Identifying common orbweavers from the underside - Wiki

This is an experiment with a tutorial that can be edited by anyone. It’s also intended to be the kind of thing discussed here. Sometime in the future (May 2019 at the earliest) iNaturalist will have a place for content like this on taxon pages. When that time comes, I will transfer all the content in this tutorial over to iNaturalist, so you needn’t fear losing any work you put into this.

Most often spiders are photographed from above, but observations of orbweavers often only have a photo of the underside of a spider on its web. This is a tutorial about how to identify common orbweavers from the underside.

Vocabulary


Photo © Mark Nofsinger

Note: starting off with orbweaving spiders common in the U.S. and Canada. Feel free to add species, edit existing species, reorganize, and modify or delete this comment.

Araneidae (Orbweavers)

Araneus ( Angulate and Roundshouldered Orbweavers)

Araneus diadimatus (Cross Orbweaver)


Photo © Joe Bartok

Among the most commonly observed orbweavers in North America. Introduced from Europe. White marks bracketing underside of abdomen. Marks straighter, more angular than Larinioides. Black inside the overhang of the bracket marks, dark brown between. In poor light the dark brown may be indistinguishable from the black. Sternum dark, unlike Neoscona.

Araneus marmoreus (Marbled Orbweaver)


Photo © peggyo

Yellow marks bracketing underside of abdomen. Legs orange closer to body. Strong color variations in this species; some are all black-and-white instead of yellow, orange, and black, which can make identification difficult.

Araneus nordmanni (Nordmann’s Orbweaver)


Photo © psweet

Underside of abdomen has two white dots on a black background.

Araneus trifolium (Shamrock Orbweaver)


Photo © LJ Moore-McClelland

Adults have a dark reddish underside with no bracketing marks. (Juveniles different?)

Argiope (Garden Orbweavers)

Argiope aurantia (Yellow Garden Spider)


Photo © Erin Faulkner

Legs usually solid black at tips, solid brown near body (compare with banded A. trifasciata), but sometimes have banded legs. Underside of abdomen difficult to distinguish from A. trifasciata, but epigyne may be distinguishable? Sternum has a single solid yellow stripe, with no distinct lobes or separate yellow marks.

Argiope trifasciata (Banded Garden Spider)


Photo © aarongunnar

Legs always banded, never bicolored like A. aurantia. Underside of abdomen difficult to distinguish from A. aurantia, but epigyne may be distinguishable?. Sternum has a large central yellow stripe like A. aurantia, and sometimes distinct lobes or separate yellow marks in pairs on either side of the central stripe, unlike A. aurantia. It can be worth the effort to get a picture of Argiope sp. from another angle, since the top and sides of the abdomen are instantly identifiable.

Larinioides

Larinioides cornutus (Furrow Orbweaver)


Photo © Dan Toth

White marks bracketing the underside of the abdomen are more curved than on Araneus. Underside of abdomen black throughout, unlike Araneus diadimatus. Sternum dark, unlike Neoscona. Very difficult to distinguish from other species in the same genus.

Larinioides sclopetarius (Grey Cross Spider)


Photo © Jeremy Hussell

White marks bracketing the underside of the abdomen are more curved than on Araneus. Underside of abdomen black throughout, unlike Araneus diadimatus. Sternum dark, unlike Neoscona. Very difficult to distinguish from other species in the same genus, especially L. patagiatus. However, Grey Cross Spiders are much more common on buildings than the other species. E.g., they’ve been photographed on webs on the outside of windows 50+ stories above ground on skyscrapers in Chicago.

Mangora


Photo © Matt Claghorn

Mangora are tiny and mostly found in wooded areas, and are thus less commonly photographed than Araneus, Larinioides, or Neoscona, but are quite common once you know where to look. With the exception of M. placida, they have a green sternum. All have relatively large hairs or tufts on their legs. The center of their web, where the spider rests, has a dense net with lots of small cells instead of the fewer larger cells in most species’ webs.

Mangora gibberosa (Lined Orbweaver)


Photo © Joe Bartok

Sometimes (but not always), creates an opaque circle or disc of silk to rest on. Web otherwise similar to other Mangora. Green sternum unlike M. placida. Black lines on undersides of legs close to the body, unlike M. maculata and M. spiculata.

