Fundamental differences in the optical structure of the eyes of nocturnal and diurnal mosquitoes
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Ó Springer-Verlag 1999
ORIGINAL PAPER
M. F. Land á G. Gibson á J. Horwood á J. Zeil
Fundamental differences in the optical structure of the eyes of nocturnal and diurnal mosquitoes
Accepted: 17 March 1999
Abstract We have studied the anatomy and optics of the eyes of a range of mosquito species from the wholly dark-active blood-feeding Anopheles gambiae to the diurnal plant-feeder Toxorhynchites brevipalpis. Consistent with studies by Satoà in the 1950s, we ®nd that darkactive and crepuscular species have short fused rhabdoms with a conical construction. This maximises the amount of light the rhabdoms receive from the almost hemispherical wide-aperture lenses. Toxorhynchites, on the other hand, has long narrow rhabdomeres that are separated from each other over their entire length, and so resemble the open rhabdoms of advanced ¯ies (Brachycera and Cyclorrhapha). These ®ndings are con®rmed by studies of the pseudopupil, whose form indicates the layout of the rhabdomere tips in the focal plane of each ommatidial lens. In anopheline species the pseudopupil is a single undivided ellipse, indicating a fused rhabdom structure, whereas in Toxorhynchites there is a ring of six outer elements surrounding a central one. This means that each rhabdomere views a separate direction in space, and our measurements indicate that, as in higher Diptera, adjacent rhabdomeres share their ®elds 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 of less M.F. Land (&) á J. Horwood Sussex Centre for Neuroscience, School of Biological Sciences, University of Sussex, Brighton BN1 9QG, UK e-mail: [email protected] Fax: +44-1273-678535 G. Gibson Pest Management Department, Natural Resources Institute, University of Greenwich, Chatham, Kent ME4 4TB, UK J. Zeil Centre for Visual Sciences, Research School of Biological Sciences, Australian National University, Canberra, ACT 2601, Australia
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. Key words Mosquito á Eye á Vision á Neural superposition á Optics
Introduction The life-styles of mosquitoes cover a very wide range of feeding habits and light environments, from wholly nocturnal blood-feeding species such as Anopheles gambiae, the scourge of sub-saharan Africa, to the large but innocuous diurnal nectar feeder Toxorhynchites brevipalpis, from Australia and New Guinea. The average light intensities experienced by these insects during their periods of activity dier by a factor of at least 106, the dierence between sunlight and moonlight, and it is reasonable to suppose that the structure of the eyes should re¯ect this dierence. In a recent study (Land et al. 1997) we found that A. gambi
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