Candidates for extraocular photoreceptors in the cockroach suggest homology to the lamina and lobula organs in beetles

Using light- and electron microscopic methods, we describe two novel putati ve extraocular photoreceptor organs in the optic lobes of the cockroaches L eucophaea maderae and Blaberus craniifer. The lamina organ is an elongated structure distal to the first optic chiasm, adjacent to the anterior edge o f the lamina. The lobula organ is situated on the anterior distal surface o f the lobula. In cross sections through the pigment-free organs, cell bodie s are arranged in a closed or open circle and are interconnected by desmoso mes. They send protrusions with rhabdom-like microvilli into a common, cent ral space apparently filled with extracellular matrix. A different cell typ e gives rise to electron-dense lamellae, which also extend into the central space and partly join to form a common lamellar bundle. Axonal processes e xtend from the microvillar cells and run along the outer surface of the org ans to the neighboring optic neuropils. The organs receive multiple efferen t innervation from neurosecretory axons, Both organs show strong immunostai ning with an antiserum against Arabidopsis cryptochrome 2 that is associate d with the lamellated structure in the central lumen, The specific features of the organs suggest that they are homologous to similar organs in the op tic lobe of beetles and may serve a role as extraocular photoreceptors for light entrainment of the circadian system. (C) 2001 Wiley-Liss, Inc.