ULTRAVIOLET REFLECTANCE OF SPIDERS ANDTHEIR WEBS

9 Pages·2002·9.3 MB·English

by  Samuel Zschokke

2002. The Journal of Arachnology 30:246-254
ULTRAVIOLET REFLECTANCE OF SPIDERS AND
THEIR WEBS
Samuel Zschokke: Department of Integrative Biology, Section of Conservation
Biology (NLU), University of Basel, St. Johanns-Vorstadt 10, CH-4056 Basel,
Switzerland. E-mail: [email protected]
ABSTRACT. To determine the reflectance of spider webs and spiders under ultraviolet (UV) light,
UV
spiders and their webs were photographed under normal (white) light and under light. It was found
that all silk in araneoid webs reflect slightly more UV light than white light; i.e., they had a positive UV-
brightness. However, the often cited, particularly high UV-brightness of stabilimenta could not be con-
UV
firmed. Spiders differed in their UV-brightness, with most spiders reflecting less light than white light.
Based on the knowledge of the visual system of insects and invertebrates it is suggested that the main
function of stabilimenta is predator defense. However, drawing a final conclusion requires more knowledge
on the way potential predators and prey perceive spiders, spider webs and stabilimenta.
Keywords: Stabilimentum, camouflage, predator-prey, spider silk, visibility
The function of stabilimenta in orb-webs is al. (1994) argue that silk of primitive spiders
the subject of an intense debate. Originally, it and cribellate silk of uloborids have a high
UV
was suggested that stabilimenta serve to sta- reflectance whereas derived (araneoid) ae-
bilize the web, hence the name stabilimentum rial web spinners produce viscid silks that are
UV
(Simon 1893). More recent studies suggest spectrally flat or have a low reflectance.
that in most species, stabilimenta serve a vi- Watanabe (1999) measured the reflectance of
sual function towards prey and/or predators of the stabilimentum of a uloborid species and
the spiders, also reflected in the fact that no found that it was fairly flat, with a slightly
UV
spider species that removes the web during the higher reflectance in the range. The high
UV
day is known to build a stabilimentum (Her- reflectance of stabilimenta has been con-
berstein et al. 2000). However, whether prey sidered by several authors to be an attractor
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or predators are the intended viewers of sta- for prey (Craig Bernard 1990; Tso 1998;
bilimenta remains hotly debated. Results of Watanabe 1999), whereas other authors have
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several studies that showed that stabilimenta questioned this function (Eisner Nowicki
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attract prey could not be confirmed by others. 1983; Blackledge 1998b; Blackledge Wen-
The function of stabilimenta to deter or con- zel 1999, 2000).
fuse predators is equally disputed, especially Most, if not all, spiders that build a stabi-
since it is not easily amenable to experiments limentum sit on the hub of the web (Scharff
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(for a review see Herberstein et al. 2000). Coddington 1997). However, the appear-
UV
Predatory spiders that have recently been ance under light of the web together with
shown to use stabilimenta to find their prey the spider has been documented only once
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spider (Seah Li 2001) are quite certainly with three pairs of photographs of a single
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not the intended viewers of the stabilimenta. species taken in the field (Craig Bernard
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Craig Bernard (1990) assessed the re- 1990). Taking comparative pictures in the
flectance of spider silk by measuring the re- field is problematic since lighting is neither
flectance of individual silk strands for wave- constant nor controllable. The aim of the pres-
lengths between 340 and 700 nm at 10 nm ent study is to compare the appearance of spi-
UV
increments. They concluded that cribellate ders and their webs under light and white
sticky silk and stabilimenta, but not other silk light under standardized lighting conditions.
types of araneoid orb-webs, have a high re- In particular I asked the following questions:
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flectance in the ultraviolet (UV) spectrum. In 1. can the results of Craig Bernard (1990)
a later study using the same method, Craig et and Craig et al. (1994) be confirmed using an
246
ZSCHOKKE—UV REFLECTANCE OE SPIDERS AND WEBS 247
50 cm
—
Figure 1. Layout of the ‘black box’ (modified
&
after Langer Eberhard 1969) used to take the
pictures, seen from above. B = box lined with black
velvet, C = camera, F = fluorescent bulbs (vertical)
W
and = web with the spider. The bulbs used were
UV UV
either bulbs for the treatment or white
bulbs for the white light treatment (cf. Fig. 2).
