I have noticed these black and yellow solitary bees making use of the 'bee hotel' which I put up in the spring. The larger holes were occupied and sealed several weeks ago and I am very pleased to see these little ladies making full use of the smaller holes. I would be pleased if you could provide an identification so that I can accurately label them on Flickr. I think the last picture might amuse you. She was easing herself into the cell on her back!