Mangora placida (Tuft-legged Orbweaver)


Photo © jawinget

Brown and black on the underside where other Mangora are green. Size, tufted legs, and web similar to other Mangora.

Neoscona (Spotted Orbweavers)

Neoscona arabesca (Arabesque Orbweaver)


Photo © Julie Filiberti

Pair of white marks on either side of underside of abdomen. Dim white marks on underside of abdomen towards head, unlike N. crucifera. Sternum pale surrounded by dark, unlike Araneus and Larinioides.

Neoscona crucifera (Spotted Orbweaver)


Photo © Even Dankowicz

White marks bracketing underside of abdomen often resemble the bottom half of square brackets, with a well-defined, angular corner. Upper half of brackets usually connected to the lower half unlike N. oaxacensis, and the part nearer to the head is usually about the same brightness as the part further away, unlike N. arabesca. Sternum pale surrounded by dark, unlike entirely dark Araneus and Larinioides.

Neoscona domiciliorum (Red-femured Spotted Orbweaver)


Photo © Joe Walewski

Found in the south-eastern US. Femurs (the parts of the legs nearest the body) are bright red. White marks bracketing underside of abdomen similar to N. crucifera, but much more often detached from each other. Sternum pale surrounded by dark, unlike entirely dark Araneus and Neoscona.

Neoscona oaxacensis (Western Spotted Orbweaver)


Photo © Tony Iwane

Found in the west coast and southwestern states of the U.S. and in almost all of Mexico. White marks bracketing underside of abdomen similar to N. crucifera, but much more often detached from each other. Sternum pale surrounded by dark, unlike entirely dark Araneus and Neoscona. Coloring highly variable, ranging from the very dark body and banded legs in the photo above to very pale.

Tetragnathidae (Long-jawed Orbweavers)

Leucauge

Leucauge venusta (Orchard Orbweaver)


Photo © molanic

Usually makes a horizontal web instead of a vertical one. Orange V-shaped mark on underside of abdomen, point towards head, highly reflective from some angles but not others.

Tetragnatha


Photo © JeremyHussell

Easily identified to genus by the combination of very long, thin legs, long abdomen, and prominent jaws. However, there are at least 15 widespread species in N. America, and there is not much known about how to identify them from photos with the exception of T. viridis (it’s green).

19 Likes

Fantastic post!

2 Likes

Thanks! It’s a work in progress. Additional content would be welcome!

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Orbweavers from above seem to be very variable within a species. Is the view from below generally less variable/more reliable for ID? If I see two such views in observations that differ in the pattern on the abdomen, is it conclusive that they’re not the same species, genus or family?

e.g.,
https://static.inaturalist.org/photos/19305814/original.jpg?1528381535
https://static.inaturalist.org/photos/11521908/original.jpg?1509362896

I would say the ventral view is much less variable, and for the majority of species it will be diagnostic. There will be some species where it isn’t though, so care needs to be taken.

With secondary characters (pretty much anything non-genitalic or eyefield) I would consider them in clusters, where the more you have the more certain you can be of the ID, but without dissection and/or genitalic structures you can never really be 100% certain.

Yes, I’ve generally found the undersides to be less variable than the top of the abdomen in most species. That doesn’t mean there’s no variation: a few species have lots, while the majority seem to have only one or two color morphs. Either way, it can be quite useful at times to have a shot of the underside to confirm the ID. And this guide exists because sometimes a shot of the underside is the only evidence available.

For the two photos you provided, I don’t recognize either of them, I’m afraid. The first one might be an Eustala. I haven’t really studied the undersides of that genus yet.

I think it is all the more useful because most “Spider Books” will show you the dorsal view, and maybe a lateral, but they seldom show you the ventral, and as Jeremy states, ventral is often the only view one can get without disturbing the critter. For what it’s worth, a similar problem exists for lepidoptera, in that adults are well presented in books, but the caterpillars not so much.

Thanks everyone. The two example pictures were both from different RG observations of eustala anastera. I was looking at these to compare to my own observation and found some variability in the ventral view of the RG observations. It looks like I’ll have to look a little deeper before drawing any conclusions.