—
Figure 2. Relative spectral irradiances of sun-
alternative technique and 2. what is the ap“ light (open circles), and of the artificial lights used
UV UV
pearance of web building spiders under (black squares: fluorescent bulbs Sylvania black
light ‘F15W/BLB -T8’, crosses: Osram ‘L 15W/12-
light. —
950 Daylight’). Curves were normalized to have
METHODS
their maximum at 1.0. The gray bar at the top in-
Spiders were collected in the wild and ac- dicates the range of wavelengths visible to humans.
climatized to laboratory conditions where they
built webs in acrylic plastic frames (30 x 30 aperture of f = 16 was used. Negatives were
X 5 cm). The frames with the spiders were developed commercially, which resulted in
placed at the front of a ‘black box’ (Fig. 1) slight differences in development between
and photographed there. Each spider in its films. However, both pictures of one object
UV
web was photographed under light and were always on the same film and therefore
under white light (Fig. 2). Unless indicated underwent identical development.
otherwise, whenever ‘white’ is used in the Negatives were scanned with a Polaroid
present paper, it implies white to the human ‘SprintScan35’ slide scanner. After scanning,
UV
eye. Light was switched between and contrast was enhanced by 20 steps with Adobe
white by exchanging the fluorescent bulbs, Photoshop. To allow exact comparison,
thus ensuring that, apart from the spectral dis- brightness and contrast of the picture were
tribution, lighting was identical in both treat- further adjusted in such a way that the bright-
ments. Pictures were taken with a Nikon F ness value of the gray card and the brightness
mm UV
camera with a 105 lens and, where value of the dark background were the same
M2
necessary, a Nikon macro adapter on Ko- in both pictures of each pair.
dak Tri-X Pan 400 ASA BAV film (this film The reflectance of spider, silk and stabili-
has a high sensitivity down to 300nm; Kodak, menta was estimated by measuring their
UV
pers. comm). For pictures taken under brightness value (in percent, ranging from 0
light, a UV transmitting ‘black’ filter was = black to 100 = white) at the same position
UV
placed in front of the lens. Pictures under in the two pictures using the utility Apple
light were exposed for 3 sec and those under DigitalColour Meter on pictures that were
white light for 1 sec. These exposure times suitably enlarged or reduced with Photoshop.
resulted in the same shade of gray when tak- For the measurements of the brightness values
ing a picture of a standardized Kodak gray of the different kinds of silk, four pairs of
card, which reflects 18% of the incident light measurements were taken for each picture and
across all wavelengths. The gray card was silk type. The brightness of each spider was
photographed together with the spider at the measured three times, once on the cephalotho-
A
edge of all pictures (visible at lower edge of rax and twice on the abdomen. measure-
Figs. 9, 10, 13, 14). For all spider pictures, an ment of the absolute brightness of the silk or
THE JOURNAL OF ARACHNOLOGY
248
—
UV
Table 1. Reflectance of spiders under light and under white light. Values for debris and egg sac
CH = KE = LK = MX
stabilimenta are given in the text. Abbreviations: Switzerland; Kenya, Sri Lanka;
= Mexico; SG = Singapore; ZA = South Africa.
