Latrodectus geometricus [Brown Button Spider]

in Arachnids7 months ago (edited)


As promised, another spider post!

This week will be focusing on my favourite spider, the brown button/widow spider. I'll be referring to them as button spiders because that's what they're called in South Africa, but just so that there isn't confusion, know that button spiders are widow spiders. It's just a geography thing. They all belong to the same genus.

The brown button spider is one of the only four of South Africa's medically significant spiders, though the least venomous of them. The other three are the black buttons, the violin spider (also known as a recluse spider), and the six-eyed sand spider (my unicorn).

While medically significant, button spiders are shy and web-bound, making bites very rare. They prefer to run away if they can or curl up and pretend to be dead, only biting if they're squeezed, as a self-defence mechanism 2.

L. geometricus so good at survival that they often displace the local black button species in the countries they were introduced to, like in Australia and the USA 3, 4. Brown buttons are found on every continent except Antarctica. They are local to South Africa, however, and while we can't say for certain, it's thought that brown buttons originate from our country 4, 2. This is further supported by the presence of another brown button species, Latrodectus rhodesiensis (Zimbabwean brown button), which looks exactly like L. geometricus.

• LATRODECTUS GEOMETRICUS. •


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Big Bertha, the brown button spider, with her mate

The photo above is a shot of Big Bertha, a brown button spider who used to live under my tarantula table. Alas, they live only until around 2 years old. The smaller spider in front of her was one of the males who mated with her. Don't let the black colour fool you, she is not a black button spider at all. Brown buttons, specifically L. geometricus, darken with age and/or environment 2. Generally, the darker the female, the older she is but this is not always the case.

The easiest way to identify a brown button spider is the distinct hourglass marking on the underside of their abdomen. No female black button spider in South Africa has that marking in adulthood. The hourglass can range from yellowish, to orange, and bright blood red. Even if they appear to be solidly black, if there's an hourglass, they're a brown button spider in South Africa.

This does confuse a lot of people, especially because the black colouration with the hourglass makes our brown buttons look like the American Southern Black Widow (Latrodectus mactans), but those spiders are not here. Some brown button spiders mature into an almost pure black colour with red hourglass marking.

Big Bertha here was an older female, she was much more brown in previous months and the geometric patterns on her abdomen were more defined than in the photo.

She was also considerably large, and had a post-maturity molt. I can't be certain, but from my observations of a lot of brown button spiders, this only seems to be possible if the female doesn't mate shortly after maturing. I've also noticed it tends to occur in the dark variant L. geometricus females.

• GENUS •


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A better look at the faded geomteric pattern on the abdomen of L. geometricus, dark variant

Latrodectus is an iconic genus, mainly thanks to the wide-spread knowledge of the black widow spiders. They can be identified by their long first leg pair, large and round and bulbous abdomen, and their messy webs. They belong to the family Theridiidae, which are commonly called comb-footed spiders, or tangle-web spiders.

Sometimes, Latrodectus can be confused with another Theridiid genus, namely Steatoda (false button or cupboard spiders). Unlike Latrodectus, Steatoda sp. tends to have a band of white or red going across the front of the abdomen, and they also have prominent "dimples" on the top of the abdomen.

Not all Latrodectus sp. have a marking under their belly, so looking for the hourglass shape isn't reliable for identifying the genus.

If you're able to see them, you'll find that the eye arrangement in Theridiid spiders occurs as two curved rows of 4 almost equally-sized eyes. A spider's eyes are the best way to determine the spider's family, but not the genus. With the brown button spider, the identification is easy: if in South Africa, simply look for the orange or red hourglass marking.

In other countries where the black button spiders have the hourglass, a good way to identify L. geometricus is by the geometric pattern on the abdomen. It's always there, even in the black variant, you'll just need to find it. Otherwise, a give-away is the brown colouration on the legs, especially in the tibia and metatarsus sections of the legs. Black button spiders with the hourglass are purely black in their legs as adults.

