic legs or wings back on course.

1. Heinze, S., & Homberg, U. (2007). Maplike representation of celestial E-vector orientations in the brain of an insect. Science, 315(5814), 995–997. https://doi.org/10.1126/science.1135531
2. Pfeiffer, K., & Homberg, U. (2014). Organization and functional roles of the central complex in the insect brain. Annual Review of Entomology, 59, 165–184. https://doi.org/10.1146/annurev-ento-011613-162031
3. Seelig, J. D., & Jayaraman, V. (2015). Neural dynamics for landmark orientation and angular path integration. Nature, 521(7551), 186–191. https://doi.org/10.1038/nature14446
4. Stone, T., Webb, B., Adden, A., Weddig, N. B., Honkanen, A., Templin, R., Wcislo, W., Scimeca, L., Warrant, E., & Heinze, S. (2017). An anatomically constrained model for path integration in the bee brain. Current Biology, 27(20), 3069–3085. https://doi.org/10.1016/j.cub.2017.08.052
5. Hulse, B. K., Haberkern, H., Franconville, R., Turner-Evans, D. B., Takemura, S.-Y., Wolff, T., Noorman, M., Dreher, M., Dan, C., Parekh, R., Hermundstad, A. M., Rubin, G. M., & Jayaraman, V. (2021). A connectome of the Drosophila central complex reveals network motifs suitable for flexible navigation and context-dependent action selection. eLife, 10, e66039. https://doi.org/10.7554/eLife.66039