Solving the Murder Mystery in the Spider House

This is part two of a previously published post, Murder Mystery in the Spider House.

It started like it did last time. It was a sunny summer day here in Pittsburgh. The windows were open and the sheer curtains were billowing in the breeze. One of the windows doesn’t have a screen and I usually welcome the flies in as they end up being caught as free spider food. I was busy redding up the house (this means tidying up in Pittsburghese) when I noticed a spider on a dragline coming off the back of the rocking chair in the spider room. This was weird since spiders are pretty quick to get where they’re going and this one was just dangling there; hardly moving. Upon closer inspection I could tell it was an immature orb weaver because of its shape and size (and time of year – mid July). It was possibly Araneus diadematus going by the markings and it was in distress – it could only slightly wiggle some legs. I looked back at the rocking chair where it came from thinking it was extremely odd to have this kind of spider IN the house, even with a wide open window. That’s when I noticed the crime scene; a one inch cocoon made of mud was fastened to the back of the rocker. I hadn’t noticed it before. And I knew they were back….the spider-killing mud dauber wasps had returned!

This exceptional, half paralyzed, orb weaver managed to wriggle its way out of the mud nest before it was sealed! I have to give props to the survival instinct of this spider! Unfortunately, the sting of a mud dauber wasp is irreversible. The spider was incapacitated enough that it wouldn’t survive.

I prepared to crack open the nest. In my first blog about the mud dauber wasp, I was a gumshoe at this stuff. I had accumulated enough background to get an immediate mental picture of the scene I was about to uncover. I knew in that little clay tomb, I would find a host of paralyzed spiders and one very hungry mud dauber larva.

Inside the rocking chair nest were nine spiders: three adult Emerald jumping spiders (Salticidae), four running crab spiders (Philodromidae), a beautiful, green orb weaver (Araneus cingulatus?), and a “Jane Doe” spider I could not identify. Feeding on one of the Philodromids was a fat, legless larva. Click on this link to see the vid.

Suddenly, as I was examining the nest, the murderer returned to the scene of the crime! Everything about this is so coincidental – a spider-eating wasp is killing spiders right under a spider lover’s nose! I heard what sounded like a mini electric saw – the sound the mud dauber makes when it’s building a nest. It was coming from the right side of a bookcase which is draped by a cloth. I slowly lifted the cloth edge and there she was! A wasp mom was busy building! I assumed she was responsible for the rocking chair nest, too. She flew up, surprised at the sudden exposure, and helicoptered around to get her bearings. These wasps are not aggressive toward observers and they are solitary hunters, so there were no other wasp accomplices. She flew directly out the window which is about ten feet away and around a corner. These wasps are very visually oriented and know exactly what they’re doing and exactly how to get in and out. They don’t meander and crash into things like disoriented paper wasps. If a landmark is moved, like a piece of furniture, they re-orient themselves by going back a few feet and trying the route again. Their flight is silent and you don’t even see them because they are so direct. You’ll think you saw something whiz by, but then it’s gone. Without a vigilant cat standing guard at the entry/exit giving it a swat, it’s hard to be certain you saw anything at all!

The next time she came in (I could hear her buzz-saw-building sound before I saw her), I waited for her exit from under the cloth with a clear container and was able to catch her to get a few blurry snapshots.

Notice that she has the long, thread-like waist which is characteristic of mud daubers. Also, notice that she is mostly brownish with no yellow markings. She was definitely the suspect, but I did not know her specific identity. I figured I’d let her go and continue to observe her. Later, I would open the nests and note the spiders she brought back. Last time this happened, there were some pleasant surprises albeit at the expense of the spiders’ lives.

Later that same day, it started to rain and I had to shut the window. The unfinished nest was never finished. The next few days were sunny so the screenless window was again opened, but the wasp didn’t come right back. Then I heard that familiar buzzing sound that announced her arrival. This time, she was actually building under my spider lab desk! It was as if she was mocking Team Spider! Since she had chosen a more open nesting site, I was able to really get some clear photos of her.

Now I could see she did have some yellow stripes on her thorax and abdomen, but was mostly brown. I left her to her business of back and forth trips, my curiosity stimulated. I never did see her carrying any spiders. I’m guessing there is no sound associated with stockpiling the nests and since they don’t make a sound in flight, I missed all of that action.

