A. L. and her friend, M. S. asked:
Bugman,
Just yesterday, I witnessed a spider fall from a tree onto concrete.
I know that if I fell the same distance, proportionally, I would definitely
die. However, the spider I witnessed simply crawled away. Could you
please explain this. Thank you.
and we said:
COOL! What a
very cool question! There's a couple of interesting things going on here... surface-area-to-volume-ratio
and chitin.
The surface area of an object that gets impacted upon by air pressure is going
to effect the rate at which it falls. Just imagine an open piece of paper vs. a
balled-up piece. Which falls faster? It's sort of like there's air pushing up
from the ground. Think about the difference between a parachute that does open
(lots for the air to push against) vs. a parachute that doesn't open - the air
just whooshes right by while the parachutist crashes to the ground.
An object's volume is going to effect how much it weighs and how fast it
falls... but if that volume is spread out over a large surface area, the
resistance of the air pushing up will slow it down. So a spider that has a
relatively small volume compared to large surface area, will fall slowly enough
to not slam like we would - we're big, clumsy beasts.
Now the other nifty adaptation spiders have is CHITIN. Chitin is a protein that
is found only in Arthropods (the group of animals in which insects and arachnids
like spiders belong) and - believe it or not - Fungi! Who knows how it only gets
produced in these very different groups?
Depending on how much chitin is in the exoskeleton (outer "skin") of
an Arthropod it might have a hard or soft shell. So a hard, shiny beetle has
more chitin in its exoskeleton than a caterpillar does. Also, spiders that are
more likely to fall often have hard, chitinous abdomens. Spiders that live
mostly on the ground, like tarantulas, often have soft, low-chitin abdomens
which can hold lots of insulating fat bodies and water.
So spiders - and most other bugs that might fall a lot have a couple of neat
adaptations to not getting flattened when they do fall. HOW COOL!
And also a really cool question! Thanks again for Askin' BUGMAN!