What does Theridion eat? Asked & answered

Yesterday, I posted a photo of Theridion on a signpost, and unclefrogy asked:

the question I have is what kind of prey is also attracted to that area and what time of the day because that spider sure looks well fed.

A good question! Theridion is a cobweb spider, and cobwebs are optimized for catching prey on the ground, unlike orb webs which are better for catching flying prey. These particular spiders are on a metal post 1.5 meters off the ground, so they’re unlikely to catch grounded prey. But I was passing by this same signpost today, and saw that the spider had been successful!

That’s a dead dessicated Dipteran by my finger (I had to poke my finger in to stabilize the victim — it’s windy today, and everything on silk was vibrating madly). So…they’re catching gnats and flies and mosquitos that encounter the tangle of cobweb silk.

Also, don’t insult the spider. The Theridiidae all have those nearly spherical abdomens, so she is a beauty among her species.

Signpost spider

For years, one of the earliest signs of the spider season is the appearance of spider silk criss-crossing these metal signposts around campus. I rarely see any of the animals making the silk — they tend to hide in the holes that puncture the posts. But today I spotted one hanging out in a visible place! They are spiders in the subfamily Theridiinae, probably in the genus Theridion. I do not know why they favor this one peculiar habitat. These black metal posts get really hot in the sun, so these spiders must like it hot.

They have quite pretty patterns on their abdomens.

Mother of spiders!

Show me a 500 million year old chelicerate, and I’ll be happy for a day. Look at this beauty, Megachelicerax cousteaui, excavated from a Utah fossil bed.

Anatomic reconstructions of the dorsal (left) and ventral (right) morphologies. b, Artistic reconstructions by M. Hattori illustrating oblique views of the dorsal (top) and ventral (bottom) morphologies. The sanctacaridid-like morphology of the posteriormost body region is speculative. gi, gill (that is, a set of gill lamellae); te, telson.

Pretty cool, right? The best part of it is that pair of appendages at the very front of the animal — those are chelicerae, the biting/chomping/chewing/venom-injecting bits of a modern spider, that make them distinct from insects, which only have antennae at that end. That makes this the oldest known chelicerate ever discovered. It was a swimming marine animal, and doesn’t have the legs we associate with spiders — chelicerae evolved first, legs much later.

Also, this isn’t just the mother of spiders, but is also the mother of a huge family of cousins: horseshoe crabs, eurypterids, as well as spiders.

Megachelicerax documents the oldest stratigraphic occurrence of chelicerae (that is, uniramous, unichelate deutocerebral appendages) and bridges the simple body and limb organization of Cambrian megacheirans with the more derived anatomy of post-Cambrian synziphosurines and crown-group chelicerates. a, Simplified consensus topology based on Bayesian analysis (Mk model, 4 chains, 5,000,000 generations, 1/1,000 sampling resulting in 5,000 samples with 25% burn-in resulting in 3,750 samples retained); detailed results and comparison with parsimony provided in Extended Data Fig. 6. The numbers in parentheses correspond to the total number of podomeres and the number of chelae, respectively, present in the deutocerebral appendage. Taxa whose names are in bold font are illustrated in b–l. b–l, The morphology of the anterior body region in select taxa. b, Fuxianhuiid Chengjiangocaris kunmingensis (Cambrian, Stage 3). c, Artiopod Olenoides serratus (Cambrian, Wuliuan). d, Megacheiran Yohoia tenuis (Cambrian, Wuliuan). e, Megacheiran Haikoucaris ercaensis (Cambrian, Stage 3). f, Megacheiran Leanchoilia superlata (Cambrian, Wuliuan). g, Mollisoniid M. plenovenatrix (Cambrian, Wuliuan). h, Habeliid Habelia optata (Cambrian, Wuliuan). i, M. cousteaui (Cambrian, Drumian). j, Synziphosurine Dibasterium durgae (Silurian, Wenlock). k, Xiphosurid Limulus polyphemus (recent). l, Eurypterid Slimonia acuminata (Silurian, Llandovery–Wenlock).

That is one wildly successful tree. It just goes to show that you can go on to do great things even if your face looks like a nest of spiky clawed jointed tentacles.

Rudy Lerosey-Aubril, Javier Ortega-Hernández. A chelicera-bearing arthropod reveals the Cambrian origin of chelicerates. Nature, 2026; DOI: 10.1038/s41586-026-10284-2

Screaming for joy?

I can relate to this National Park Service message.

Our favorite part of spring is walking into spider webs and screaming every time. What’s yours?

I wish. I haven’t seen any webs yet, although I have spotted some individual strands of silk. Warm weather is coming, though!

Down, down, in the dark

Shirtsleeve weather, the sun is shining bright, and there are stirrings in the darkness. I prowled about my yard, searching for spiders, but the best I could find was spider-sign — they’re out and about, leaving strands of silk in crevices and corners, but I saw none.

That is, until I turned to the ever-reliable compost bin. I found even more silk everywhere in there, but to find an inhabitant I had to bend over and stick my head upside down deep into the bin, way down low until I was look just above the edge of the decaying plants, and there at last I found one, a familiar old friend, Steatoda borealis.

S. borealis is entirely black in body color, and she was on the side of a black bin, in shadow, deep in darkness, so getting any kind of photo was difficult. But there she was, my first Theridiidae of the new season.

This compost bin is a favored spot. I think they snuggle down in the layers of rotting glop and overwinter there, and then they’re the first to reappear once the weather well and truly breaks. It’s kind of sweet to think of them sleeping down in the dark, in the mulch, all winter long, waiting to reemerge.

I’ve had few opportunities to photograph spiders

I haven’t been showing off my spiders much lately. There’s a reason for that. They still think it’s winter, so they’re all hunkered down in their mossy, silk-covered nests, and they don’t come out much, and when they do, they’re shy and not very photogenic. Here’s the best black widow shot I could get today.

I didn’t do myself any favors by providing them with a fairly cluttered environment, but they seem to like a space where they can hide and only come out to grab some food, and then retire quickly back into their refuge. They’re very retiring little ladies.

Minnesota is warming up, though. The snow is mostly gone, and I occasionally see jumping spiders sunning themselves, so I’m hoping to go hunting native spiders again soon.