TABLE 3.

Key interneurons and their putative-related processing in the central complex (CX).

Internal signal	External signal	Putative processing in CX	Related identified interneurons#	Related neuropiles#	Reference numbers
					Honey bee	Bumble bee	Sweat bee	Locust	Fruit fly (Drosophila)
Circadian	Sun brightness and chromatic gradient	Head direction with respect to visual landmarks	EPG/PEG (CL1)	EB (CBL), PB, Gall	(Homberg, 1985; Hensgen et al., 2021; Kaiser et al., 2022)	(Sayre et al., 2021)	(Stone et al., 2017)	(Homberg et al., 2011; Bockhorst and Homberg, 2015; Pegel et al., 2018)	(Seelig and Jayaraman, 2015; Hulse et al., 2021)
Circadian	Polarized light	Map-like representation of E-vector. Extracting a reliable current head direction	Δ7 (TB1)	PB	(Homberg, 1985; Kaiser et al., 2022)	(Sayre et al., 2021)	(Stone et al., 2017)	(Heinze and Homberg, 2007; Homberg et al., 2011; Pegel et al., 2018)	(Hulse et al., 2021)
Self-motion	Polarized light	Angular velocity detection	PEN (CL2)	PB, EB (CBL), NO	(Hensgen et al., 2021)	(Sayre et al., 2021)		(Heinze and Homberg, 2009)	(Green et al., 2017; Turner-Evans et al., 2017; Hulse et al., 2021)
	Optic flow in flight (speed)	Navigational computation and path integration	PFN (CPU4)	PB, FB (CBU), NO	(Hensgen et al., 2021; Kaiser et al., 2022)	(Sayre et al., 2021)	(Stone et al., 2017)		(Hulse et al., 2021)
		Steering	PFL (CPU1)	PB, FB (CBU), LAL	(Homberg, 1985; Hensgen et al., 2021)	(Sayre et al., 2021)	(Stone et al., 2017)	(Homberg, 1994; Bockhorst and Homberg, 2015)	(Hulse et al., 2021)

^#The nomenclatures of the related identified interneurons are in accordance with those of the bumble bee (Sayre et al., 2021). The parentheses in related identified interneurons and neuropils show the corresponding name used in the other insect. All related interneurons were morphologically identified in the honey bee CX (Hensgen et al., 2021; Kaiser et al., 2022). Each EPG/PEG keeps the animal’s head direction with a visual landmark in the fruit fly (Seelig and Jayaraman, 2015). Each EPG/PEG (CL1 in honey bee) has preferred azimuth angles of the unpolarized light spot (Pegel et al., 2018). Δ7s (TB1s in honey bees) are organized in map-like representation to polarized light in PB (Heinze and Homberg, 2007). PEN (CL2 in honey bees) is responsive to the rotation velocity caused by self-motions in the fruit fly (Green et al., 2017; Turner-Evans et al., 2017) and to the polarized light in locusts (Heinze and Homberg, 2009). PFN (CPU4 in honey bee) receives both optic flow signals in the NO and the current heading signals in the PB and output to the FB (Stone et al., 2017). Stone et al. (2017) speculated this neuron holds a memory signal for the home vector in the computational model. PFL (CPU1 in honey bees) sends steering signals to contralateral LAL which is the dendrite region of descending neurons to the thoracic CPG (Homberg, 1994). CBL, lower unit of central body; CBU, upper unit of central body; CL, columnar neuron; EB, ellipsoid body; FB, fan-shaped body; LAL, lateral accessory lobe; NO, noduli; PB: protocerebral bridge; TB, tangential neuron of PB; CPU, the neurons with smooth endings both in a single column of the PB and columns of the dorsal most layer of the CBU and axonal fibers with varicose endings in the LAL.