Spiders

How do taxi drivers find prey passengers? Some prowl the high streets at random, intercepting commuters who thrust a waving arm. This approach requires good eyesight. Other drivers with strained visual receptors may prefer to lurk at high traffic locales like hospitals, airports, hotels and bars, where a regular stream of clients can be expected and can be snapped up.

Some drivers do the day shift; others pound the nightways. A few seem to specialise in phone-bookings, while it’s said that some even cherry-pick their passengers, ferrying only cross-straits travellers (yes, there is actually a taxi service to Johor Baru!), airline crews or other professions with specific habits and hangouts.

Thus began Joseph Koh’s spiel on spiders and their hunting strategies, which he gave on 8 February at the Botanic Gardens’ Visitor Centre. Mr Koh (left) is a home-grown arachnophile who has written a book (in the excellent if somewhat dated BP Science Centre nature in Singapore series, available online here) on Singapore’s spiders and continues to pursue research in classifying and studying the huge variety of species found on the island, with a particular focus on interesting aspects such as spider genitalia. He even has a few species named after him, although I wager his namesakes would run away just as eagerly as any other webspinner at his approach. 

Mr Koh’s opening served to illustrate how different spiders, like individuals in many human occupations, adopt highly disparate strategies to seek out prey animals and have developed morphological and behavioural traits that complement their hunting methods.

All spiders are carnivorous, feeding on prey such as insects, other invertebrates, small vertebrates and even other spiders. To immobilise prey, spiders inject venom from a pair of fangs. The venom is a neurotoxin that causes paralysis, allowing the spider to ‘vomit’ out digestive enzymes that cause the prey’s innards to liquefy. The spider then sucks up the fluid, leaving behind a dry husk. Fortunately, no local species is known to have venom that is fatal to man, although some have painful bites. There is also no truth in the urban myth that the bite of certain spiders will endow human victims with the ability to swing from roof to roof wearing leotards and a ridiculous mask. 

Spiders are classified under the taxonomic order Araneida in the class Arachnida which also houses scorpions, mites, ticks, whip scorpions and just about any anthropoid you might come across that does not have six legs. There are at least 30,000 named spiders (and thousands more still undescribed). Singapore alone harbours more than 380 species, although Mr Koh reckons the real number to be twice as large, given the regularity of sightings that turn out to be new records.

Broadly, spiders either spin webs or hunt. Web builders are typically sedentary creatures with a fixed territory. They have relatively poor eyesight, relying primarily on tactile senses for navigation and prey detection. They “taste by touch”, using their sensitive legs to detect movement and vibrations on their web that indicate the presence of entangled prey.

Web spinners
Different web spinners have their own approaches to handling prey. Some wrap the struggling animal in a cocoon of silk (using their fourth leg to skillfully twirl the thread ejected from their spinnerets at their abdomen around the prey like a live mummy) before delivering the immobilising venom. Others bite first and ask questions later. A number of species like their meal well crushed before partaking of the resulting amorphous lump of organic matter. Others like their entrées whole, leaving behind clean shells of once buzzing insects.

Web architecture is part a spider’s extended phenotype. The classic orb web is spun by members of the families Araneidae, Tetragnathidae and Uloboridae. In Araneidae are the so-called spiny spiders (genus Gasteracantha) which build vertical orb webs. The spiders, which have spikey extensions that can be longer than the body itself, sit boldly at the hub of their webs during the day, as they are unpleasant tasting, with appropriately gaudy warning colourations. 

Orb webs are both beautiful and complex structures. This site has good visuals on the elements of an orb web and the process of spinning, but essentially the web is made up of an anchored frame, with radial lines that connect the frame to the centre of the orb. Most of the web is made from different types of non-sticky silk from the spinnerets. Only the prey-snaring capture threads that form concentric spirals around the hub are made from sticky silk.

Some orbs have a free zone (a gap between the spiral threads) in the hub that allows the spider to easily move from one side of the web to another. There are also spiders who choose locations near or above water. A number of spiders, rather than sitting squarely on the hub, hide nearby and use a signal line attached to the web that vibrates when a prey hits home. A further variation are webs with hubs that are built around a leaf. One unusual vertical orb web spinner is Poltys illepidus, the so-called tree-stump spider for its uncanny resemblance to the broken end of the stick. It assumes this camouflage position in the day, building a new web every night and consuming it at daybreak to reingest the valuable proteins. 