A day or two later, the new landlord arrived to check the windows for an upcoming inspection. We closed the window so he could assess. The wasp, with her directness and speed, must’ve previously snuck in to make a nest stop because she was trying to go out of the window while we were talking. Confused, she would hover at the window in front of us like a hummingbird for a few seconds and go back to try the route again. “Oh, we need to let her back out.” I said nonchalantly as if the wasp were a common, domestic household pet waiting to be let outside. He opened the window and she silently flew out. I explained that I was doing an observation and that she was a solitary wasp so there was no danger of a wasp infestation! I hope he thought it was as funny as I did!

I left for vacation a day or two later knowing there were THREE nests under the spider lab desk to investigate when I got back. Unfortunately, one week away was too long. When I carefully opened the nests upon my return, the spiders had all been eaten. There was not one morsel left in all but one nest which had some random jumping spider parts. This is why there is no such thing as “vacation” for investigators. However, my focus turned to learning more about the suspect.

I had grossly overestimated the length of the larval phase. Mud daubers go through a traditional metamorphosis: insatiable appetite, spider devouring, fat grub, larval phase > mummy-like, packaged robot, pupal phase > adult spider paralyzer phase. Two of the larvae were already in amber-colored pupae cocoons. I assume the cocoon is made from some sort of secretion. It’s very alien-like.

After about a week, the pupae started taking on the wasp shape. I read that there are insects that will parasitize mud dauber nests – other types of wasps, velvet ants, and flesh flies. What a change of course it would’ve been after all of this, that something else emerged from the nest! But, all seemed normal from what I could see through the transparent, orange, cellophane-looking wrapper cocoons. I thought it was so cool how the pupae were neatly packaged in perfectly symmetrical forms. Nature is so amazing! There was very little movement, but I did observe one of the pupae rotating its body via little nubs on its abdomen. Week by week, the pupae gained pigment and I could see wings forming. I checked on the three of them daily.

During this time, Pittsburgh had a few back to back hot and humid days and I finally broke down and grudgingly turned on the air conditioner. That meant all windows closed, so I had to take the fan out of the bedroom window. Behind the fan, clinging to the screen, was a dead a mud dauber wasp. Seriously, this is almost the SAME thing that happened in Murder Mystery in the Spider House! It looked like the mama wasp or at least was the same type of wasp.

I had not seen the mama return since I had left for vacation – or I should say I hadn’t heard her. Having a dead specimen allowed me to really get a look at it under the microscope and hopefully, I would be able to figure out the identity and have a name for the record file. According to Bugguide.net, a very important resource (I appreciate yinz!), she could be one of three mud daubers we find around here:

Genus Sceliphron – Black Mud-dauber Wasps (as opposed to Blue Mud Dauber Wasps)

• Species assimile – Clayman’s Mud Dauber Wasp
• Species caementarium – Yellow-legged Mud-dauber Wasp
• Species curvatum – Asian Mud-dauber Wasp

The Clayman’s Mud Dauber wasp is said to be found only in Texas (Bugguide and iNaturalist), so that was an easy elimination. The Yellow-legged wasp IS found in Western PA, but the profile didn’t fit. The Yellow-legged wasp has a longer, more slender body with bright yellow stripes on its thorax and legs. The wasps I was finding had dull yellow stripes on the abdomen, blackish, brown legs (no yellow), and maybe a spot of the dull yellow on its head and thorax, but nothing flashy. That left the Asian mud dauber (Sceliphron curvatum) an introduced species that was “spreading fast”. Sure enough, iNaturalist had observations across the north eastern US with four sightings in Pittsburgh. The profile matched, but I wanted to know without a doubt, so I dug up some literature and found a paper that translated into “Sceliphron curvatum (F. Smith 1870) in Europe with an identification key for the European and Mediterranean Sceliphron species”. I used google translate and did my best!

The German paper compared one of their native wasps to the Asian wasp and provided illustrations that I used to compare with my wasp. Okay, now I was able to accept that I, indeed, had Sceliphron curvatum, the Asian mud dauber. The same paper also mentioned that this wasp has the odd behavior of building nests inside people’s houses on, in, and around furniture, drapes, and other odd places. Well, they can add rocking chairs and spider lab desks as “other odd places”, too!