UV-brightness
silk
dry sticky stabili-
Species Origin spider silk silk mentum
Achaeraranea liinata (Clerck, 1757) CH -15 15
CH -2
Agelenatea redii (Scopoli, 1763) 12 10 11
Arachnura sp. LK -14 12
Araneus diadematus Clerck, 1757 CH -14 14 11
CH -8
Araneus qiiadralus Clerck, 1757 10 2
MX -2
Argiope argentata (Fabricius, 1775) 8 6 6
CH -5
Argiope bruennichi (Scopoli, 1772) 6 3 8
Argiope ver.v/co/or (Doleschall, 1859) SG -18 7 8
CH -1
Cyclosa conica (Pallas, 1772) 6 12 9
ZA -3
Cyclosa insulana (Costa, 1834) 14 12 8
MX
Cyclosa turbinata (Walckenaer, 1842) 15 11
MX
Cyclosa walckenaeri (O.P.-Cambridge, 1889) -11 9 12 11
ZA -5
Cyrtophora citricola (Forskal, 1775) 8
Micranthena gracilis (Walckenaer, 1805) MX -14 12
KE -2 -1
Nephila senegalensis (Walckenaer, 1842) 5
ZA -2
Nephila sp. 23 15
CH -18
Steatoda triangulosa (Walckenaer, 1802) 7
Uloborus sp. ZA -23 16 25 11
MX
Verrucosa arenata (Walckenaer, 1842) 2 7 9
CH -1
Zilla diodia (Walckenaer, 1802) 16 15
LK
Zosis geniculatus (Olivier, 1789) 4 18 26 16
the spider in comparison with the gray card itive UV-brightness (ecribellate sticky silk: +
was not possible because the brightness of the 9, araneid and nephiline dry silk: +11; theri-
silk largely depends on its position relative to diid dry silk: +11; uloborid stabilimentum:
the light source (cf. brightness of radii within + 13; araneid silk stabilimentum: +9), cribel-
Figs. 11, 12) and because the position of the late sticky silk had the highest UV-brightness
gray card differed from one pair of pictures to +25), dry silk of uloborid webs also had a
(
the next. high UV-brightness (+17), whereas detritus
The relative reflectance of the silk and the stabilimenta (wrapped prey remains and shed
spiders was then assessed by subtracting the skins) of Cyclosa conica and C. insulana both
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brightness value under white light from the showed a neutral UV-brightness (+1 0),
UV
brightness value under light. I will use the and the egg sac ‘stabilimentum’ (Levi 1977)
C
term UV-brightness for this difference. Posi- of turbinata even had a negative UV-
tive UV-brightness values indicate higher brightness (-6). Similarly, the cocoon of A.
UV
brightness under than under white light. versicolor was also found to have a negative
Nomenclature of orb-web elements follows UV-brightness (-9). —
Zschokke (1999). Reflectance of spider. Most spiders ap-
UV
peared darker under light than under
RESULTS
— white light, i.e., had a negative UV-brightness.
Reflectance of silk. I found that all silks However, there was some variation between
(dry silk, cribellate and ecribellate sticky silks, species, ranging from fairly low UV-bright-
UV
stabilimenta) reflected light better than ness to neutral UV-brightness (Figs. 5-12, Ta-
white light, i.e., had a positive UV-brightness ble 1). Only a few of the species analyzed
UV
(Table 1; Figs. 3, 4). showed different patterns under light
Most kinds of silk had an intermediate, pos- compared to white light: the bright yellow ab-
—
ZSCHOKKE—UV REFLECTANCE OF SPIDERS AND WEBS 249
—
Figures 3-4. Stabilimenta and hub decorations of Zilla diodia (A), Argiope bruennichi (B), Zosis
UV
geniculatus (C) and Cyclosa conica (D) mounted on microscope slides photographed under light (3)
and white light (4). The stabilimentum of C. conica is a pure silk stabilimentum. The small bright spot
to the right of the stabilimentum (arrow) is the wrapped remains of a fruit fly Drosophila sp., as it is
sometimes incorporated into the stabilimentum.
dominal spots of Nephila senegalensis disap- itive UV-brightness of 20 and 4 respectively,
UV
peared under light (Figs. 9, 10) and ju- whereas the other seven flowers had a nega-
venile Micrathena gracilis showed dark spots tive UV-brightness ranging from almost neu-
under UV light that were not visible under tral (-2, Barbarea vulgaris) to —70 {Leucan-
white light (Figs. 7, 8). On average, the ab- themum vulgare). Some of the UV-bright to
domen of spiders had a lower UV-brightness UV-neutral flowers showed dark, distinct pat-
UV
(-8) than the cephalothorax (-4). terns under light, which are thought to
Reflectance of background vegetation. serve as guiding lines for visiting pollinating
As a control of my approach and to compare insects (Figs. 13, 14; Jones & Buchmann
reflectance patterns and UV-brightness of 1974).
