Thursday, July 23, 2015

Muddy bassinets

Do spiders eat wasps? Do wasps eat spiders? We've been discussing this in the comments on yesterday's post. Conclusion; yes, and yes.

Even baby wasps eat spiders.

The porch of the building behind the washrooms at Reifel Island Migratory Bird Sanctuary is a quiet corner, rarely visited. There, swallows tend their nests in the eaves, and moths sleep on the walls, undisturbed. And in the shade at the top of the inner wall, black and yellow mud dauber wasps build their baby incubators.

A dozen little cradles, all in a row.

Half a dozen. A new nursery, under construction.

Each little tube contains one wasp egg and provisions for the growing larva. The female wasp collects mud from the edge of the pond nearby, and molds it into tubes on the wall, working from both sides, making ribs which meet in the middle and are cemented shut.

As each tube is finished, she goes grocery shopping, bringing back a half-dozen or more spiders, live, but paralyzed by a shot of wasp venom. They will never wake up. In each tube, then, she lays one egg, and seals the tube up with mud. When the eggs hatch, the growing larvae will eat the spiders, still alive up to that point.

The two open tubes in the photos above are still unfinished; the others all contain comatose spiders and developing eggs.

Here's a mother wasp, working on an open tube.

Once most of the eggs are laid and provisioned, she starts daubing the whole mass with another layer of mud. This will dry hard and seal in the babies until they're ready to break out.

Adding spiders to the latest tube. The spider hiding on the left doesn't realize her danger!

Of course, nothing is foolproof. Other species of wasps, ichneumonids* and cuckoo wasps**, look for mud dauber nurseries under construction, and lay their eggs inside while the mother is off catching spiders. When the invaders' eggs hatch, they eat the spiders and the mud dauber larvae, too, then burrow their way to the surface of the mound.

Look at an old mud dauber nest and you can decipher what happened to the offspring. A large hole chewed out at the end of a cell means an adult mud dauber successfully emerged. Small holes along the length of the cell mean some kind of parasite came out instead. (From BugEric)

About names: the common name is simply Black and Yellow Mud Dauber. Its scientific name is Sceliphron caementarium; hard to pronounce, but it makes sense: caementarius means "mason, builder of walls." Of cement, of course.

* Ichneumonids. Christopher Taylor called them Icky Newmans. A good mnemonic, and it helps with pronunciation, too!

** Cuckoo wasps. Brilliant green; I saw a fair number at Reifel Island on Tuesday. I'll look again at those nests later on, to see who were the final hatchlings.


  1. Wow learned something new today. So many people hate spiders and I usually mention how spiders often eat other spiders.

    Now wasps are on their side as well

  2. My wife is a potter. Her studio is an old cabin frequented by all manner of wildlife including mud daubers. Mud daubers are not as sure-footed as one might think. Paralyzed spiders falling out of the air is not a rare event during the summer months.

  3. I can't claim credit for the 'icky newmans' label for ichneumonids (no 'y'). I lifted it straight from a Calvin and Hobbes cartoon, where an attempt by Calvin's parents to get him to read more backfires after a trip to the library leaves him spouting a graphic description of what an ichneumon does to a caterpillar.

  4. Christopher, I don't know how that "Y" snuck in there. Fixed. Thank you. I never saw that Calvin cartoon; sounds very Calvinesque.

    Marvin; raining spiders! I'd be wearing a hat, for sure!


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