A common question I've received is: how thin is the material on your Vibram Five Fingers? Or do your Vibram Five Fingers make your feet hot?
The material is very thin. Per the FAQ on Vibram FiveFingers dot com:
The upper is made with a thin, abrasion-resistant stretch polyamide fabric.
But since "thin" is a pretty vague term and photos of VFFs are never in cross-section, it's difficult, if not impossible, to get an appreciation for the thin-ness of the "polyamide fabric" (just a synthetic fabric). However, above is a photo of my KSOs where I'm pinching the two types of material. If you can mentally divide that width in half, you'll get a decent appreciation for how thin the fabric is and as you can tell, it's pretty thin!
One note, the fabric beneath the velcro strap at the top is much thicker, which is great b/c it provides cushioning and protection against the tightened strap.
If you're still uncertain about the fabric thin-ness, I've got one more way to explain it. Last night, I was hanging out with Nathan on his porch. I was wearing my KSOs. At some point, I was bitten not once, but twice on my left foot by a mosquito! I couldn't believe it at first because I had the KSOs on, but sure enough, when I took them off, I had a mosquito bite on my little toe and another on my mid, upper-foot where the meshed fabric of the KSOs is!
Thus, put simply, the fabric on Vibram FiveFingers is so thin that a mosquito can still suck your blood through it. In short, even though the fabric is good at keeping stuff out and good for keeping your feet cool and protected, it's not so good at stopping mosquitoes.
Go figure. It's a trade off.
Note: This response applies to Classics, Sprints, and KSOs, but not Flows, which use neoprene fabric.