Love 'em or hate 'em, you've got to admit that spiders are
some pretty impressive, well-equipped animals. They have a
top-notch sensor array, a built-in construction set, a heavily
armored body and a highly effective venom-injection system.
How many other creatures can claim that?
These remarkable adaptations have made spiders some of the
most successful carnivores in history. In their 400 million
years of existence, they've spread over every continent and
mastered nearly every environment on Earth. Today, there are
about 40,000 known spider species, and potentially thousands
more we haven't discovered yet. This is pretty astounding when
you consider that there are only about 4,000 different species
in the entire mammal kingdom.
In this edition of HowStuffWorks,
we'll find out how these unique animals spin webs, attack prey
and walk straight up walls. We'll also look at some
particularly interesting spider species, including spiders
that swim, spiders that jump from branch to branch and spiders
that can kill a person.
Spider Basics Spiders look a lot like
insects, but they're actually part of an entirely different
class of animals, called Arachnida. Spiders make
up the order Araneae within this class, which also
includes mites, ticks and scorpions.
Click on the buttons at the bottom of the image to
see how spiders fit into animal
While spiders vary considerably in size, shape and
behavior, nearly all species share a basic set of
They have eight legs, made up of seven segments each.
They feed primarily on insects.
They can inject venom into their prey.
They can produce silk.
They have a pair of small appendages on the head, called
Their bodies are divided into two sections, the
cephalothorax and the abdomen, joined by the
thin pedicel. The cephalothorax -- a fused head and
-- distinguishes spiders from insects, which have a separate
head, thorax and abdomen.
Spiders have the same basic bodily systems as people, but
they don't work in the same way and they're arranged
differently in the body. The cephalothorax contains the brain,
stomach, eyes and mouth, and the abdomen contains the heart,
digestive tract, reproductive organs and lungs.
Spiders have two different types of respiratory systems --
trachea and book lungs (most species have both,
but some have one or the other). Compared to human lungs,
these respiratory systems are incredibly simple. Trachea are
just long tubes that run from a slit in the exoskeleton
through the body. Air flows in, oxygen diffuses
into the blood and carbon dioxide diffuses into the air. The
spider's normal movement provides all of the necessary energy
to push air in and out. Book lungs are a series of very thin,
leaf-like structures (like the pages in a book). The inside of
each leaf is filled with blood, and the outside is exposed to
air. As in the trachea, the book lungs exchange oxygen and
carbon dioxide through diffusion.
The spider's blood, called hemolymph, circulates
oxygen, nutrients and hormones to the different organs in the
body. Unlike humans, spiders have an open circulatory
system. The spider's simple heart -- a tube
surrounded by a muscle, with a one-way valve on each end --
pumps blood into the body cavity, all around the spider's
organs. Organs get oxygen because they're soaking in blood.
One of the most amazing things about spiders is how much
they can accomplish with such a small brain. The spider's
central nervous system is made up of two relatively simple
ganglia, or nerve cell clusters, connected to nerves
leading to the spider's various muscles and sensory systems.
The simple instructions encoded in these nerve cells give
spiders all the information they need to undertake complex
tasks, such as building webs and attacking prey. Some species
even exhibit learning behavior. If something isn't working --
a web in a particular spot, for example -- the spider will
give up the activity and try something new.
Photo courtesy MicroAngela Electron microscope image of one of a
most spiders have multiple pairs of eyes, vision is a
secondary sense in the vast majority of species. Most spiders
primarily interact with the world through tactile sensation.
They are covered in highly sensitive hairs that pick up even
low-level vibrations in whatever the spider is standing on
(the ground, floor, leaf or web, for example). Many spiders
have additional hairs, called trichobothria, which pick
up vibrations in the air (sound). Most spiders also have a
sense of taste and smell, which play a role in feeding and
Many spiders have special adaptations that allow them to
walk easily along relatively smooth or vertical surfaces. The
end of each leg is covered with thick brushes of hair, and the
end of each hair is covered in tiny microscopic "feet." All
the tiny feet grip the small bumps on whatever the spider is
walking on, allowing the spider to move easily over most
terrain (the smooth surface of the bathtub being a notable
exception for many species).
