ISSN:
1432-1351
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Medicine
Notes:
Summary The afocal apposition optics of butterfly eyes was examined from both a geometrical optics and a wave optics point of view. We used several different species of butterfly but put special emphasis on a common Australian nymphalid,Heteronympha merope. From the anatomy of the retina, the optics of isolated components of the eye and the ophthalmoscopy of the intact living eye we derived the following. 1. The proximal part of the crystalline cone behaves as a powerful lens which, according to our measurements of optical power, turns the complete optical system into an afocal telescope with an angular magnification of 6.4 (inHeteronympha). The rhabdom tip lies in the exit pupil of the telescope and is imaged into the cornea with a magnification of 9.1 (in the same species). 2. Using light reflected from the eye's tapetum, we studied the waveguide mode phenomena of the rhabdom. Different butterflies showed either one, two or three waveguide modes, depending on the rhabdom diameter. The mode patterns were observed at four different optical planes: at the cornea, at infinity, at the back focal plane of the corneal lens — which, for this measurement, was optically neutralised — and at the plane of the deep pseudopupil. 3. During light adaptation the closure of the pupil caused the modes to disappear in sequence, starting with the highest order. The behaviour of the fading modes indicates that the pupil acts by absorption rather than by a change of refractive index around the rhabdom. 4. The modes were used to measure the waveguide parameter of the rhabdom, from which its refractive index was deduced to be 1.36. 5. The distinction between near-field and farfield versions of the mode patterns provided further evidence in favour of an afocal optical system. Two different interpretations of the butterfly optical system are discussed and we present a hypothesis to explain how both afocal apposition and refracting superposition optical systems evolved in insect eyes.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00606122
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