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.
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.
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 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.
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
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 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.
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).