Echolocation: Bats are recognized as small, nocturnal mammals that
are able to truly fly, not merely glide as the flying squirrel
demonstrates, and form the second largest mammalian order,
outnumbered only by the rodent order, which has more species.
However, bats are also known specifically for their use of
echolocation to navigate and catch prey in the darkness: they are
constantly releasing ultrasonic waves during the whole time they are
awake, using it to enhance their sight, which exceeds the sight of
homo sapiens on its own. By emitting and receiving ultrasonic sound
waves, they retain the ability to see further distances than many other
organisms and can derive information which is much more refined.
Although not all bats use echolocation, such as some megabats
(large bats), many bats, many known as "microbats" do use sonar, however, the use versus
no use suggests different ancestors.

More info at:http://members.fortunecity.com/anemaw/bat.htm
Scroll down about halfway to Physics Application

and using echolocation they do not percept hairs - hairs do not reflect ultrasound. therefore there is a grain of truth in the stories of bats being trapped in women's locks

Dear vika: inividual hairs might not reflect ultrasound, but the head the hairs are
attached to would. And remember how little mass the moths have that the bat
locates by echo of its cries bouncing back off them.

I heard just yesterday that mother bats know their young by smelling them out. An elementary school teacher said she lets her kids (the baby bats) wear little pounches of various scents: peppermint, vanilla, etc.

The "mother" bat little kids have their eyes blindfolded and they have to find their babies.

dear wwh,

many night moth are quite hairy because it reduces probability to be discovered by bats. surely, if something like this has evolved bats have problems locating a hairy object

actually human mothers also recognize the smell of their babies... in a hospital study done of new mothers, almost half could not identify a newborn photo of their own baby (the study was done in UK-- in many urban hospital in US, the racial groups of the children would be evident by skincolor, eyefolds, and hair, and unless the nursery population was very high, there might not be enough babies of any one ethic/racial group!) but all of the mothers correctly identified which blanket their baby sleep in over night.

mother's often recognize kids clothing by smell as well as by sight, and a curious thing happens when kids reach sexual maturity... they start to stink.. most parents find their teen age kids smell, (and tell them to bath more frequently-) it a build in way to help prevent incest.

parents don't stop loving their kids, but will often complain about how they stink.. and many times encourage the kids to start doing their own laundry, or make other changes, it is especial evident when kids come home from college, and their laundry "stinks"... (girls are less offensive to mothers, boys less offensive to fathers)

the same sort of thing happens with animals. a female tiger will either recognize(by sight) , or be repelled by the smell of her own litter, and will not later mate with one of her own.

(this can be overcome if they are keeped penned together in a zoo or other unnatural places.)

how little mass the moths have

In the case of ultrasound it isn't always* the mass that's important but the dimensions of what's doing the reflecting. If the object (say, a hair) is so small that it is smaller than the wavelength of the sound, it won't reflect the sound back to the bat, so it is "invisible" to the bat. Another possibility is that the hairs scatter or absorb sound instead of reflecting it: think of the difference between a lake on a still day (reflects very well) and a choppy day (reflections go every which way and not all the light is reflected in the same direction).

* not to say that mass isn't important but that there are other important factors, too