|
And what role do spiders play in Malaysia's ecology?
Spiders are arthropods, which also include various other "critters"
such as insects, centipedes and millipedes.
Nonetheless, in evolutionary terms, like humans the arthropods
have been extremely successful, and form one of the earth's great
families. There are a huge number of arthropod species distributed
all over the world, and many can be found at close quarters with
human beings. To date, over 30,000 species of spiders alone have
been recorded.
The venom of 99.8% of the world's known spider species is harmless
to humans. "Apart from a small number that can cause pain and
itching in allergic people, there are no more than five species that
are a genuine danger to human life,
As for the most poisonous of all spiders is the black widow
Macrothele taiwanensis, a funnel-web spider whose venom also
contains a dangerous nerve toxin, lives mainly underground and is
rarely seen.
The spiders that like to "participate" in the noisy life of
humankind represent only a tiny proportion of the enormous number of
spider species. Most spiders stick to the wild.
Forest track, spider kingdom
Lifting up a damp stone, you may find different species
of spider crouching beneath it. Lycosa coelestis, a type of wolf
spider, scuttles off in a flash, carrying its egg sac on its back.
The young of a Pardosa takahashii-another wolf spider-who are riding
on her body, all fall off in a panic, but fortunately they have
inherited a handy turn of speed, and in an instant they climb back
on board, whereupon their mother carries them off into the
undergrowth.
Amid the foliage, Leucauge magnifica, a type of orb-web spider with
a white underbelly and a green back, is quite unperturbed by the
presence of humans. With its long legs stretched out like an
octopus, it sits resolutely on its web built on palm grass,
patiently waiting for a meal to arrive. On a nearby tree branch
above head height, a light-green Oxytate striatipes, a type of crab
spider, has attached her egg sac to the underside of a leaf. The
female spider remains crouching over the eggs, guarding them night
and day.
On a moss-covered rock face beside the path, a dangerously poisonous
Macrothele taiwanensis occupies a small hole, across the mouth of
which she has stretched a web smaller than the palm of a hand. Lying
in wait in the mouth of the hole, when a passing insect disturbs the
web, she grabs it with lightning speed and pulls it inside.
Hairy legs and silk
Both spiders and insects are arthropods, but they have very
different body patterns. Insects have six legs and a distinct head,
thorax and abdomen, but spiders' bodies are divided into only a "cephalothorax"
(combined head and thorax) and abdomen, along with eight hairy legs
that may be many times longer than the size of their bodies. They
are very unique in form, and have some remarkable physical
attributes. Spiders have fully developed digestive, circulatory,
respiratory and nervous systems. Their legs have seven segments
each, with claw-like end segments used for gripping and climbing.
The fourth pair of legs is usually used for handling the spiders'
silk. The fine hairs of varying lengths on their legs serve as taste
and smell organs, and are also used for hearing and for judging
orientation. They are highly sensitive to air flows on the ground,
on water or across the spider's web, and are thus indispensable
survival tools for hunting and for escaping predators.
Apart from their all-purpose hairy legs, the way spiders spin silk
and weave webs is also one of the wonders of nature. Spider silk is
a composite mixture of proteins. It is mostly white or translucent.
In the movie, Spiderman shoots out silk from his wrists, but in real
life, spiders' spinning methods are more elaborate and complex, and
they secrete different silks according to different needs. Numerous
silk glands in their abdomens link into silk ducts that open into
spinners. Some of the threads they spin are sticky to catch prey,
while others are used to build nests; and the silks they use to make
egg sacs or to wrap and immobilize prey are also different.
Spiders all have remarkable spinning abilities, but they can be
divided into two categories: web-building and non-web-building
species. Web-building spiders like to build a web in a fixed
position to catch food. If not disturbed they can spend the whole
year on their web, and their temperament is calm and composed. As
for those spiders that are not in the habit of building webs, but
which roam freely on the ground, on trees, on plants and on walls,
they always reel out a "dragline" behind them, which they secure
every so often to the surface they are on. When they jump from a
high place or are blown by the wind, this dragline provides the best
of safety lines. The jumping spiders that are often seen in human
habitations, and the much maligned laya, are both non-web-builders
of no fixed abode.
Up into the treetops
Web-building spiders' ability to catch food by building webs is
closely bound up with the evolution of their main prey-insects. As
insects developed from crawling on the ground, to climbing up into
the branches and leaves of trees, and finally flying through the
air, spiders' skills in using their silken webs also advanced by
leaps and bounds. Spiders were originally ground-living animals, but
they later began to stretch their webs high in the air, and to
produce all kinds of carefully crafted webs in response to different
insects' habits and characteristics.
Spiders have an average lifespan of two to three years, and only
breed once, generally in spring or summer. At this time, the male
spiders leave their webs in search of partners, and because in most
species the female spiders are larger than the males, the males do
indeed have to be extremely cautious. "They have to first perform a
mating ritual, to avoid being mistaken for food themselves," says
Chen Shyh-hwang.