flowers with that of spiders and their webs, I
DISCUSSION
photographed nine different flowers and a va-
—
riety of plants using the same method as I Reflectance of silk. It is striking, that
used for spider pictures, albeit with a smaller most silk types showed a very similar UV-
=
aperture (f 32). brightness of around +10, the only exceptions
Stems and leaves generally appeared some- being sticky (cribellate) and dry silk in ulo-
UV
what darker under light than under white borid webs, and detritus and egg sac stabili-
My
light (UV-brightness 5, Figs. 13-16). The menta of Cyclosa spp. study thus could
UV-brightness of the flowers varied consid- confirm that cribellate sticky silk has a higher
erably (Figs. 13, 14). Two flowers {Geranium UV-brightness than ecribellate sticky silk
&
sanguineum and Echium vulgare) had a pos- (Craig Bernard 1990; Craig et al. 1994).
250 THE JOURNAL OF ARACHNOLOGY
—
UV
Figures 5-12. Spiders and central part of their webs photographed under light (left) and white
light (right): 5, 6. Argiope hriiennichi; 7, 8. juvenile Micrathena gracilis’, 9, 10. Nephila seuegalensis’, 11,
12. Zosis geniculatiis.
ZSCHOKKE—UV REFLECTANCE OF SPIDERS AND WEBS 251
—
Figures 13-16. Plants under UV light (left) and white light (right). Stems leaves, and many flowers
UV
appear darker under light than under white light. The flowers of the dandelion {Taraxacum officinale,
yellow colored, A) the bloodred cranesbill {Geranium sanguineum, purple colored, B) and the winter cress
UV
{Barbarea vulgaris, yellow colored, C) show distinct, contrasting patterns under light, which are
thought to serve as guiding lines for visiting pollinating insects. Note that the two flowers that appear
white (Oxeye daisy, Leucanthemum vulgare, D) or whitish (Clematis, Clematis sp., E) to us are among
UV
the darkest under light. Pictures taken with a smaller aperture than, but otherwise identical set-up as
A
the spider pictures. colored version of Fig. 14 can be found at http://faeulty.vassar.edu/suter/joaserver/.
However, my study could not confirm that sta- ered wavelengths down to 300 nm
bilimenta have a higher UV-brightness com- (wavelengths < 340 nm may not be very rel-
&
pared to other silks in orb-webs (Craig Ber- evant biologically, since few insects are sen-
nard 1990). It is not clear why my results sitive to wavelengths < 340 nm; Briscoe &
differ from those of earlier studies. There are Chittka 2001). 4. The reflectance measure-
several possible explanations: 1. The differ- ments of Craig & Bernard (1990) may have
ence in UV-brightness between the stabili- been biased since the diameter of some of the
menta and other silk types is too small to be spider’s silks (0.4-4 jam; Craig 1986; Vollrath
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detected using photographs. 2. The measure- Kohler 1996; Zschokke 2000) lies in the
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ments of Craig Bernard (1990) and Craig range of the wavelengths of visible light (0.4-
et al. (1994) considered each wavelength sep- 0.7 p.m), and the interactions between light
arately, whereas the measurements in the pres- and such thin objects are rather complex
ent study are integrations over a range of (Craig 1988; Nishiyama et al. 2001). 5. Since
&
wavelengths. 3. Craig Bernard 1990 did not the reflectance of silk depends on the incident
consider wavelengths shorter than 340 nm and angle of the light (cf. radii in Figs. 11, 12),
UV &
their measurements suggest that reflec- Craig Bernard’s measurements, which used
tance of stabilimenta drops off below 360 nm; the same incident angle of light for all mea-
whereas measurements in my study consid- surements, may not be representative, espe-
252 THE JOURNAL OF ARACHNOLOGY
&
cially if there is an interaction between the sensitive to red (Briscoe Chittka 2001).
