Hmm. I had made a great post on UV-light on one of these forums a while back, but it seems to be missing now. It probably should have been archived. I'll try to summarize it again.
UVA:
Here's some info about UVA:
http://www.emaxhealth.com/66/7215.html
UVA is mostly transmitted through glass and screens (and is not blocked by normal sunscreen and it penetrates deeper into the skin and causes aging and melanoma). Our crabs are probably a small amount of natural UVA even indoors in crabariums. A UVA light on a crabitat will provide extra.
Reptiles see in the UVA range, so they have increased activity when exposed to UVA.
UVB:
Here's my source on the UVB-info
http://www.uvguide.co.uk/index.htm
Basically:
chicken wire (very coarse mesh): Reduces UVB by 7%
mesh: Reduces UVB by 15-30%
flyscreen (very fine mesh): Reduces UVB by 50%
Aquarium glass: Reduces UVB by 98.5%
Plexiglass acryllic: Reduces UVB by 20-40%
Combination of house windows and aquarium means that the hermit crabs are getting absolutely 0% natural UVB. If you place a UVB light ontop of a glass or plastic lid, likely only 2% of the UVB is reaching the crabs.
Reptiles use UVB to synthesis Vitamin D3
So, in order to effectively use a UV-light, you need to have the light over a screen or open-portion of the top of your crabarium.
As for lights reducing humidity, it's not something you have to worry about if done right. The heat from lights increases the saturation point of the air, so for a given amount of water vapor in the air, the relative humidity will decrease. However, a properly setup crabitat should have good pools of water and a moist/sandcastle substrate. The radiation from the light will evaporate more water from the pools and surface, increasing relative humidity.
The hotter air will increase air pressure, so the air will try to escape out of the tank, rising out of gaps in the lid. You'll simply have to reduce the gaps in the lid. I always have my tank lid proped open about 15-degrees (PI/12 radians), and the relative humidity stays 75-80%. Of course it helps that I have two tropicaire humidifiers running to keep moist air coming in. But for my ISO, I just prop the lid or de-prop the lid as necessary to regulate the humidity.
The only thing you'll have to do is refill the water pools more often, since they'll evaporate faster, and maybe occasionally mist the substrate (or poor small amounts of water down the sides) to keep the substrate moist.
15w fluorescent UV-lights add a negligible amount of heat. You should put them on timers so that the only come on during daylight hours so as to keep proper day/night cycles for your crabs.
Some crabs will only come out at night. But I've always got a number of crabs that enjoy basking. They will climb up and get as close to the light as they can and sit there all day.
And on top of it all, crabs need higher temperatures that most people's houses provide, and overhead lighting (in my opinion) is the best way to provide temperature increases. Moonglo/nightglo bulbs can provide 24/7 heat, and can be found in 15 and 25w packages so as not to overheat/overevaporate the tank.
And just because I'm trying to set a guinness book of world records here or something, I'd like to mention that my original pair of 25w moonglos are still running! They've been on continuous for 24,500 hours now! They were only rated for like 2000.
For heat, it's fine to go through the glass. Some people have had their glass lids crack from the heat, but mine hasn't yet. Keeping the glass lid over the tank also helps prevent moisture from escaping.
However, you'll need to have the UV light over a mesh or open part. The hood should provide enough enclosure so as to prevent much air from escaping.
