Links
Insects and oxygen
I have heard that oxygen at concentrations much higher than the current 21 percent present fire hazards (self-correcting situation in an ecosystem). I have seen no source so far confirming that the oxygen levels in those times actually were that high.
More on ancient atmospheres here. Note other informative and interesting links on this subject, including why dinosaurs like lower oxygen levels, under "relevant stories.)
Spiders on Skylab