incident angle of light and wavelength. 6. If Second, insects differentiate colors primarily
some silk types are fluorescent, this could re- through their color contrast and not through
UV
sult in an over-estimation of their reflec- brightness (Fukushi 1990; Backhaus 1991;
&
tance using the method of Craig Bernard. Chittka et al. 1992; Chittka et al. 1994). To
Whichever method is used to measure silk insects, objects that we perceive as white and
UV
reflectance, one conclusion remains the same: that also reflect light (i.e., have a flat spec-
all silks in orb-webs (including stabilimenta) trum), have the same color as the background
UV
reflect more light than the background (e.g., leaves, bark, soil), all appearing achro-
UV
vegetation, which reflects little light, and matic at the center of the insect color space,
UV
therefore appears darker under light (Frol- since insects are not able to detect red light,
ich 1976; Chittka et al. 1994; Figs. 15, 16). which we use to distinguish white objects
However, the analysis presented in this paper from leaves or soil. As a consequence, there
shows that the UV-brightness of silk stabili- are very few white (i.e., white for humans),
menta is much smaller than that of some flow- insect pollinated flowers that also reflect in the
UV
ers, and it is therefore questionable whether wavelengths (Kevan et al. 1996; see also
stabilimenta can attract pollinating insects Figs. 13, 14). Silk stabilimenta probably also
UV
through their reflectan—ce. fall into this category: to humans they appear
Reflectance of spiders. Spiders varied in white and they reflect UV light. We may
UV
the way they reflect light compared to therefore conclude that stabilimenta, being
white light. There seems to be a trend for the achromatic, are not very conspicuous to in-
more colorful species (e.g., A. bruennichi, A. sects (Blackledge 1998a).
argentata, N. senegalensis, V. arenata) to Our eyes have a maximum resolution of 0.3
UV
show a neutral brightness, whereas the min of arc. To be able to see a typical spider
more cryptic species (e.g., Arachnura sp., thread with a diameter of two p.m with the
which tries to mimic a dead leaf, Micrathena naked eye would require us to approach it to
gracilis, which resembles a ball of dirt) seem a distance of less than two cm, at which dis-
to have a lower UV-brightness. One can spec- tance we are not able to focus on it. We can
UV
ulate that the reduced reflectance under therefore perceive spider threads only if there
light, which is comparable to that of the back- is a large contrast between the thread and the
ground vegetation, is part of the camouflage background compensating for the lack of spa-
of this spider. Due to the simultaneous posi- tial resolution of our eyes. In a similar way,
tive UV-brightness of silk, and the negative or the apparent size of all fixed stars in the sky
neutral UV-brightness of the spiders, the spi- at night falls below our eye’s resolution, but
ders with a hub stabilimentum appear more we can nevertheless perceive many of them,
UV
cryptic under light than under white light thanks to their great contrast to the dark sky.
(Figs. 5, 11). — Since spider silk is white, the best way for us
Visibility of webs. The visibility of the to achieve the necessary high contrast to see
web is crucial for the spider: the web should single threads, is to view them brightly illu-
be simultaneously invisible or attractive to the minated against a dark background. The spa-
spider’s prey and invisible or deterring to the tial resolution of insect’s eyes is roughly 100
spider’s potential predators (Blackledge times poorer than our own (Wehner 1981),
1998a). Many of these potential prey or pred- which would require the insects to approach
mm
ator species (e.g., insects and birds) are known the web to less than a to be able to see
to have UV receptors (Menzel & Backhaus it. It is not known, how and under what cir-
&
1991; Finger Burkhardt 1994), and conse- cumstances the insect eyes can make up for
UV
quently, the reflectance of spiders and the lack of resolution to see spider threads.
their webs must be considered. At the same However, Rypstra (1982) and Craig (1986)
time, color perception and spatial resolution have reported that Drosophila sometimes
of the visual systems of potential prey or pred- change their flight path as they approach silk
ator species must be taken into account. strands, suggesting that they are able to detect
Insect vision differs fundamentally from them.
that of humans and other vertebrates. First, It is not quite certain how insects perceive
many insects can detect UV light but few are stabilimenta, which have a fairly flat spectrum
ZSCHOKKE—UV REFLECTANCE OF SPIDERS AND WEBS 253
and which could therefore be expected to ap= and foraging success in Argiope aurantia andAr-
pear rather dull and colorless to them. In one giope trifasciata (Araneae: Araneidae). Journal
experiment, Blackledge & Wenzel (2000) of Zoology 246:21-27.