But do spider "brides" really make a meal of their grooms after
mating? Chen explains that in Taiwan, apart from the sheet-web
spiders (family Linyphiidae), most spider couples are able to walk
away from their nuptial encounters unscathed. "In fact, spiders
interact in a great variety of ways. For instance, Philoponella
prominens, a type of feather-legged spider common in central and
southern Taiwan, even lives in mixed social groups of both sexes;
they build their webs alongside each other, and coexist peacefully,"
says Chen. Biological interactions are complex mechanisms. When male
spiders mate they are generally at the end of their natural lifespan
anyway, so even in the cases where the female spiders do eat the
males, it is not simply a case of ruthless treachery as people may
imagine.
After mating, the female spider usually looks after her eggs alone
until they hatch. The mother spider wraps the eggs tightly in a
waterproof egg sac made up of many layers of silk, and when the
spiderlings hatch out they have to break their way out, like
silkworms emerging from a cocoon. Depending on the species, spiders
may lay as few as a dozen eggs, or as many as 1000. The eggs take
around a month to hatch, and it is then another week before the
young spiders can leave their mothers and live independently.
At least 1000 species
As biologists' understanding of spiders has advanced, in recent
years both spider silk and spider venom have become hot topics of
research, in the hope of developing new industrial products.
Researchers in Thailand are working hard to develop bullet-proof
vests using spider silk; and based on analysis of the chemical
structure of spider venom, scientists in Germany have isolated a
protein which may be useful for treating heart disease.
In Taiwan, spider research started late. A decade or so ago when
Chen Shyh-hwang was revising the species list of Taiwanese spiders,
he discovered that of the more than 300 species already described at
the time, the bulk had been recorded by early adventurers, and of
the 65 species newly recorded in the previous 20 years, almost all
had been found in surveys by Japanese scientists.
Taiwan's spiders fall into over 40 families. Relatively large
numbers of species have been recorded in the families Psechridae (psechrid
spiders), Oxyopidae (lynx spiders) and Theridiosomatidae (ray
spiders), but in other families such as Lycosidae (wolf spiders),
Salticidae (jumping spiders), and Heteropodidae (brown huntsman
spiders-which include the familiar laya), only three to four species
have been formally described in each. Chen Shyh-hwang's own
experience of field surveys shows that if one only takes the time to
study any of these families in more depth, one can find two to three
times as many species as were already known. Furthermore, due to the
isolation of habitats that results from Taiwan's being an island
with mountainous topography and great geographical diversity, the
proportion of endemic (unique) species among Taiwanese spiders is as
high as 10%.
"In just one spider survey zone on Mt. Chiuchiu in Nantou County, we
discovered 200 species," says Chen Shyh-hwang. He states that the
number of spider species thus far discovered in Taiwan stands at
between eight and nine hundred, but over half of these have not yet
been properly identified, so the next major task to be tackled is to
compare the specimens of newly discovered spiders, classify and name
them, and include them in illustrated field guides.
"For instance, the sheet-web spiders are a big family, but at
present there's no-one in Taiwan who knows how to identify them."
Chen freely admits that many spiders are small and very similar in
appearance, so that their identification takes ample literature,
large collections of specimens, and great powers of discernment.
Hence spider research in Taiwan is still in its early stages. In
Chen's own laboratory alone there are 100 species of spider awaiting
classification.
Nature takes care of itself
Chen Shyh-hwang has already published many papers describing new
species and reporting species newly recorded in Taiwan. Apart from
his efforts to fill in the gaps in Taiwan's spider map, in
coordination with other zoologists he has set up sampling plots to
observe changes in spider populations on Mt. Chiuchiu in Nantou,
which was denuded of large areas of vegetation by the earthquake of
21 September 1999.
Although the observations are still in progress, Chen has discovered
that as the denuded areas are recolonized, first by herbaceous
plants and shrubs and then by fast-growing broadleaved trees, the
spider populations are also quickly recovering. "The largest number
of species, and the largest populations, are of spiders that prefer
a grassland habitat. But in the areas where there is still no
vegetation cover, and there is no place to hide except under stones,
the numbers are lowest, due to the limited habitat."
Spiders' diversity is underlined by their ability to survive in very
different natural environments.
The destruction of wetlands may have a long-term impact on
damp-loving spiders like Leucauge magnifica that build their webs by
rivers or ditches. But overall, spiders are small and reproduce
prolifically; as long as they have fixed locations where they can
build their webs.
Perhaps what we humans can do for spiders is to put away our
ignorance and fear, open our minds, and-just as we admired the feats
of Spiderman on the movie screen-appreciate the variety and vitality
of spiders in nature.
p.086
In nature, no two spider webs are exactly alike. The brightly
colored Argiope aetheroides-a type of golden orb-web spider-adds a
cross-shaped "stabilizer" to its web which in fact reflects
ultraviolet light to attract insects into the web, like moths to a
flame.
p.089
Chrysso trimaculata, a golden comb-footed spider, carries her egg
sac with her wherever she goes.
p.089
Rhene atrata, a jumping spider commonly seen in Taiwan's lowlands,
is a non-web-builder that roams where it pleases.
p.090
A female Chrysso trimaculata and her spiderlings. Different spider
species produce anything from a dozen to a thousand offspring at one
time.
|
OECOBIIDAE Tiny House
Dwellers / Wall Spider (Oecobiidae) - Oecobius
