06 February 2010

Sitting Pretty:
Colour morphology and hunting success in orb web spiders

Why do some orb web spiders display striking colours and patterns, while others are drab and brown? Bright colours, it seems, help to attract tasty prey to their webs, but also provide easy targets for ferocious predatory wasps. What spiders have done, according to Dr. Tso I-Min of Tunghai University, is to evolve optimum trade-offs in morphology, behaviour and mutualistic relationships that balance the scales of reward and risk.

Golden orb web spiders (Nephila spp.) are common, and to some shocking, sights on forest fringes and even in well-vegetated parks such as the Botanic Gardens, where they construct massive webs strong enough to snare large insects such as cicadas and by some accounts, even small birds. The huge females (the males are midgets barely a centimetre long) are usually found hanging head down at the hub or centre of their webs, and despite their size, are docile creatures who attempt to hide under a nearby leaf if disturbed by curious humans.

Many orb web spiders are night-time hunters who take shelter by day and emerge to spin a new web each evening. Nephilas, as well as a few other builders of orb webs such as the St. Andrew’s Cross spiders (Argiope spp.) and those in the genus Cyclosa are active during the day. Besides their different shift, diurnal orb web weavers are noticeably much more colourful than their nocturnal cousins. Some also decorate their webs with vivid patterns of silk called stabilimenta (see photo below).

Why should these spiders, which rely mostly on touch to find their way around and communicate with members of their own species, boast such striking patterns? Dr. Tso I-Min, a behavioural ecologist at Tunghai University in Taiwan, has been investigating how body colour influences the foraging success and survival of orb web spiders and shared some of his findings in a recent seminar at the National University of Singapore.

According to Dr. Tso, conspicuous body colouration has been shown by many studies to help attract insects to the webs of spiders such as Nephila, Leucage, (which the Taiwanese call orchid spiders), Argiope and the spiny genus Gasteracantha (pictured left). It has been suggested that the spiders’ colour patterns act as visual lures as they resemble the floral parts of certain flowers, to which many insects such as bees, flies and moths are attracted.

A common hue among diurnal spiders is yellow, which Dr. Tso notes is the colour of pollen, an important source of protein for many insects. But if yellow is an insect magnet, he asked, shouldn’t the spiders have wholly yellow bodies?

Dummies on the web

Nephila pilipes, which in Taiwan is known as the giant wood spider, is a golden orb weaver that ranges from India and Okinawa to Northern Australia and the Western Pacific, where islanders are known to use its robust web as sticky fish nets. The species bears prominent yellow spots on the underside of its legs as well as gold spots and markings on the ventral side of the abdomen. The base colour is black, which according to Dr. Tso, insects cannot tell apart from the surrounding vegetation. Lacking the resolution of mammalian eyes, bugs can only distinguish the yellow pattern on the spider’s underside (see slide above).

Dr. Tso’s work involved field tests to find out if the spider’s colours affect its foraging success. To eliminate the possibility of non-visual influences such as scent and sound, he used cardboard dummies instead of live spiders (“it’s hard to paint living spiders” he noted), and compared the success of webs with various dummies to empty but intact webs, from which the owners were temporarily removed.

Taking care to ensure that the colours of the dummies matched the exact shades and reflectance spectra of actual spiders, Dr. Tso’s team created dummies with colour patterns identical to the spiders as well as dummies that were all yellow or all black. The dummies were then placed on webs and recorded on video for 8 hours a day to monitor their performance.

After pouring through thousands of hours of footage, some broad observations emerged. Compared to empty webs, those with life-like dummies attracted significantly greater numbers of prey insects as well as predators such as wasps that seek out spiders to feed their young. (Juvenile Nephilas, which are possibly more vulnerable to large spider-hunting wasps, build barrier webs around their main web as possible lines of defence.)

The all black dummies registered much lower success rates. However, the all yellow dummies were significantly more effective than the realistic dummies at luring insects. So why don’t we see all yellow spiders in life? The answer, Dr. Tso thinks, may be because the all yellow dummies also drew significantly more hits by predators. This predation pressure, he suggests, could be a factor that limits the amount and intensity of colour signals in diurnal spiders that attract prey. The colourful morphology of Nephilas thus reflect a possible trade-off between hunting success and their own vulnerability to visually-cued predators.

Parawixia dehaani, a nocturnal orb web spider related to Neoscona, but with bright orange spots on the upper side of its abdomen.