Even casual observers in local parks and reserves would find it hard to notice the large orb web spiders Nephila maculata and N.  pilipes on their huge, yellowish webs that can be a couple of metres wide. To protect the main web from unsuitable catches or bumbling ducks monkeys, barrier webs are spun nearby. The spiders sit on the hub facing downwards and the vertical web is fittingly asymmetrical, with the lower portion covering a much greater area than the upper. Despite their size and ferocity, Nephilas can fall prey to a species of wasp that lays its egg on the spider's back. When the larva hatches, it feeds off the living spider and as a penultimate gesture, causes the spider to spin a final web that has no prey-catching ability but is strongly secured, after which the larva dishes out the coup de grace (by literally sucking the life out of the spider) and pupates in the safety of the web.

Horizontally suspended webs are built by spiders in the genus Tetragnatha, which are known as ‘big-jaw’ spiders. Webs are also ‘decorated’ by some spiders, which place debris to camouflage the traps, or decoys such as egg sacs that mimic the spider’s appearance (perhaps as a strategy against predatory wasps). A silken band may also be used to hide the spider.

Orb webs with lace-liker zigzag bands (stabilimentum) are built by Argiope spiders, which align their legs to the stabilimentum, which may be 4-armed or 2-armed. Stabilimentum in the form of a lacy network can also be found in the hubs of some webs. They also serve as sunshades for some spiders. 

Cyrtophora spp. or tent spiders build webs that are 3-dimensional, with an inverted dome at the bottom on which insects that fly into the web fall into. Psechrus singaporensis also builds a 3-D web with a tent with a tubed retreat. Other builders of 3-D webs are members of the family Therididae, to which the infamous black widow belongs, and the Pholcidae family, the so-called daddy longlegs which can often be found in dusty cornices. Eschewing the typical practice of depositing eggsacs, daddy longlegs  carry their relatively large eggs in their jaws.

The family Linyphiidae contains spiders which spin horizontal sheet webs. Insects that fly into the upper tangle web are shaken onto a mat below and grabbed. The sheet web wolf spider (Hippasa holmerae) is one whose traps are commonly seen (but disregarded) on well-turfed lawns, especially in the early hours of the morning, when they sparkle with droplets of dew. A silken funnel hides the spider while it waits.

Other web designs include webs in other webs, orbs that are spun over the surface of a leaf and orb webs with silken retreats. An extreme adaptation is seen in the reductionism practised by species such as Phoroncidia studo, a spider in the family Theridiidae that uses a single horizontal sticky line of up to a foot long that snares insects which may be attracted to chemical lures emanated by the spider or the thread.

Another spider, Argyrodes flagellum looks like a thin strand itself, lining its legs up like a stick insect to form a straight line and producing a simple Y-shaped trap. Other members of its genus are not spinners but tiny stealers that hang around the webs of other spiders like jackals and help themselves to fresh catches. In his book, Mr Koh also notes that the males of these spiders ‘plug’ their females with a resin after mating to prevent the entry of other suitors.

Finally, there are spiders which cast nets such as Dinopsis, which holds its stretchable net using its front legs and flings it onto prey that wander in range. These spiders are also known as ogre-faced spiders for their unusually large eyes which are a thousand times more sensitive than even the sharp eyes of jumping spiders. They hunt at night, hanging the net on a nearby spot during the daytime slumber.

Long-legged hunters
It’s probably not well-known that some spiders hunt by the sea. Locally, Idioctis littoralis is an ambusher that hides in a waterproof tunnel dug into mud lobster mounds. Desis martensi is a low tide hunter with large jaws, shown above with what appears to be a peeled shrimp. In the vicinity of freshwater habitats can be found some species of wolf spiders (family Lycosidae) as well as the fishing spider Thalassius albocinctus (family Pisauridae, pictured below), which uses its legs as bait to attract small fish. It then plunges into the water and drags the prey ashore.

