| The exoskeleton of spiders
doesn't grow with them, so all spiders have to molt (shed their
skin) to grow. They only molt when young, and cease to molt once
become an adult.
Spider Molting
Like people, spiders move by contracting muscles attached
to a skeleton. But instead of an internal skeleton covered
in flesh, spiders 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 proteins 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).
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 separate them.
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. |