Spiders have thick brushes of hair, each
ending in tiny microscopic feet that let it walk up
arachniphobes, large, hairy tarantulas are as bad
as it gets. But in fact, tarantulas, a loose term for
large, hairy spiders in the mygalomorphae
suborder, are relatively harmless to humans. Most spend
their time in the brush or in holes, waiting for their
prey to wander along. They do not spin webs, and they
aren't venomous to humans. Their fangs can puncture the
skin, however, and some species can shoot barbed hairs
from their back.
Spider Structure and Growth Like people,
spiders move by contracting muscles attached to a
skeleton. But Instead of an internal skeleton covered
in flesh, spiders have have an exoskeleton -- a stiff
support structure on the outside of the body. Exoskeleton
segments are connected together with joints so the spider can
move them back and forth.
Muscles attached on the inside of the exoskeleton contract
to move the legs inward, but spiders don't have any muscles to
extend the legs back out again. Instead, they have to force
bodily fluids (mainly blood) into the legs to push them
outward. If a spider loses too much body water, it can't
generate the necessary hydraulic
pressure to push its legs out. This is why you sometimes see
spiders on their backs with their legs curled up.
The exoskeleton is made of several layers of
cuticle, a composite material containing various
and chitin, a long-chain polysaccharide (sugar).
The chitin and protein molecules are arranged in long chains,
in successive layers, like the grain in plywood.
This structure makes cuticle extremely strong, as well as
highly effective at keeping the spider from drying out, but
the material does have one serious drawback. While it's
flexible enough for movement, it can't expand like human bones
and tissue -- in other words, it can't grow. In order
to increase its size, the spider has to form a new, larger
cuticle exoskeleton and shed its old one (this is called
Molting occurs frequently when a spider is young, and some
spiders may continue to molt throughout their life. At the
appropriate time, hormones tell the spider's body to absorb
some of the lower cuticle layer in the exoskeleton and begin
secreting cuticle material to form the new exoskeleton. The
new exoskeleton is typically folded to some extent, so
it can expand once the spider sheds the older one. The spider
also secretes a molting fluid between the old
exoskeleton and the new one. Once the new exoskeleton is
finished, the spider absorbs the molting fluid. This creates a
gap between the two exoskeletons, which makes it easier to
Micaria romana: While these spiders have
eight legs and two body segments, like all other
spiders, they superficially resemble their primary prey,
To shed the old exoskeleton, the spider has to bust out
from the inside. It increases its heart rate to pump a lot of
hemolymph (the spider's blood) from the abdomen into the
cephalothorax. The pressure expands the cephalothorax, which
pushes on the old exoskeleton until it cracks. The spider
flexes its muscles until the old exoskeleton falls away.
Typically, the spider does most of its growing immediately
after losing the old exoskeleton, while the new exoskeleton is
highly flexible. The new exoskeleton is also very soft in this
stage, making the spider particularly vulnerable to attack.
Many species will lower themselves on a silk line during the
molting process, so they're out of reach of predators while
the cuticle material hardens.
Jumping spiders, as you might expect,
have the ability to jump great distances -- as far as 50
times their own length. They don't have particularly
strong muscles in their legs; they actually spring
forward using hydraulic pressure. A powerful
muscle in the cephalothorax squeezes fluids from the
body into the legs to make them expand.
With more than 5,000 species around the world,
jumping spiders are one of the more common spider
varieties around. They're characterized by large eyes,
which help them spot potential prey at a good distance.
In contrast to web-spinning spiders, most jumping
spiders hunt sort of like cats, stalking their prey and
then springing on them at high speed.
Spinning Silk The main thing that
distinguishes spiders from the rest of the animal kingdom is
their ability to spin silk, an extremely strong fiber. A few
insects produce similar material (silk worms, for example),
but nothing comes close to the spinning capabilities of
spiders. Most species build their entire lives around this
Scientists don't know exactly how spiders form silk, but
they do have a basic idea of the spinning process. Spiders
have special glands that secrete silk proteins (made up of
chains of amino acids), which are dissolved in a water-based
solution. The spider pushes the liquid solution through long
ducts, leading to microscopic spigots on the spider's
spinnerets. Spiders typically have two or three
spinneret pairs, located at the rear of the abdomen.
Photo courtesy MicroAngela Electron microscope image of a spider's silk
Each spigot has a valve that controls the thickness and
speed of the extruded material. As the spigots pull the
fibroin protein molecules out of the ducts and extrude
them into the air, the molecules are stretched out and linked
together to form long strands. The spinnerets wind these
strands together to form the sturdy silk fiber.