It’s different at night

Unfortunately, the story doesn’t end here. Was there more to the Nephilas’ habits that could shed more light on this possible link between colour and survival? A reluctant grad student found himself monitoring the daily cycle of the spiders and thus revealed that Nephilas are in fact active at night as well. The spiders rest when the sun sets, but after midnight, sneakily start to build new webs where they sit until the following evening.

In an unpublished experiment to see how colour affects foraging success of Nephilas at night, Dr. Tso repeated the dummy tests. But in addition to black and yellow dummies, a blue dummy was added, as blue and yellow cannot be told apart in the monochrome of the dark, despite their different chromatic signals. Another addition were realistic dummies with the yellow markings altered to form a prominent central patch on the body.

Unlike the daytime experiments, all yellow dummies failed to attract significantly more prey than realistic dummies. Blue dummies fared worse, while those with the altered patch showed little change in success from the life-like dummies, indicating that as far as insect eyes are concerned, the patterns don’t make much of a difference.

As a counter measure, Dr. Tso also studied a strictly nocturnal orb web weaver, the ghost spider (Neoscona punctigera, photo shows a Singaporean Neoscona species). This animal has a drab brown body, save for two yellow ventral spots. Suspecting that they act as visual lures, Dr. Tso used pens to cover the spots and found that spiders that suffered this indignity were significantly less attractive to prey than undefiled individuals.

Going further, he created dummies in various colours: brown, black, yellow and grey, as well as realistic dummies, dummies with fused yellow spots, all black and all yellow dummies. At least 30 of each dummy were field tested, along with a control of 28 empty webs, leading to 1,464 hours of footage enjoyed by TV-loving students.

Again, realistic dummies proved far better hunters than empty webs. They were only slightly more effective than dummies with fused spots, suggesting that mere pattern differences do not matter to prey. But they were much more successful than the uniformly yellow models, hinting that a defined or symmetrical pattern may better mimic the way some flowers have ‘lane markers’ that help guide insects to their nectar.

Visual signals thus appear to have a significant impact on foraging success in both day and night, although there are major differences. Dr. Tso noted that predation by wasps was not a threat at night, and so all yellow dummies suffered no attacks. But whereas predation pressure may be one reason why Nephilas are not more yellow, hunting success may be what favours yellow spotted ghost spiders over hypothetical all yellow ones.

Spiders with a different spin

A curious twist to the colour code of orb web spiders comes in an ongoing project involving two cohabitating species. Argyrodes fissifrons is a small, silvery spider (left photo shows a Singapore species of Argyrodes) often regarded as a kleptoparasite of the far larger red tent spider (Cyrtophora unicolor, pictured above), which lacks bright colours and hides on a dry leaf in its web. But given the earlier findings with other orb weavers, Dr. Tso wonders if there is more to the relationship than a simple case of kleptoparasitism.

To test the idea, he quantified the colour signals of both spiders, removed the smaller species and compared these webs with controls that have both species. Preliminary results suggest that webs without Argyrodes caught fewer prey than those with the ‘kleptoparasite’. The differences found so far are not statistically significant, but if a clearer pattern emerges, it could suggest the existence of a body colour mediated mutualism between the two species, in which the larger spider avoids the risks associated with a brightly coloured body by tolerating the presence of freeloaders that help attract prey for their host. A possible study in contrast crops up in the closely related Cyrtophora mollucensis, a brightly spotted spider that sits openly in a similar tented orb web.

Cyclosa ginnaga is a small orb weaver with a very bright abdomen and a different strategy for survival. The spider adds decorative stabilimenta (photo shows the stabilimenta of a local species of Cyclosa) on its web, which Dr. Tso suspects help to make the web more attractive to prey as well as deflect the attention of predators which may target bits of silk instead of the actual spider.

By applying dark powder on the stabilimenta, both stabilimenta and spider, and comparing the consequences with untainted controls, Dr. Tso found that spiders with darkened stabilimenta suffered more attacks and caught fewer prey than those on unpowdered webs. This species may thus represent another approach in finding an optimal balance between having colours that contribute to hunting success and a means to reduce or deflect the predation risks of such prominent patterns. The trade-off, it seems, may be an uncertain one, as the spiders do not always construct stabilimenta with each new web. As for the Cyclosas' larger cousins, the colours of utmost success are likely to come at a cost that keeps the spiders from becoming flashier than they already are.

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