This group of bees is homeless for a completely different reason. Unlike the male Lipotriches, even the females of this species don't build nests. They don't collect pollen (the bee in the photo is just taking a drink of nectar for some energy). They don't raise their offspring at all. In fact, the lifestyle of these bees is completely different to what most people think of when bees are mentioned. These bees have taken a page - and a title - from the book of a peculiar group of birds: the cuckoos.
We all know how cuckoo birds lay their eggs in the nests of other birds. Cuckoo bees do the same thing, laying their eggs in the freshly provisioned nests of bees such as Blue banded bees and Teddy bear bees (Amegilla spp). This phenomenon is known as kleptoparasitism (i.e. parasitism by theft).
So that's why the bee on the left has to sit all alone out in the cold. I've once come across a dead shrub that was used as a night-time roost by half a dozen or so cuckoo bees. During the day they patrol areas visited by Amegilla bees (and in sunlight the blue really stands out and they can be seen from a long distance), then follow them to their burrow and eventually enter it (presumably when the host is absent) and lay an egg inside. Some cuckoo bees are equipped special mandibles used to kill any host offspring already present. Strangely enough, there have been reports that the host offspring isn't always killed, and it can make it through to adulthood - though it's malnourished and emerges at 1/4 of the size of a normal individual.
Apparently in bees this sort of thing is relatively common - up to 20% of all bees may be kleptoparasites. Australia has about 24 known species including this particular one, Thyreus nitidulus (other genera include Coelioxys and Inquilina which parasitise leafcutter bees and reed bees respectively though I've not seen these guys). Other Thyreus species have similar colourations - startling blue or white patches on a black background. Why such bold colourations in a creature that relies on stealth in order to follow its host and avoid detection? One hypothesis is that the colouration helps the bees maintain territories by being able to recognise conspecifics. Personally I've come up with another theory, though it requires some explanation.
Bees and other insects have completely different vision to us mammals. In fact, without going too much into it, they have two separate visual systems that kick in depending on the angle subtended by the object. But that's not important, what matters is how these vision systems lead to perception and recognition by the bee. There's been some interesting research done by scientists in Thaiwan on perception of Golden Orb-weaving spiders by bees. To cut a long story short, all those bright coloured spots you see on the spider may be the only thing the bee sees contrasting with the spider's background, which doesn't present a spider-shaped image, which means the bee doesn't recognise it as a danger, which leads to it blundering into the spider's web. That's a gross simplification, so read the original paper.
My point is, it's possible that there is a similar system at work here - if the bee only sees a few disconnected dots, it may not be able to recognise the cuckoo bee for what it is. The point is that it's important not to assume that what we see as blaringly obvious colouration may look completely different to a bee.
Well, this article has gone way over my word limit. Time for some refs then I'll call it a day.
Tso, I. M., Lin, C.-W. and Yang, E.-C. (2004). Colourful orb-weaving spiders, Nephila pilipes, through a bee's eyes. Journal of Experimental Biology 207(15): 2631-2637.
Dollin, A. (1999). The cloak and dagger cuckoo bees - "the bad relations". Aussie Bee 10: 4-5.
Edit: I should point out is that there is even another group of hymenopteran kleptoparasites: the cuckoo wasps (family Chrysididae). These creatures are brightly coloured too, with spectacular metallic colouration. Check out chrysis.net for the eye candy if nothing else.