In its plesiomorphic state the insect ommatidium consists of eight retinula cells forming a fused rhabdom. It has long been observed that, in contrast to this pattern, Heteroptera have open rhabdoms. However, there has so far been no comprehensive and comparative study of heteropteran ommatidia. For this reason, we investigated the rhabdom structure in 36 species from all higher groups of Heteroptera, as well as from Coleorrhyncha and Auchenorrhyncha as outgroup representatives. In addition we surveyed the data of earlier authors, which brings the number of examined species to a total of more than 70. All examined Heteroptera do have open rhabdoms, with a system of six peripheral and two central rhabdomeres. Outgroup comparison shows that the open rhabdom is an autapomorphy of the Heteroptera. As for the rhabdom structure within the Heteroptera, we found further autapomorphic patterns in Corixidae (Nepomorpha), Gerromorpha, and Leptopodomorpha. Finally, the Cimicomorpha and Pentatomomorpha share a special pattern of the two central rhabdomeres, which we call V-pattern. This is a new synapomorphy of these two taxa.
Ultrastructural data from 108 species of Chrysomeloidea show that all rhabdom‐patterns can be assigned to one of two basic patterns. The insula‐pattern: two central rhabdomeres (Rh 7/8) are spatially isolated from the six peripheral ones (Rh 1–6). The ponticulus‐pattern: Rh 7/8 fuse at two sites with the ring of Rh 1–6. The distance between the two systems may prevent optical or electrical coupling in the insula‐p. The structure of the ponticulus‐p may allow electrical coupling as well as contrast‐intensifying lateral filtering. Potential relative polarization and absolute sensitivities differ interspecifically between homologous cells and intraspecifically between Rh7/8 and Rh 1–6, and between Rh 7 and Rh 8. The Bruchidae show only the insula‐p, the Chrysomelidae and Cerambycidae both. The distribution of the two patterns is subfamily‐specific within the Chrysomelidae, but not in the Cerambycidae. Identical patterns must have developed convergently within the Chrysomeloidea. Both basic patterns are subdivided in different subfamilies or tribes.
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