Photo courtesy Ed
bruennich, an orb web weaver, spinning
spiders have multiple silk glands, which secrete different
types of silk material optimized for different purposes. By
winding different silk varieties together in varying
proportions, spiders can form a wide range of fiber material.
Spiders can also vary fiber consistency by adjusting the
spigots to form smaller or larger strands.
Some silk fibers have multiple layers -- for example, an
inner core surrounded by an outer tube. Silk can also be
coated with various substances suited for different purposes.
Spiders might coat fiber in a sticky substance, for example,
or a water-proof material.
Spider silk is incredibly strong and flexible. Some
varieties are five times as strong as an equal mass of steel, and
twice as strong as an equal mass of Kevlar.
This has attracted the attention of scientists in a number of
fields, but up until recently, humans haven't been able to get
much out of this natural resource. It's simply too hard to
extract silk from spiders, and each spider has only a small
amount of it.
This may change in the near future. Researchers at a
company called Nexia
Biotechnologies have genetically modified goats using
silk-producing genes from spiders. The hope is that a small
number of goats will be able to produce a large amount of silk
material in their milk. Engineers will be able to put this
material to work in aircraft, bullet-proof
vests and artificial limbs, among other things (check out
page for more information).
Actually forming silk material is only the first half of a
silk-spinning process. In the next section, we'll find out how
spiders put silk fiber to work.
Using Silk While all spider species spin
silk, they do a number of different things with the fiber once
they produce it. Not all species spin webs, and many don't use
their silk in hunting at all. Just like human building
materials, spider silk has a wide range of applications.
Photo courtesy Ed
Nieuwenhuys Purse web
spiders coat a hole in the ground with silk and lie in
wait for passing
One of the most common silk uses is the dragline. As
some spiders move from place to place, they lay out a thin,
dry thread behind them. Just like a mountain climber, the
spider uses the thread as a safety line. If it gets in
trouble, it can quickly backtrack on the line to get to
Another common silk-use is nursery building. In most
species, female spiders will spin a thick, protective cocoon
for their developing eggs and sometimes the spiderlings
once they've hatched. Some species will leave the cocoon
unattended while the young spiders develop, and others, such
as wolf spiders, will carry the cocoons around with them.
The most familiar use of silk, of course, is web building.
Web structure varies a good deal from species to species. Some
spiders build totally disorganized cob webs, some form long
funnels out of silk sheets and some work as a colony to form
huge masses of silk sheets around plant life. Some spiders,
such as the net-casting spider, will form a small web
between their legs and quickly wrap up any insect that gets
The best-known web is the orb web, the intricate
design spun by most garden spiders. This web is one of
the most remarkable structures in nature, and its construction
is among the most incredible animal activities you'll ever
see. We'll examine this process in the next section.
spiders, or Argyroneta aquatica, are one of the most
amazing animals on the planet. These spiders breathe air
just like other arthropods, but they live most of their
lives underwater. The trick is the spider's diving
To build the diving bell, the spider first forms a
web platform underwater, typically connected to a plant.
Then it carries air bubbles down under the web, on the
underside of its body. It releases the air bubbles under
the web to gradually build a stable underwater air
pocket -- the web keeps the air bubbles from rising to
the surface. The spider ventures out to catch fish,
tadpoles and other pond animals, and brings the prey
back to the diving bell to eat it. It also mates, lays
eggs and raises spiderlings in the bell.
The Orb Web An orb-spinning spider puts its
elegant traps together pretty quickly, proceeding easily from
step to step according to the instruction manual preprogrammed
into its brain. The diagram below shows the major steps.
Every web begins with a single thread, which forms the
basis of the rest of the structure. To establish this
bridge, the spider climbs to a suitable starting point
(up a tree branch, for example) and releases a length of
thread into the wind. With any luck, the free end of the
thread will catch onto another branch. If the spider feels
that the thread has caught onto something, it cinches up the
silk and attaches the thread to the starting point.
It walks across the thread, releasing a looser thread below
the first one. It attaches this thread on both ends and climbs
to its center. The looser strand sags downward, forming a V
shape. The spider lowers itself from this point, to form a
Y-shape. This forms the core support structure of the web.
The spider easily grips the thin threads with special
serrated claws, a smooth hook and a series of barbed hairs on
the end of its legs. As it walks along the initial structural
threads, it lays more frame threads between various
anchor points. Then it starts laying out radius threads
from the center of the web to the frames. The spider does not
coat the frame and radius threads with sticky material, since
it needs to walk across them to get around the web.
