**Originally published on the Texas EEB Club blog.**
Selenopids. Ever hear of them? Probably not. I had never heard of them until spring 2021, when I was applying for my REU at California Academy of Sciences. I worked there for 9 weeks and I’m still working on my project I started there, so let me tell you about these funky little guys.
Selenopidae is a family of spiders consisting of 9 genera and roughly 200 species. A selenopid is a spider (or species of spider) that belongs to the family Selenopidae. Selenops is the largest genus of selenopids and members of this genus are found worldwide. Some of their common names include: flatties, wall crab spiders, and wall spiders. Like one of their common names suggests, they are flat!
They are found in tropical and subtropical regions around the world, and the species that occur in the United States are found in the deserts of southwestern North America in Utah, Arizona, West Texas, Baja California, and Northwest Mexico.
One of my favorite facts about them is that they don’t build webs, and instead they catch their prey and need to be fast. Because of this, they’ve evolved the fastest turning strike in the animal kingdom. In fractions of a second, they detect movement of prey behind them, rotate themselves by pivoting on 3 of their 8 feet, and strike. Their prey have no time to process what hit them!
The Selenops debilis group is one species group of selenopids. There are 8 nominal species in this group, and I am looking at 3 of these that occur in the North American desert southwest region: S. debilis, S. nesophilus, and S. actophilus.
Most arthropods are differentiated by their genitalia, and selenopids are no exception. But, there’s a big problem. There’s A LOT of variation between individuals of one species, making it nearly impossible to actually differentiate S. debilis group spiders by morphology alone.
Type specimens are specimens of a species that represent that species, and the female and male type specimens of S. debilis were found in San Jose del Cabo, Baja California and described by Banks in 1898. Later, the S. nesophilus type specimens were collected from the same place and S. actophilus types were collected from very nearby locations.
The type specimens’ genitalia for S. debilis, S. actophilus, and S. nesophilus look like this:
Holotypes are the most formal type specimens and are the individuals originally based upon the species was named, and when Muma revised the S. debilis group in 1953, the male holotype was lost, so Muma designated a neotype (a specimen chosen to be the holotype when the holotype is destroyed or missing) from Arizona based on form and coloration. BAD BAD MUMA – This neotype was found hundreds of miles away from the original type locality and within range of another species! This problem combined with the genitalic variation made things even more complicated for understanding species differentiation!
Blah blah blah ok so now you must be thinking, I read spider genitalia in the title, when is he ACTUALLY gonna talk about spider genitalia? Now. Now I’m gonna talk about spider genitalia.
You might have heard of pedipalps (shortened to “palps”), the “limbs” near the mouth of the spider used to taste and smell. Well in sexually mature males, the palps have an extra structure on the end of the tarsus (the last segment of the palp furthest away from the body), known as the palpal bulb. This palpal bulb is responsible for transferring semen, and the structures making up this vary in size and shape, making up the genitalic variation I’ve mentioned.
Selenopidae is a part of a clade of spiders known as the RTA clade, which consists of nearly half of all known spider species and 39 total families, including wolf spiders, jumping spiders, and crab spiders. The main synapomorphy of this clade is the presence of the retrolateral tibial apophysis (RTA) on the tibia of the pedipalp. RTA shapes are very diverse and there are many overlapping characteristics and “morphs” in the S. debilis group. A consistent difference in the RTA is the dorsal section, and specifically this point at the edge of the RTA varies in shape a lot.
In female genitalia, there is even more variation than in males. Their reproductive organs are located around the middle on the underside of their abdomen (not anywhere near the spinnerets or pedipalp! They differ in the spermatheca, the posterodorsal fold (PDF), and externally. Their spermathecae are all different proportions and some are twisted. Their PDFs are sometimes curved differently at the outer edges.
Because there are so many different shapes within and between species, my advisor Sarah Crews is trying to group them in little morphs to help us group genitalia shapes in our specimens. We’re still working on if this has anything to do with their geography but this is very hard to do and everything is still very fuzzy.
References 🙂
Crews, SC (2011) A revision of the spider genus Selenops Latreille, 1819 (Arachnida, Araneae, Selenopidae) in North America, Central America and the Caribbean. ZooKeys 105: 1–182. doi:10.3897/zookeys.105.724
Dippenaar-Schoeman, A. S. and Joqué, R. (1997) biodiversityexplorer.info/arachnids/spiders/anatomy.htm
Encyclopædia Britannica (2012) “Internal organization of a spider” britannica.com/animal/arachnid/External-features#/media/1/31791/47088
Muma, MH (1953) A study of the spider family Selenopidae in North America, Central America, and the West Indies. digitallibrary.amnh.org/handle/2246/4881
