Re: why does a butterfly flaps its wing when it stands still?

Dear Maria

This question was quite difficult to answer for me for two reasons.
First: I am not sure what you mean with "while they stand still" as I know 
some different behaviors that might fit this description.
Second: while I can think about several reasons for this behavior, in some 
cases I just do not know – and I am quite sure, nobody really knows.

Anyway:
Behavior: a butterfly sits or "stands still" on solid ground (e.g. plant) 
with open wings, not moving for about 10 seconds, then closes wings, then 
not moving for several seconds, then opens wings again … and so on.
This can have several reasons, most of which you can experience yourself if 
you want to do little experiments:

A) Temperature control
You sure know already, what happens if you go outside on a bright sunny day 
– you get hot. If you have a dark (metal) plate, lay it flat in the sun. 
Feel or measure the temperature after a few minutes. (Careful – it might be 
hot). Then pop it up and support it so that it can not fall down again and 
measure after a few minutes again. In a very similar way butterflies can 
regulate their body temperature by opening and closing their wings. The more 
surface is exposed to the sun rays, the easier the animal gets warm. 
(Especially effective with dark colored butterflies.) Closing the wings 
reduces the surface that is exposed to the sun and the animal cools down.

B) Prevent exhaustion of single muscles
Just try to hold your arms outstreched for some minutes. A while you are 
fine, but soon you will find it more difficult with every second that goes 
by. Sooner or later you have to give in. Now repeat the experiment watching 
a clock and hold your arms up for ten seconds then giving them a rest for 4-
5 seconds, repeat holding and resting for a while. You will find that you 
can hold up your arms much longer. The reason is, that every muscle needs 
periods of rest. No muscle can work without break. Otherwise it gets tired 
and starts hurting. The same might be true for the muscles that hold the 
wings open or closed. Doing so  could well tire a certain set of muscles in 
a butterfly, so the flapping is necessary to give this set its rest, while 
using a different set of muscles to do the work for a while.

C) Prevent "cramps"
Remember your last really long trip in a car? Even at rest, "not moving" 
sooner or later starts to feel uncomfortable, the legs might start to hurt 
and in worse cases you might get a cramp. The same might be true for 
butterflies. Not moving might feel very uncomfortable after a while, even if 
the muscle does not have to work.

D) Fun
It might sound strange to you, but I believe, that even insects can 
experience something like fun. (No reason for this believe, I just worked 
long enough with insects to become convinced about that.) They might just 
like to move for the fun of it like us people like to do some sport.


Second behavior: a moth sits still (or walks slowly around) but beats its 
wing in a quite high frequency.

A) Warm up for flight
If you want to do serious sport, you have to warm up your muscles. Without 
you can not bring top performance and even have a high risk of damaging your 
body. Similar, a good number of moths are not able to fly without worming up 
before take off. This can take several minutes. 

B) Spreading pheromone
Many moths and butterflies communicate via chemical signals. For example 
female moths attract males with pheromones. Near every surface airspeed is 
nearly zero (boundary layer). The chemicals have to diffuse a short distance 
before they reach a layer that is moving. Beating the wings produces air 
turbulences near the body, that makes the boundary layer much smaller. This 
means, more chemicals can be distributed in shorter time and this can 
increase the speed or reliability of communication between the animals. You 
do the same if you want to disperse a bad odor in flapping a newspaper 
something.

I don’t think you asked about that, but there is a third behavior: butterfly 
stands still in the air, while flying. This is another form of "standing 
still" and just to be complete I list it here. A few species of butterflies 
can stand in the air like a helicopter or hummingbird. The reason for the 
wing beat is clear: It would plainly fall down doing nothing.

Referencs and more information:
Most of the general muscle information comes from human and animal 
physiology textbooks. Any school library should have some. 
For more information on pheromones: try the keywords    insect and pheromone 
at the google search engine - i found lots of information.
Still standing flight: see  thinkquest  and here the page with
 photographs of a moth flying. 

Hope that helps
Dr. Jürgen Ziesmann