After building all the radius threads, the spider lays more
non-stick silk to form an auxiliary spiral, extending
from the center of the web to the outer edge of the web. The
spider then spirals in on the web, laying out sticky thread
and using the auxiliary spiral as a reference. The spider eats
up the auxiliary spiral as it lays out the sticky spiral,
resulting in a web with non-sticky radius threads, for getting
around, and a sticky spiral for catching bugs.
The spider sits in the middle of its web, monitoring the
radius threads for vibrations. If an insect gets caught in any
part of the web, the spider will feel the motion through the
radius threads and make its way to the vibration source. In
this way, the web extends the spider's sensory system over a
much wider area. The spider might also leave the web, to
retreat to a separate nest, while monitoring the web via a
connected signal line.
Web-spinning spiders have an innate ability to tell the
difference between vibrations from insect prey and vibrations
from other sources (a leaf falling into the web, for example).
Many species can also distinguish the characteristic
vibrations of dangerous insects, such as wasps, from their
When the orb web has deteriorated and is no longer useful,
many spider species will destroy it, eating up all the threads
so it can recycle the raw silk material. Spiders may leave the
heavy bridge thread so that they can easily rebuild the web at
a later point.
In the next section, we'll find out what spiders do when
they catch their prey, whether in a web or on the ground.
Feeding Spiders are predators, above all
else, so hunting and killing is where they really shine. In
the bug world, spiders are fairly fearsome animals -- they're
the tiny equivalent of wolves, lions or sharks.
Different spiders employ different hunting strategies. As
we saw in the last section, some species build intricate webs
to ensnare passing insects. Other spiders, such as the various
wolf spiders, seek their prey out on the ground. Trap
door spiders dig holes, cover them up with dirt doors hinged
with spider silk, and lie in wait for passing prey. Similarly,
some spiders hide inside flowers to catch feeding insects by
Photo courtesy Ed
Nieuwenhuys Two examples
of crab spiders camouflaging
themselves to catch insects off guard: Misumena vatia,
the yellow crab spider above, can change its coloration
over a couple of days to match flowers and other
Whatever their hunting strategy, the vast majority of
spiders follow the same basic killing and feeding procedure.
The spider's primary weapon is its chelicerae, a pair
of jointed jaws in front of the mouth. Each jaw has two major
parts: the basal segment, the bulk of the jaw, and the
sharp fang housed inside of it.
Normally, the fang is retracted inside the basal segment.
When the spider catches its prey, it swings the fangs out into
the animal's body. The fangs work something like hypodermic
needles. They have a small hole in the tip and a hollow duct
inside. The duct leads to the venom gland, either inside the
basal segment or farther back in the cephalothorax. When the
spider pierces its prey with the fang, it squeezes out the
venom, injecting the animal with enough neurotoxin to paralyze
or kill. This makes it safe for the spider to feed on its
prey, without the risk of a struggle.
In the mygalomorph spider suborder, which includes
the various tarantulas, the chelicerae are positioned so that
the fangs swing forward into the prey, like an axe. In the
dominant araneomorph suborder, the chelicerae swing in
toward each other, like a pincer. For the mygalomorph system
to work effectively, the prey has to be on ground or another
solid surface -- the spider has to sandwich the prey between
something else and the fangs. The araneomorph system works
whether or not the prey is on solid ground -- the chelicerae
simply push against each other.
After paralyzing its prey, some spiders may wrap it up in
silk to make it easier to transport back to the nest. Some
species actually cover the prey in silk before injecting the
venom, making it easier to attack. A female spider may carry
wrapped prey back to its young spiderlings, and a male may
bring the wrapped prey to a female as a courtship gift.
Most spiders don't eat their prey whole; instead, they
expel digestive enzymes onto or into the animal to
liquefy it. Some spiders use their fangs to inject the
digestive fluid directly into the animal. This sort of spider
liquefies the animal's insides, leaving the exoskeleton more
or less intact. Then it sucks the liquefied remains into its
stomach through hairs on its chelicerae and mouth, which act
as a filter. Other species chew their prey up with serrated
"teeth" on the chelicerae before vomiting digestive fluid on
the body and sucking in the liquid remains.
A very small
percentage of spiders are venomous to humans.
