// Twitter Cards // Prexisting Head The Biologist Is In: Mystery of the Amazon (2/2)

Saturday, December 21, 2013

Mystery of the Amazon (2/2)

This mysterious biological structure turned up in the Amazon in the summer of this year. It was discovered by grad student Troy S. Alexander, while he was working in Peru. He posted some of his photos (at left) to Reddit, hoping that someone would be able to help him identify the maker of the structures… but it turned out that nobody on the whole of the internet-aware Earth had seen such a thing. Well… almost nobody. Another Reddit user (Julgr) had photographed (at right) the same structure on the far side of the Amazon in French Guiana.
The best guess that the internet community came to was that the maker was an unknown Cribellate spider and the structure was to protect its eggs.



The widely spaced, though sparse reporting, suggests the spider is widespread in the Amazon region.
The Cribellate spiders fall into 21 families. By filtering for families found in the Amazon, and then for those with maternal behavior which amounts to finding a good place to leave their eggs… I was able to reduce the number of likely families responsible for this structure to the Deinopidae and the Dictynidae.
The Deinopidae are typically long, stick-like spiders. This body form doesn't seem consistent with the size and shape of the structure, so I favor the idea that the creator is a spider from the family Dictynidae.
The size of the structure suggests it was 'designed' to discourage predation by ants, a very common predator of baby spiders.


A research expedition to identify the creature has recently returned from Peru. They found numerous examples to study.

Roughly half of the examples were found on Cecropia trees. Cecropia are a specialized ant-pant, where the tree provides food and homes for the specialized partner ants. The ants' part of the bargain is to keep plant-eating insects (and light-stealing plants) under control.

After routinely checking on the structures they found, three spiderlings were found to have appeared (one per corral). It can be very difficult to identify the adult species from the appearance of spiderlings, as they have lots of growing to do an often will change color or shape along the way.
The researchers posit that it might be a jumping spider (Salticidae family), as it has a prominent pair of forward-facing eyes, even though the eye placement isn't quite right for the jumping spiders.

The Dictynidae (and Deinopidae) also have a pair of prominent forward-pointing eyes. (Example at left : Chorizomma subterraneum) The researchers did find an adult spider (photo at right) near some of the structures. Although the posted photos don't show much detail, this spider is broadly consistent with the Dictynidae.
I await further identification of the spiders, both adults and juveniles, that were found.



An interesting observation made by the researchers is that the central spires appear to contain two eggs. The eggs differ in color, with one being more white and the other being more yellow.
The difference in color of the two eggs and the final hatching of only one spiderling suggests that one of the eggs may have been sterile and would have been used as food by the just-hatched baby.


Part 2