The first picture is of a bee home we don't recommend - it was something donated to us and it has bees in it...so we've kept it around.
The second bee home, with the wire over it, has nesting trays, which fit together to create nesting holes. The trays are super easy to take apart and clean out, which can be an important step in interrupting fungi and pests that attack the bee larva.
Here is a post from Robbie, where they did just that, took the trays apart and cleaned them out. Robbie wrote:
After lunch Fred wanted to get us out of the heat so we cleaned up a leafcutter bee box for their new season to pollinate. We removed the cocoons from the tubes and checked for parasites. Once they were cleared, we cleaned the frames with a wire brush and put the box back together. The cocoons were put in a mesh bag above the tubes for the larvae to hatch and repeat their cycle.
Robbie's first picture above shows cleaning the trays. (I'm not sure if that dark colored critter is a dried out leafcutter larva or not - I think they are usually a milky white color.) Though he said that the larva in the second picture above was a parasite that invaded the cocoons.
Our bee house and trays for the leafcutter solitary bees were a gift from Crown Bees. Certainly a world-wide expert on solitary bees and their amazing pollinating power. See also Crown Bees' Native Bee Guide (in the permies digital market).
The third solitary bee home that you can just barely see in Coco's second picture above, was build by Leif at our PEP1 gathering last month!
He completed a PEP BB for it here and posted these photos of the elegantly simple design:
Built a mason bee hotel today, from scrap 1x4s and finish nails, using a hand saw to cut the chunks of board. The reeds were courtesy of Jocelyn, and it was mounted today to the base camp garage!
Thanks for another bee hotel, Leif! Ours has been filling up each summer!
Note that the common term for solitary bees is "mason bees" though mason bees are just one of many, many types of solitary bees. Mason bees pack their nesting tubes with mud and are more prolific and common in slightly warmer zones like the Pacific coast. If I recall, mason bees hatch in the spring, when temperatures are consistently above 50F or so during the days.
Leafcutter bees pack their nesting tubes with--you guessed it--leaves(!) and are more common and prolific inland where the summers get hot and there are colder winters. They hatch in late spring or summer, only after several days in a row of 70F or above. Leafcutter bees prefer a smaller sized hole than mason bees, too, I think.