Lynx spiders (family Oxyopidae) are day hunters with a characteristic hexagonal eye arrangement. The family Ctenidae or wandering spiders are nocturnal hunters with long legs. Indoors, some local households may have the dubious pleasure of harbouring a few huntsman spiders (family Sparassidae), especially in kitchens and bathrooms. Apart from houses, different species of huntsman spiders prowl specific habitats from plant foliage to tree trunks. Two-tailed spiders (family Hersiliidae) are hunters on tree trunks that have two long spinnerets which extrude silk that is used to wrap and immobilise prey.

A major and very successful family of spiders are the jumping spiders – Salticidae, which number at least 4,000 species. In this group are the black and white fighting spiders (Thiania bhamoensis) beloved of schoolboy gamers a generation ago, as well as the several mottled grey housefly catchers that are at home in human dwellings. Jumping spiders are day hunters with excellent eyesight, thanks to an eye arrangement (see picture below) that grants almost 360 degree vision and two pairs of frontal eyes that act as stereoscopic wide angle and high resolution telephoto lens respectively. Mr Koh describes their stalk and pounce hunting technique as cat-like.

Many species of jumping spiders are sexually dimorphic. Their visual acuity has resulted in elaborate recognition and courtship rituals that employ specific paedipalp signals as well as the development of remarkable head patterns and decorative features that look like beards and funky hair-dos.

One tribe of jumpers have become mimics of ants, with a first pair of legs that are waved about like ant antennae. They even have an abdomen that is ‘tightened’ in the middle to mimic the belt between an ant’s thorax and abdomen. Spots that look like the eyes of ants also feature on the cephalothorax. Ant mimicry may discourage predators, for instance, some spiders boast with long jaws that mimic the biting mandibles of ant soldier classes. Myrmarachne plataleoides is one species that mimics the dreaded tree-nesting kerengga, which my duck assiduously avoids thanks to certain unhappy past encounters. (Going a step further, the crab spider Amyciaea lineatipes both mimics – with egg spots on its abdomen – as well as preys on kerengga ants by leaping on the victim and dangling it helplessly in mid-air).

Another group of jumpers are the Portia spiders, which prey on web-spinning spiders. They are debris-like creatures that make use of their prey’s own prey-detecting mechanism, plucking the web lines to draw out the host spider. Some researchers have called Portias eight-legged cats for their seeming ability to anticipate the movements of their prey and dexterity in manoeuvring to a position that offers an optimum attack point, even taking detours that indicate a certain level of ingenuity that one would not associate with a creature less than a centimetre in length.

Another specious family is Thomsidae – the crab spiders. Many species in this group are well-camouflaged on specific flowers, on which they sit motionless and wait for prey. With bees a common target, they have appropriately fast-acting venom. Two genera of crab spiders are obligate pitcher plant dwellers that intercept insects which enter the pitcher. They have a thick layer of hair on the body that repels the pitcher plant fluid. 

Spitting spiders (family Scytodidae) are nocturnal hunters with an enlarged cephalothorax housing special glands that pump out glue and venom onto prey. And bizarrely, Ordgarius or bolas spiders swing a line with a sticky wad of silk at the end. The spiders give off the scent of female moth pheromones which attract lusty males that are whacked with the line and dragged up for consumption.

Not found in Singapore but of interest to arachnophiles are the assassin spiders (family Mimetidae) of Australia, South Africa and Madagascar. Just last week, researchers from the California Academy of Sciences announced the discovery of nine new species from Madagascar. Assassin spiders are tiny (2 mm) but have oversized fangs (and incredibly long necks) that are used to stab other spiders. 

What’s the use of spiders, beside their obvious utility in consuming flies, mosquitoes, butterflies and other bugs of ill ilk? Mr Koh suggests that the chemistry of spider webs may offer a template for biosteel with high tensile strength as well as material for bullet-proof vests and surgical thread. But while the strength of spider silk is well-known, the trouble is getting sufficient quantities beyond the minute volume needed for a little orb. Some scientists have managed to genetically engineer goats that produce spider silk proteins in their milk. All very well and good, but the next step is how one separates the silk from the slurry....

Links for further reading
Journal of Arachnology online.

Comments

[Hai~Ren]

Very, very nice and comprehensive.

[YC Wee]

I enjoyed Joesph's first talk at NParks Biodiversity Centyre. Now reading through your account, the talk vividly comes back to me. Great write up, Budak.

[pepe]

WHAT IT DOOKIE SPIDERS!!

Kool