Twilight orientation to polarised light in the crepuscular dung beetle Scarabaeus zambesianus
Marie Dacke1,*,
Peter Nordström1 and
Clarke H. Scholtz2
1 Department of Cell and Organism Biology, University of Lund,
Helgonavägen 3, S-223 62 Lund, Sweden
2 Department of Zoology and Entomology, University of Pretoria, South
Africa
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Fig. 1. Set-up used for experiments on the orientation to polarised light. The
black open circle represents the polarising filter (shaded) in its holder, and
the white dotted circles represent four symmetrically placed legs, height 10
cm. The set-up was equipped with a magnetic compass to orient the e-vector
produced by the filter (double-headed arrow) in a west-easterly direction,
90° to the e-vector on the evening skylight (dotted double-headed arrow).
The turn made by the beetle ( ) in response to the shifted polarisation
pattern experienced on entering the filter was measured from the track drawn
from filming the beetle. Open arrows mark the direction of movement. Note that
the beetle rolls head down and backwards.
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Fig. 2. Schematic drawing of the orientation of the beetles on encountering a
barrier (N=15). Open arrows mark the direction of movement. Beyond
the barrier, seven of the beetles turned to the left of the previous course,
and eight turned to the right. Absolute mean angle of deviation from the
original course is 16.9°.
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Fig. 3. (A) Scanning micrograph of the dorsal and ventral eye of S.
zambesianus. For correct orientation, the anterior (ant) direction of the
animal is indicated. A lateral view of the head of the beetle shows the
canthus (can) that totally separates the eye into a dorsal and a ventral part.
The asterisks mark the border of the dorsal rim area that covers approximately
half the dorsal eye, narrowing towards the ends. (B) Scanning micrograph of
the dorsal eye and canthus. Scale bars: 500 µm.
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Fig. 4. Electron micrographs of the rhabdoms in the dorsal and ventral eyes. The
rhabdoms are formed by seven receptor cells but differ in their shape and
microvillar orientation in different eyes and eye regions. The rhabdoms of the
dorsal rim area (A) are heart-shaped with orthogonal microvilli, while the
rhabdoms in the rest of the dorsal eye (B) and in the ventral eye (C) are
flower-shaped with several microvillar orientations. Scale bar: 5 µm.
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Fig. 5. A light microscopical section through most of the dorsal rim area (DRA).
The double-headed arrow marks the orientation of the section (D, dorsal; V,
ventral). The black bars, indicating the microvillar orientation of retinula
cells 2–7 (see the enlarged cross-section, orientation indicated in this
case by a white bar), form a fan-shaped pattern across the DRA. Besides the
shape of the rhabdoms, no distinct border in distribution or orientation of
the ommatidia indicates the limits of the DRA. The flower-shaped rhabdoms
(FSR) in the remainder of the eye are seen at the bottom of the image. Scale
bar: 50 µm.
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Fig. 6. Schematic drawing of theoretical paths travelled by `leaving' (A) and
homing (B,C) animals when negotiating a barrier (black rectangle). In C, the
animal is picked up and displaced sideways (broken line) as it reaches the
barrier.
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Fig. 7. Cross-sections of DRA-rhabdoms in the crepuscular beetle Scarabaeus
zambesianus (A) and the diurnal beetle Pachysoma striatum (B).
Note the difference in the size of the rhabdom and amount of pigmentation,
both morphological adaptations to the time of activity. Scale bar: 2
µm.
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© The Company of Biologists Ltd 2003
