Fundamental differences in the optical structure of the eyes of nocturnal and diurnal mosquitoes

We have studied the anatomy and optics of the eyes of a range of mosquito s pecies from the wholly dark-active blood-feeding Anopheles gambiae to the d iurnal plant-feeder Toxorhynchites brevipalpis. Consistent with studies by Sato in the 1950s, we find that dark-active and crepuscular species have sh ort fused rhabdoms with a conical construction. This maximises the amount o f light the rhabdoms receive from the almost hemispherical wide-aperture le nses. Toxorhynchites, on the other hand, has long narrow rhabdomeres that a re separated from each other over their entire length, and so resemble the open rhabdoms of advanced flies (Brachycera and Cyclorrhapha). These findin gs are confirmed by studies of the pseudopupil, whose form indicates the la yout of the rhabdomere tips in the focal plane of each ommatidial lens. In anopheline species the pseudopupil is a single undivided ellipse, indicatin g a fused rhabdom structure, whereas in Toxorhynchites there is a ring of s ix outer elements surrounding a central one. This means that each rhabdomer e views a separate direction in space, and our measurements indicate that, as in higher Diptera, adjacent rhabdomeres share their fields of view with one of the rhabdomeres in the immediately adjacent ommatidia. This in turn means that in the diurnal type of mosquito eye there is a basis for neural superposition, but the fused construction of anopheline rhabdoms precludes this. The Aedes species studied were similar to Anopheles but with lenses o f less extreme aperture, and Sabethes cyaneus, a diurnal blood-feeder, was intermediate in structure, with fused conical rhabdoms in the centre of the eye and unfused rhabdomeres around the periphery.