Venomous means the spiders may inject humans with
dangerous poison (generally called venom in this
context), whereas poisonous means the spiders
would be harmful if eaten. The effects of spider venom
vary depending on the species, age and sex of the
spider, and on the age and health of the bitten person.
Neurotoxins in the venom may affect the human
nervous system, causing dizziness, difficulty breathing,
nausea, blurred vision
rigidity, among other things. The venom may also kill
tissue surrounding the bite. Generally, if a bite victim
gets medical attention, they'll suffer minimal damage.
If left untreated, a spider bite can kill, though this
is very rare.
In North America, the most famous dangerous spiders
are the black widow and the brown recluse.
Both spiders can potentially kill, but the danger is
slight for healthy adults. The spiders are both
reclusive by nature, and will only bite if they feel
threatened. Check out this
site for more information on dangerous spiders.
Spider Sex The male spider's primary
objective in life is to impregnate one or more female spiders
before other males can. As it turns out, this is no easy task
in most species.
The first obstacle is actually finding a female spider.
Most spider species are completely solitary animals, meaning
they live and feed on their own, and they are generally spread
out over a wide area, making an available female relatively
scarce. The male spider has the daunting task of tracking down
a sexually mature, receptive female in the area before other
males can get there.
Photo courtesy Ed
Nieuwenhuys Males and
females of the same spider species often look totally
different. Above, a tiny male golden orb spider (follow
the yellow arrow) climbs on a giant female. Below is a
female (green) and male (brown) of the crab spider
In most species, the female makes it easier on the males by
"advertising" herself with pheromones, communicative
chemicals. Many female ground spiders will secrete a pheromone
on their drag line, the silk thread they leave trailing
behind them. When males of the same species come across the
dragline, they smell the pheromone with the chemical sensors
on their front legs and follow the dragline to the female.
Web-spinning females may release pheromones directly into
the air or coat their webs with pheromones, to make a natural
"chemical antenna." Males may also stake out developing,
sexually immature female spiders, so they can be the first to
mate after the spider's final molt.
Once the male locates a female, it has to contend with any
other males in the area. In species where the female spins a
pheromone-coated web, the male's first order of business is to
destroy the web to cut off the signal attracting any other
males. If other males are present, the spiders in most species
will fight it out for the right to copulate with the female.
After taking care of any other male contenders, the
spider's next task is to deal with the female spider itself.
Male spiders are generally much smaller than females in their
species, making them easy prey. The male has to signal to the
female that it is a spider of the same species, not food or a
potential predator, and that it intends to copulate. This is
Photo courtesy Ed
Nieuwenhuys An Araneus
diadematus male approaches a
Courtship varies considerably among different species. Many
web-building spiders will use vibration as a means of
courtship communication. The male may strum a unique signal on
a thread connected to the female's web to identify itself and
get across its intentions. Many spiders with better eyesight,
such as various wolf spiders and jumping spiders, will "dance"
to court the female.
Once the female recognizes the male's courtship behavior,
she will position herself for sex, signaling to the male that
she is receptive, or she will make it clear that she is not
receptive (by shaking her web, for example, or just crawling
away). If the male is desperate to mate, because all the
females in the area will soon lay their eggs, he may proceed
anyway, with full understanding that the female might kill
Both the male and female reproductive organs are at the
rear of the abdomen, but spiders don't mate by coupling these
organs. Instead, the male deposits some sperm onto a small web
and picks it up on the end of his pedipalps. When the
female is in position, the male deposits the sperm in the
female's genital opening. The female stores the sperm in
receptacles near the ovaries. When she is ready to lay her
eggs, months down the road in some species, she uses the sperm
to fertilize them. Some spiders may lay hundreds, even
thousands of eggs in one shot.
As we saw earlier, some spider species will encase their
eggs in a silken pouch and abandon them, and others will stay
with them until the babies hatch. Many wolf spider species
carry their hatched spiderlings around on their back until the
spiders are mature enough to take care of themselves.
Photo courtesy Ed
Nieuwenhuys This crab
spider keeps its spiderlings safe in a silk
The spiderlings continue molting, growing larger until they
reach sexual maturity. Then the entire cycle begins again --
males seek out females, and females lay eggs. Most spiders
have a fairly short lifespan, ranging from a few months to a
couple of years. But some spiders, such as various female
tarantulas, can live as long as 20 years. These spiders lay
eggs many times throughout their life, and they molt annually,
mainly to replace damaged body parts.
For much more information about spiders and other
interesting animals, check out the links on the next page.