TWENTY days later, after checking and checking, the wasps seemed to finally be “done”. The exoskeletons were darkened and I could see fully developed wings.

I just happened to be taking a look at “#3” when it started moving…a lot! It took about ten to fifteen minutes of struggling and buzzsawing before the wasp emerged. The cocoon was not encased in the mud nest since I had broken it open, yet the wasp ignored that easy way out! #3’s long legs would poke out through the hole as it wriggled to free itself from the cocoon. Even though it would’ve been easier to just exit via the hole I had made in the nest, the wasp did what it was pre-programmed to do and chewed through the top and squeezed out of a tiny exit hole. Once the wasp was out, I guess I was expecting it to sit there for a few minutes to get its bearings or groom its wings like a butterfly or cicada, but NO! It spent one second preening its antennae and then it flew away! I was able to get it on video! Check it out here!

Luckily, the window was closed, so it didn’t fly straight outta here! I captured the wasp and put it into an enclosure. The wasp looked smaller than the mom wasp, otherwise, an identical specimen.

A day later, #4 emerged. This wasp was noticeably larger. Maybe #3 was a male and #4 was a female? I put #4 into the same container as #3 and she was instantly attacked. They both fell to the bottom of the enclosure clinging to one another. It looked very much like mating – their abdomens were touching, but only for a minute, and they separated. So the smaller wasp was the male! They lived amicably in the container for about a week while I waited for the last wasp to emerge.

While I waited, I tried experimenting with a few things. I wondered what would happen if I marked them and set them free. Would they come back? I popped the enclosure into the refrigerator for fifteen minutes to make the wasps sluggish and used a fine-tipped paintbrush with yellow and white acrylic paints to put a dot on their thoraxes. If they returned, I’d know it was them!

I read that they drink nectar in addition to killing spiders so I mixed some sugar water and used a dropper to provide small drinking pools, which they drank! I wondered, what if I put some mud in there would the female start building? Both clay and hummus soils were ignored. What if I put some spiders in there? Would prey stimulate nesting behavior? I gathered two cobweb weavers from the basement (because they’re so easy to find) and put ’em in with the wasps. The wasps were not interested and the spiders were more focused on trying to eat one another. Maybe the wasps needed a different menu item, like jumping spiders. I did read that individual dauber wasps have a spider preference and they’re usually some type of wandering spider, not cobweb weavers.

Five days later, #5 emerged. It was a male. It was small like #3. I put #5 in with the other two, but nothing significant happened. Nobody attacked anybody and I now had an enclosure that was too small for three wasps and two dueling spiders. I set them all free the next day, hoping that the female, #4 would return – how interesting THAT would be! There would be time for one more generation this season, but I did not see any of the mud daubers again.

With my investigation complete, I realize that I let the suspect get away and then raised and released its spider-murdering offspring. However, I was able to get a brief, yet intimate peek into the world of the Asian Mud Dauber wasp, and through that, a better understanding of the spider as prey instead of predator. From what I read, although the Asian Mud Wasp is introduced, it is not considered to be invasive or to be a serious threat to our local spider, but spiders do NOT have it easy by any means. Media likes to put spiders on top of the food chain as “deadly” threats to humans and mammals, which is totally not true. The mud daubers are only ONE of the spider’s many predators and parasites. When I see a fully mature spider, or even a large spider, I think about how that individual was able to outsmart the mud daubers, avoided being eaten by other equally stealthy spiders, and managed to not be parasitized by mantid flies. I can now close this case with an even greater appreciation of my spider friends and how their web extends much farther than themselves!

Bohart, R. M. (Richard Mitchell), 1913-2007. Sphecid Wasps of the World: a Generic Revision. Berkeley: University of California Press, 1976.

Ferguson, C.S., Hunt, J.H. (1989). Nearnest behavior of a solitary muddaubing wasp, Sceliphron caementarium (Hymenoptera: Sphecidae). J Insect Behav 2, 315–323. 

Schmid-Egger C (2005) Sceliphron curvatum (F. Smith 1870) in Europa mit einem Bestimmungsschlüssel für die europäischen und mediterranen Sceliphron-Arten (Hymenoptera, Sphecidae). BembiX 19: 7-28.