They’re paralyzed but still alive.
From Minibeast’s Wildlife Photos on Facebook, we have this amazing picture by Alan Henderson, which the caption explains:
Paralysed and waiting: We accidentally broke open a mud-dauber wasp nest under our house yesterday while moving some equipment. The mud cells were packed with paralysed jumping spiders of multiple species. The adult female wasp captures and paralyses the spiders and then packs them into the mud cells. She then lays an egg before closing the cell over with mud. The spiders are paralysed, but not killed so that they do not rot and the wasp larva can consume them over time. I rearranged the contents of one cell for this photograph. A wasp larva is in the centre in the process of consuming a spider.
A somewhat sketchy Wikipedia article reports describes these lovely, thin-waisted wasps:
Adults of both sexes frequently drink flower nectar, but they stock their nests with spiders, which serve as food for their offspring. Like connoisseurs, they prefer particular kinds of spiders, and particular sizes of spiders for their larders. Instead of stocking a nest cell with one or two large spiders, mud daubers cram as many as two dozen small spiders into a nest cell. They appear to know exactly what they are hunting for, and where to find it.
Black and yellow mud daubers primarily prey on relatively small, colorful spiders, such as crab spiders (and related groups), orb weavers and some jumping spiders. They usually find them in and around vegetation.
Blue mud daubers prefer immature black widow spiders and their relatives. They hunt them in dry areas, such as outbuildings, rocky areas and stone piles.
Pipe-organ mud daubers generally provision their nests with various kinds of orb weavers, but their diets includes other kinds of spiders, as well.
To capture a spider, the wasp grabs it and stings it into submission. The venom from the sting does not kill the spider, but paralyzes and preserves it so it can be transported and stored in the nest cell until consumed by the larva. A mud dauber usually lays its egg on the prey item and then seals it into the nest cell with a mud cap. It then builds another cell or nest. Missouri’s mud daubers generally have two generations per year. The young survive the winter inside the nest.
It also notes that mud daubers were responsible for at least one airplane crash.
Here’s a video of one species showing how the group got its name—they roll up balls of mud, carry them to their nest site, and then plaster the mud into a beautiful nest. They are skilled workmen—or, rather, workwomen! Quite an amazing behavior, and one coded, along with every other wasp-y behavior, within a brain the size of a pinhead!
h/t: Matthew Cobb, Alex Wild