found that they could not train bees to asso= Blackledge, T.A. & J.W. Wenzel. 1999. Do stabili-
meeta in orb webs attract prey or defend spiders?
ciate a reward with stabiiimentum silk, where-
Behavioral Ecology 10:372-376.
as they could train bees to associate a reward &
with silks that have UV reflective peaks. On Blackledge, TA. J.W. Wenzel. 2000. The evo-
& lution of cryptic spider silk: a behavioral test.
the other hand, Blackledge Wenzel (2001) Behavioral Ecology 11:142-145.
also showed that spiders in stabilimenta dec- Blackledge, TA. & J.W. Wenzel. 2001. Silk medi-
orated webs were more likely to survive at- ated defense by an orb web spider against pred-
tacks of mud-dauber wasps, suggesting that atory mud-dauber wasps. Behaviour 138:155-
the wasps were able to perceive the stabiii- 171.
&
mentum. Briscoe, A.D. L. Chittka, 2001. The evolution of
Bird vision is m—.ore similar to that o—f hu- color vision in insects. Annual Review of Ento-
mology 46:471-510.
mans, but it often like that of insectS' ex- &
UV & Chittka, L., W. Beier, H. Hertel, E. Steinmann R.
tends into the (Finger Burkhardt 1994).
Menzel. 1992. Opponent colour coding is a uni-
Consequently, stabilimenta are probably quite
versal strategy to evaluate the photoreceptor in-
conspicuous to birds. Since birds are only puts in hymenoptera. Journal of Comparative
rarely the prey of spiders, it may be concluded Physiology A 170:545-563.
that the main function of stabiiimentum is Chittka, L., A. Shmida, N. Troje & R. Menzel.
probably deterrence against birds, rather than 1994. Ultraviolet as a component of flower re-
attraction of prey; thus confirming the studies flections, and the colour perception of hymenop-
of e.g., Lubin (1975), Horton (1980), Eisner tera. Vision Research 34:1489-1508.
& Nowicki (1983), Schoener & Spiller (1992) Craig, C.L. 1986. Orb-web visibility: the influence
and Blackledge & Wenzel (1999). However, of insect flight behaviour and visual physiology
on the evolution of web designs within the Ar~
before any final conclusions can be drawn,
aneoidea. Animal Behaviour 34:54-68.
much more must be learned about the way
Craig, C.L. 1988. Insect perception of spider orb
different potential prey and predators perceive
webs in three light habitats. Functional Ecology
spiders and their webs. 2:277-282.
&
ACKNOWLEDGMENTS Craig, C.L. G.D, Bernard. 1990. Insect attraction
to ultraviolet-reflecting spider webs and web dec-
I thank the scientific photography labora- orations. Ecology 71:616-623.
tory of the University of Basel for lending me Craig, C.L., G.D. Bernard & J.A. Coddington.
the photographic equipment, Christophe Bar- 1994. Evolutionary shifts in the spectral proper-
ney for building the 'black box’, Nicole Mi- ties of spider silks. Evolution 48:287-296,
&
noretti for taking care of the spiders, J. Alvaro Eisner, T. S. Nowicki. 1983. Spider web protec-
Garcia Ballinas, Daiqin Li, Suresh Benjamin tion through visual advertisement: role of the sta-
biiimentum. Science 219:185-187.
and Fritz Vollrath for providing various spi- &
Finger, E. D. Burkhardt. 1994. Biological aspects
ders, and Todd Blackledge, Catherine Craig,
of bird colouration and avian colour vision in-
Andreas Erhardt, Hans-Peter Rusterholz, Rob-
cluding ultraviolet range. Vision Research 34:
ert Weber and an anonymous reviewer for
1509-1514.
fruitful discussions or comments on the man- Frolich, M.W. 1976. Appearance of vegetation in
uscript. The project was partially supported by ultraviolet light: absorbing flowers, reflecting
the Swiss National Science Foundation grant backgrounds. Science 194:839-841.
31^55617.98. Fukushi, T. 1990. Colour discrimination from var-
ious shades of grey in the trained blowfly, Lucilia
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