• DESCRIPTION •


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L. geometricus sub-adult female in the process of molting

Taxon:

  • Class: Arachnida
  • Order: Araneae
  • Infra-Order: Araneamorph (true spiders)
  • Family: Theridiidae
  • Genus: Latrodectus
  • Species: L. geometricus

Female

Dark variant female (Big Bertha) 20mm in body length. Leg span of approximately 35mm diagonally.

Cephalothorax:
Black carapace and sternum. Posterior carapace slightly flattened.

Abdomen:
Velvet black all over with clear bright red hourglass marking in the centre of ventral (underside) abdomen. Faded spots down centre of dorsal (top) abdomen. Abdomen bulbous egg-shape. A cream-coloured book lungs on either side of hourglass marking. Large rounded spinnerets.

Legs:
First leg pair longest, fourth leg pair slightly shorter than first. Tibia and metatarsus dark brown colour.


Male

About 5mm in body length. Leg span of approximately 7mm diagonally.

Cephalothorax:
Brown carapace with darker stripe down centre.

Abdomen:
Abdomen white egg-shaped with two grey-brown stripes parallel down middle toward spinnerets. Black spots surrounded by white rings in abdomen stripes. Stripes segmented between dots. Faint stripes running up sides of abdomen toward dark stripes. Cream colour hourglass marking on ventral abdomen near lungs. Dark spinnerets.

Legs:
Brown legs. First leg pair longest with darker joints. Pedipalps round and large, dark brown/black. Fourth pair shorter than first but longer than second or third.

• ABOUT THE GENUS •


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Drawing of the Latrodectus eye arrangement and the female's shape

Contrary to popular belief, button spiders don't always eat the males after copulation (mating). Sometimes, the male will decide to flip over onto the female's fangs when he's done and, sometimes, the female will eat him as a result. But not always. I watched Big Bertha and her mate, and when the male flipped over, Big Bertha scurried away, uninterested in making him food.

Why do the males sacrifice themselves? Current understanding and findings propose that the offspring of males who've sacrificed themselves have an overall higher chance of surviving 5. Aside from that, males, once mature, live very very short lives. But it makes sense, what better way to make your life purposeful than to ensure the female who will birth your offspring is fed and can use that energy to produce strong eggs?

• NOTES •


In my area, as I'm sure will be the case in many around the world, L. geometricus is very common. I find them almost everywhere. If not them, then I spot their egg sacs. The egg sacs are unique, and look like a sea mine. They're round with silk spikes all over. These spikes are believed to help ward off parasites and predators from the egg sac 2. Either way, the spikes make it easy for the female to secure the egg sac to the web and thus the sac is less likely to be blown or taken off the web.

In my observations, the spiderlings do not emerge from the egg sac until the mother bites open a hole when the slings have hatched. L. geometricus is also very attentive to her egg sac and protects it, but will abandon it to predators if she thinks she won't win. Each egg sac can hold around 150 eggs and sometimes all of the eggs hatch successfully 6. I've observed females creating 11 egg sacs in her lifetime.

Brown button spiders are also very tidy with their webs. They will cut loose the empty carcasses of their prey so that it drops to the ground, and I've observed them removing and replacing webbing that has droppings on. This makes sense, however, because the web is more of an extra limb to the spider than just a web.



Thanks for stopping by and reading and supporting!

And remember, spiders are friends.



 

• All images are Copyright © 2022 Anike Kirsten •


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Great, very nice description of this species of the genus Latrodectus, I would say it is one of the most common in much of the world, in my house I have several, but none with such a predominant dark brown. The good thing was that you managed to photograph even the boyfriend of the big bertha hehe. very nice and informative post! :)

Thanks! It's always a good day when someone enjoys reading about spiders.

Yeah, the dark variant brown button spider isn't so well known or seen. It's amazing how many colours they're found having. There's a paper that shows several colourations for them, which is amazing to see because of how fascinating the brown button spider is.

https://entnemdept.ufl.edu/creatures/URBAN/SPIDERS/brown_widow_spider.htm

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