Different retina-lamina projections in mosquitoes with fused and open rhabdoms.

Anopheles gambiae and Toxorhynchites brevipalpis represent the nocturnal and diurnal extremes of the mosquito light intensity range, and their eyes are structurally very different. A. gambiae has fused rhabdoms with huge acceptance angles, whereas T. brevipalpis has open rhabdoms with rhabdomere acceptance angles comparable with those of advanced (brachyceran) flies. Here, we show that the retina-lamina projections are consistent with these differences. The short receptor axons from each ommatidium in A. gambiae insert as a group between four lamina monopolar cell clusters. In T. brevipalpis axon bundles from each ommatidium undergo a twist in their passage through the nuclear layer of the lamina, and then fan out into a space the diameter of which is about twice the separation of the monopolar cell clusters. This arrangement is consistent with a neural superposition mechanism closely similar to that found in higher Diptera, but which must have evolved independently.