Re: what prevents the insects from flying high?

 First of all many insects, like migrating butterflies, fly at heights of 
100 meters or more.  Migratory locusts fly up and catch  thermals to assist 
them in their travels for miles.  Thermals represent columns of air rising 
right from the surface of heated patches of the land. These thermals even 
take them over oceans.

	There are a couple of reasons that many insects don't fly really high. 
First like you and me insects need oxygen to burn food energy to move their 
flight muscles.  As you go higher oxygen thins out so insects wouldn't get 
enough to breath or move muscles.
 
	A second reason is that insects are cold blooded and rely on the 
surrounding temperature to help them move. The higher you go in the 
atmosphere the colder it gets.  A cold insect is a very slow insect. They 
wouldn't be able to beat their wings fast enough to stay in the air. That is 
why the grasshoppers need warm fast moving air masses called thermals in 
order to move large distances several miles up.

	Lastly, insects evolved at the earth's surface air density, that's 
where their food is and where they mostly live so their bodies are designed 
to stay relatively close to the ground. 

Check out the site on Insect migration! http://www.ph.adfa.edu.au/a-drake/trews/ww_re_im.htm#top