Identifying spiny orbweaver (Gasteracantha) egg cases

An oblong silken structure with bright green threads, attached to a leaf, a wall, or even a car window. What is this? You've probably found the egg case of a spinybacked orbweaver. Here's an example:

An egg case of spinybacked orbweaver (Gasteracantha cancriformis), showing the characteristic lengthwise stripe of emerald green and neon green threads. Photographed by @casseljs in Georgia, USA.

In the southern United States, and throughout most of Latin America and the Caribbean, the colorful spinybacked orbweaver (Gasteracantha cancriformis) builds egg cases like this, often near or even upon human habitation. The female of species does not put her egg case in its web like certain other orbweavers; instead, she seeks out a leaf or -- for whatever reason -- hard human surfaces (here's one on a handrail for swimming pool steps: https://www.inaturalist.org/observations/27030562).

Here's a description of the process from the University of Florida: "After the eggs are laid on a white silken sheet, they are first covered with a loose, tangled mass of fine white or yellowish silk, then several strands of dark green silk are laid along the longitudinal axis of the egg mass, followed by a net-like canopy of coarse green and yellow threads."

Clear enough. But what if you find a similar structure without the central green stripe?

Well, orbweavers from several other genera make similar egg cases; for example:

-- Neoscona: https://bugguide.net/node/view/1450001 and https://bugguide.net/node/view/356313;
-- Eriophora: https://www.sciencesource.com/archive/Tropical-Orbweaver-SS2596916.html;
-- Araneus: https://bugguide.net/node/view/346418.

I think this argues for caution when identifying egg cases that lack the emerald green longitudinal stripe, especially if these cases are white or yellow without any green color. Those could belong to another orbweaver species. Some egg cases are green but do not have a distinct central stripe -- those get identified as Gasteracantha cancriformis pretty frequently (example: https://bugguide.net/node/view/1027070/bgimage). Is Gasteracantha the only orbweaver that produces green silk in the Americas? I'm not sure, but that appears to be the conventional wisdom.

In Hawaii, the situation is unclear because both Gasteracantha cancriformis and Thelacantha brevispina have been introduced onto the islands (http://www.extento.hawaii.edu/kbase/urban/Site/spinespi.htm). Thelacantha is related to Gasteracantha, and I'm not sure how to distinguish the egg cases of the two species. Here are the current examples in iNaturalist (and I won't put much stock in the IDs, which seem to be made without much evidence): https://www.inaturalist.org/observations?place_id=11&project_id=spiny-orbweavers-gasteracantha-and-kin&subview=grid&term_id=1&term_value_id=7&verifiable=any. Perhaps those of you in Hawaii or within the native range of Thelacantha know how to tell the difference?

Similar egg cases are recorded in Asia, where multiple Gasteracantha species (plus Thelacantha and related genera) live in sympatry. I have no idea how to identify these to species; perhaps some of you who live there do know. Here's an example from @wallacechen in Taiwan:

How can you help advance our knowledge on these questions? Here are some ideas:

  • If you live in a place with one or more of these species, document what's happening in as much detail as you can and submit your observations to iNaturalist.
  • If you come across an observation like this on iNaturalist, please identify it as best you can and mark Life Stage: Egg in the observation annotations section so that it can be more easily found in the future. Feel free to tag someone for help, too.
  • If you have additional information from papers, field guides, or other sources, please post a comment below!

Tagging a few of you who may want to bookmark this to link in your future IDs: @jgw_atx @joemdo @chuuuuung @wildcarrot @botanicaltreasures @lisa_bennett @claggy @tiwane @michelotto @tigerbb

Posted on March 29, 2020 10:19 PM by djringer djringer

Comments

Great post! Learned something new today 😀

Posted by claggy about 4 years ago

How fabulous they are! I have not seen any yet, but now that I know what they look like, I will be searching for them in the parts of the world where the species occur.

Thanks so much David!

Posted by susanhewitt about 4 years ago

Nice post, thanks for writing it up!

Posted by chuuuuung about 4 years ago

Thanks! You just solved one of my mysteries. Hooray! I thought it was some kind of butterfly or moth cocoon I was unfamiliar with. I just changed the ID so it is now in the project.

Posted by botanicaltreasures about 4 years ago

Awesome info and great to request that annotation! I recommend the same for annotating butterfly/moth observations to help us get data on when they go through their stages and it pops up in a neat chart on the species page for the organism in question. I was assuming all of the yellow ones were the spinybacked orbweaver so now I know to be more cautious when adding identifications.
Well done!!

Posted by joemdo about 4 years ago

I saw just one time that eggcase in 2018, I've take a long time to know of what araneid it was.

https://www.inaturalist.org/observations/15491787

Posted by michelotto about 4 years ago

Thank you for using my photo :)

Posted by casseljs almost 4 years ago

This is great, thank you! A friend has found several. I have yet to see one.

Posted by ashley_bradford 6 months ago

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