A Dinner Dance

1060 Br J Ophthalmol 2003;87:1060 Br J Ophthalmol: first published as 10.1136/bjo.87.9.1060 on 20 August 2003. Downloaded from Coverillustration......................................................................... A dinner dance lowering plants must attract birds or compound, multifaceted eye of the insect the retina much more densely than the insects to help disperse their pollen. is known as an ommatidium. In the clas- overlying facets would suggest. Denser FThe attraction, of course, is usually sic Exnerian explanation of the com- rhabdom packing creates a more detailed nectar, but this must be advertised with pound eye, each ommatidium has a lens image at each location where this bright colours and/or scent. The pollina- as a focusing element and a collection of occurs—in effect creating a fovea. These tors seek the nectar and usually cooper- retinular cells within that ommatidium. areas of the eye also have the largest ate, quite innocently, by transporting the Each retinular cell contributes a portion numbers of facets supplying light to each pollen. Most insects, like bees, land on a of its folded membrane (rhadomere) rhabdom. The hummingbird hawk- flower, seeking the nutritious reward. that joins with its counterparts from the moth’s superposition eyes have thus Some few insects, like the hummingbird other retinular cells to create a “rhab- evolved regions where the visual image is both very bright and highly resolved. hawkmoth (Macroglossum stellatarum), dom” along the axis of the ommatidial This is developed to its greatest extent in however, do not maintain their end of cylinder. This rhabdom functions as the the frontal regions of the eyes, which are the unwritten bargain. This interesting ommatidium’s photoreceptive structure. used for close observation of the flower creature will hover above the flower, Although there are variations in the petals beneath the moth while it hovers using an unbelievably long coiled tongue neural mechanisms that occur behind (Warrant E, J Exp Biol 1999;202:497). that may be three times the length of the each visual unit, the insect brain receives Looming sensitive neurons, found in the insect to extract the nectar and cheat the a mosaic pattern of visual signals that is optic lobes of the brain, help to interpret flower of its pollinating potential. This assembled into a complete image. The and integrate the expanding and con- frenetic Old World moth is a member of tracting visual image of the moving the family Sphingidae and is primarily flower. This results in the necessary diurnal. These moths have a remarkable lateral arabesque that stabilises flight hovering flight that resembles that of a and allows the moth to maintain its hummingbird. They are quite agile and position directly above the flower in a can even fly while coupled during mat- soft summer breeze. ing. (Hummingbirds are primarily South These remarkable moths also have American, are found exclusively in the true trichromatic colour vision—that is, New World, and also hover, but they are they have been shown to discriminate completely unrelated to this moth.) objects on the basis of their spectral This fast flying and fascinating moth information alone, independently of in- is common to European gardens in the tensity (Kelber A, et al, J Comp Physiol A summer and deserves more than passing 1999;184:535). To do this, these crea- attention. Consider its difficulties. While tures have at least three spectral receptor http://bjo.bmj.com/ this moth is hovering above its intended hummingbird hawkmoth has a so called types. These include an ultraviolet type, meal, the flower may be caught by a brief “refracting superposition compound and there does not appear to be a long wavelength pigment that is stimulated gust of wind and move in almost any eye,” a very sensitive design more typical in the redder portion of the spectrum, so direction, in any of three dimensions, of nocturnal moths. However, the hum- the trichromacy is not the same as ours. creating difficulties for the feeding moth. mingbird hawkmoth is diurnal, and its A related hawkmoth (Deilephila The long proboscis must remain in bright habitat has allowed remarkable elpenor) has been shown to have noctur- contact with the small supply of nectar specialisations not known in any other nal colour vision and is the only animal on October 2, 2021 by guest. Protected copyright. at the base of the flower, and hence the superposition eye. This moth’s eye is known to have it (Kelber A, Nature moth must move with the flower, deftly aspheric and possesses areas of higher 2002;419:922). Furthermore, it has good so as to maintain that contact with the resolution and sensitivity that function colour constancy; the quality that allows base of the flower and the nectar. This much like the foveae of vertebrate eyes, animals to identify a lemon as yellow in aerial positioning is accomplished by features that violate our previous under- different coloured shades of illumina- sophisticated optical and neural mecha- standing of how superposition eyes tion. Having three different visual pig- nisms that result in surprisingly good function. The optical design of a classic ments, as you might imagine, does visual acuity and movement sensitivity. refracting superposition eye allows the decrease the absolute sensitivity of the To begin with, this hawkmoth has an lenses of a large number of ommatidia to eye, and in order to see well at night unusual and complex compound eye focus light onto each rhabdom of the temporal and/or spatial summation of that does not have the typical anatomy of retina. According to the classic theory of the photic stimulation must occur most insect eyes. Most of the dipterid Sigmund Exner (1891), this requires (Kelber A, et al, Nature 2002;419:922). species (flies) have neural superposition that the retinular cells are at a set So, this moth’s aerial ballet becomes compound eyes (BJO cover, September distance behind the lenses (thus result- its dinner dance. 2001) that are more complex than, and ing in a spherical eye), and that the IRSchwab,EJWarrant an evolutionary step up from, the simple number of rhabdoms equals the number University of California, Davis, Sacramento, CA apposition compound eyes of more of facet lenses. But this hawkmoth eye 95817, USA; [email protected] and University of Lund, Sweden primitive insects (BJO cover, March has many more rhabdoms than facets, 2001). Each individual unit of the with the rhabdoms being packed into Cover image by Karlheinz Baumann. www.bjophthalmol.com.

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