I've read -- and perhaps it isn't true -- that the reason bats aren't diurnal is that their enormous membranes catch too much sun and they'd get heatstroke right away.  Pterosaurs.  Very heat-tolerant?  Covered in reflective scales or fuzz?  More nocturnal than we usually picture them?

PS I am a pathetically huge fan of pterosaur.com.  Thank you thank you thank you.

Although most bats are nocturnal, not all are -- for example, it seems that the Samoan fly fox Pteropus samoensis is largely diurnal.  So it seems there is nothing inherent in the bat body-plan that constrains them to a nocturnal lifestyle.

By the way, I was surprised to notice, when reading the Wikipedia article on flying fox bats (http://en.wikipedia.org/wiki/Pteropus) that the largest of them Pteropus vampyrus, has a wingspan up to 1.83 meters (six feet!) but a mass of only 1.5 kg.  My first instinct is that this is proportionally much lighter than we usually estimate for pterosaurs, but it turns out I was wrong.  An azhdarchid with a 12 m wingspan is 12/1.83 = 6.56 times as large in linear dimension, so if it was similarly proportioned (admittedly a gross oversimplification) then it would be 6.56^3 = 282 times as massive as the bat.  282x1.5 kg = 423 kg.

This is in the right ballpark for the mass of large azhdarchid pterosaurs according to the recent work of Mark Witton (2008): he got a figure of 250 kg for the largest individual in his study, which had a 10 m wingspan.  In fact, (10/1.83)^3 x 1.5 kg = 245 kg, which is frighteningly close to his figure.

I don't know what to make of this.  My uninformed impression is that bats seem proportionally more fragile than I've imagined pterosaurs to be.  I guess my uninformed impression must be not merely uninformed but also Just Plain Wrong.