You'll want to check out http://www.batcon.org/

Identifying bats in the air is much more difficult then
it is for birds. The survey work I've seen has been done
either using mist-nets or using a bat detector which can
modify and amplifying bat noises. The guys who use these
detectors recognize the calls of different bats.

Just guessing, however, it sounds like you may be seeing
the Big Free-tailed Bat (Tadarida molossa) which because
of the free tail can look kind of spidery or Pallid Bat
(Antrozous pallidus) which looks stongly marked another
strongly marked bat that is very rare in the northwest is
Spotted Bat (Euderma maculatum).

Jim McCoy wrote:
>
> While bats aren't strictly aviform, they *do* have wings and begin with the
> letter b, so I'm making bold to ask about them here. I was just pondering
> another sighting of our local bats in an adjacent woodlot, and I wonder
> whether there is any reasonable way for the untutored eye to differentiate
> small bat species on the wing at dusk. Are there any bat experts out there
> who are good at this sort of thing?
>
> Speaking of winged creatures that begin with b, I found a dying flying
> insect on my porch this morning (which meets the 'b' standard if I refer to
> it colloquially as a 'bug'). It had a long, copper-colored abdomen,
> accounting for perhaps 75-80% of its length of maybe an inch and a half, a
> black-and-white striped thorax, largish wings and extraordinarily long
> legs, not unlike a "daddy longlegs" spider. I saw several more of what I
> took to be the same insect this evening, patrolling just centimeters above a
> dirt/gravel path. Any idea what these guys are? I'm curious about what the
> adaptive value of long legs on a flying insect might be in this case. I'm
> currently reading the Origin of Species for the first time and my mind is
> even more than usually inclined to wonder about such things just now...
>
> Thanks for any suggestions on either front.
>
> JMc
>
> Jim McCoy
> jfmccoy at earthlink.net
> Redmond, WA

--
Mike Patterson Alas, to wear the mantle of Galileo,
Astoria, OR it is not enough to be persecuted
celata at pacifier.com by an unkind establishment,
you must also be right.
---Robert Park
http://www.pacifier.com/~mpatters/bird/bird.html