Blue Mud Dauber Wasp
When I was a kid, I used to find these blobs of mud stuck on our farm buildings, mostly under the eaves or in other spots where they would be protected from the rain. When I broke them open from time to time, they were either filled with paralyzed spiders, or had a wasp pupa inside. I’d often watch them being repaired and stocked by wasps that looked like this:
This is another old picture that I took in 2007, and the wasp was large enough that I tried getting the pictures without the macro lens, which is why the pictures came out a bit blurry.
[Note added in 2014: I've since taken some better pictures of blue mud dauber wasps, if you want to see the details better. They are here.]
I also refrigerated it, which made it pretty much act as if it were dead until it warmed up again. This seems to be a trait of hymenoptera – ants, bees, wasps, and sawflies don’t seem to take cold temperatures very well.
Anyway, it is a Blue Mud Dauber, genus Chalybion. The metallic blue color is quite striking, and the elongated, very thin waist is very distinctive.
While they are wasps, and they can sting, they are solitary wasps and don’t generally sting people except in extreme self-defense. They mainly save the stinging for the spiders that they catch to feed to their grubs. What they do is this: first, they find, refurbish, or build out of mud a tube about one-quarter of an inch in diameter (roughly 5 mm). Then, they go and find spiders that they ambush, and quickly sting just so. They don’t kill the spider outright, because then it will start to rot right away. Instead, they just paralyze it, which keeps it fresh. Depending on how big the spiders are, she’ll nab five or six of them and stuff them into the tube. Then she’ll lay an egg on them, cap the tube off with more mud, and start stocking the next tube. The egg then hatches, and the grub eats the paralyzed spiders as it grows. Then, come around fall, it pupates and waits for spring, at which time it finishes, pops out of the nest, and flies off to do in some more spiders. It’s a lot of work to build new nest tubes out of mud, so frequently they’ll just clean out an existing old nest and start over. They often take over the abandoned nests of other kinds of mud daubers (particularly nests of the related Black and Yellow Mud Dauber), or sometimes just take advantage of tubular openings that they find.
Whether these wasps are good or bad to have around kind of depends on your opinion of spiders. I’ve read that they will actually prey on black widows, if that’s the sort of thing that concerns you. Around here, though, the spiders they eat are really the sorts of things I’d rather keep around. Their mud nests are kind of ugly, too, and make things awkward when you need to repaint buildings.
I’m not quite sure why they specialize in spiders, it seems like it would be safer to go after things like caterpillars. The only reason I can think of, is that spiders are much less likely to have chemical defenses and toxins than caterpillars do. It may also be that spiders are easier to paralyze for some reason, maybe it’s easier to hit their master nerve ganglion.
 Most solitary wasps are pretty mellow that way, because there’s no percentage for them in stinging you unless you are actually killing them. See, if you have a whole nest of social wasps, then it is worth their while to sent a few out to discourage large mammals like us, because even if the defenders get swatted and killed, they are still likely to save the nest from being molested. The survivors can then continue taking care of the grubs, and the colony goes on. A solitary wasp doesn’t have that luxury, though. If she stings you to try to drive you away from the nest, and gets herself killed, then there’s nobody to stock the nest and the whole thing is a loss. It’s actually better for her to abandon a threatened nest and save herself, than to try to defend it. But, if you grab the wasp and try to kill her, she no longer has anything to lose, and that’s when she’ll sting you.
 My grandmother had these wind chimes that had kind of lost their chime, they just went “clunk, clunk” when the wind blew instead of ringing. I noticed that the weren’t working, looked into the tubes, and found that I couldn’t actually see through them. So, I took a piece of wire and rammed it through – and found out that the tubes were packed with mud. It seems they were just the right diameter to appeal to the mud daubers, and they’d used them for nests.
 Although, there’s one thing about spiders that live next to rivers and lakes. If the spider mainly eats insects that have aquatic larvae, those larvae are likely to have accumulated mercury from the water – particularly if they were predatory larvae preying on other, smaller animals. So, spiders in those conditions can have a very heavy mercury load. This is evidently a problem for birds that eat spiders, because then they have an even larger mercury load, to the point where it can kill them.
 What happens with mercury is this: you start with a very tiny quantity in the atmosphere, that eventually gets caught in the rain and carried into rivers and lakes. The mercury then converts to a form that living things will take up. Plants grow, and some of this mercury gets into their tissues. An animal then eats the plants, and the mercury gets into their fat, which holds onto it. So, they end up retaining pretty much all the mercury that they eat, concentrating it to a higher level than it was at in the plants. Then something else eats the first animal, and concentrates the mercury still more. So, if you have a chain that goes something like algae => water flea => carnivorous caddisfly => bluegill => bass => you, there are four stages of mercury concentration before you get it. If we assume that each stage of concentration increases the mercury levels by a factor of ten, then you are looking at 10x10x10x10 = 10,000 times more mercury than if you had just eaten the algae. If, on the other hand, you have algae => carp => you, there’s only one stage of concentration, so you only get 10 times more mercury than the algae, which is much better. Basically, if you want to minimize mercury in your diet, you want to eat terrestrial plants, or animals that eat terrestrial plants, and you really want to avoid eating meat from carnivores that eat other carnivores that eat aquatic animals that eat aquatic plants. Eating meat from bears that have been eating salmon that have been eating other fish would be exactly the wrong thing to do.
 Which brings up one of my favorite entries in the Bulwer-Lytton Fiction Contest:
“Terry the Tarantula and Wendy the Wasp were frolicking and cavorting together in the Flowery Meadow, (as they were the best of friends in all the Enchanted Forest of Miggly-Wompsly) when, all of a sudden, and with no warning whatsoever, Wendy accidentally stabbed Terry with her stinger, making her very sad for she knew that soon her poison would paralyze her friend and after a while her eggs would hatch inside him, and then her happy wriggling larva would slowly eat him alive, but Terry tried to smile and would have told her not to be sad as this was how the Circle of Life was continued, but he was in too much pain and, as I mentioned before, paralyzed.” (Delano Lopez, Washington, DC, in the “Children’s Story” category, 2001)