|We've learned that all arthropods have a tough
outer covering called an exoskeleton. However,
we have also learned that some arthropods, such
as "honey-pot" ants and ticks actually expand as
they collect honey or blood in their body. Is
the exoskeleton able to expand? Do these
organisms have a different type of exoskeleton
that other arthropods?
It sounds like you're really thinking about what
you have learned. That's great. It's true that
some arthropods are able to expand. As you
probably know, arthropods get their names from
having movable joints. These are places where the
hard parts of the exoskeleton are joined together
by soft tissue.
Think about wearing a
suit of armor. If the whole suit were metal, you
couldn't move your arms and legs. At the point
where, your elbow, knee or other joint is, you
have to allow for movement. Arthropods have soft
tissue there. It's hard to see this on insects and
spiders, but see if your teacher can help you find
a lobster or crab exoskeleton. You can also see
this sort of design in the hard leg protectors
that baseball catchers wear. There are hard plates
attached to a softer cloth pad.
honey pot ant expands, the soft parts get a lot
bigger, but the hard parts stay the same size. If
you have Internet access, check out the
photographs of honey pot ants at these sites:Honey_
You will see that there are one or more plates
stuck onto what the "blown up" abdomen of the
ants. That is the hard exoskeleton plate. It
didn't get bigger at all. Just the soft parts
expanded. It is now useless at protecting the ant,
but the full ants just stay inside the nest where
they are fairly safe.
By the way, their
scientific name is Camponotus inflatus. Why do
you think they were named that?
exoskeletons don't stretch, how do arthropods grow?
If arthropods had none of the thin cuticle, then
they would not be able to move. Yes, it is thin
and flexible at least for joints, and in those
animals, it is thin in other places as well.The
exoskeletons of arthropods are made of a polymer
sugar called chitin. It's the same stuff that
makes up the cuticles of annelids (segmented
worms), as well as the cell walls of fungi.
Particularly thick layers of chitin are stiffer
than thinner layers, and arthropods have made
their chitin cuticles thick enough that it
functions as a skeleton. Marine arthropods further
fortify their cuticles with the mineral calcite
(the principal component of cement), to make their
cuticles even harder. Along the arthropod body,
there are locations where the stiff, thick cuticle
becomes inpocketed and very thin, and thus much
more flexible- these are the joints. The
arthropods with flexible exoskeletons that you
just named have these thin,flexible, portions of
their cuticles able to puff out,connecting their
stiffer portions with a thinner,balloon-like portion.
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