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North America has six known species in the Genus Latrodectus. Five of them are present in the continental United States. Most of them are complex and tricky to identify, but others are pronounced. The four most common species are named as shown: Southern Black Widow (Latrodectus Mactans), Northern Black Widow (Latrodectus Variolus), Western Black Widow (Latrodectus Hesperus), and the Mexican species Latrodectus Occidentalis; Latrodectus breaks down taxonomically into approximately 31 recognized species, five of which are found in the United States; four species are native, one species (L. geometricus) was introduced.

Latrodectus hesperus - hourglass primarily complete, but it can vary a lot, from complete hourglass to broken to only one triangle to no hourglass at all, but it is likely that hesperus is actually more than one species (we will see where that lands once the revision of the genus is published). Anterior and posterior halves often proportional, posterior half (next to spinnerets) equal to but never wider than anterior half when hourglass complete; the top of the hourglass is equal to the middle never bigger when the hourglass is complete.

Latrodectus mactans - Hourglass primarily complete. Some papers and sites may mention that the hourglass can be complete, split, partial, or absent, but this is possibly because of the tricky Taxonomic time when North American widows were lumped together in L. mactans and period after where no one was completely sure what they wee looking at. Posterior half (next to spinnerets) always wider than anterior half when complete; top of the hourglass is always larger than the middle never equal or smaller.

Latrodectus variolus - Hourglass often split in middle, with both halves or one half prominent to reduced, or hourglass is partial (anterior or posterior half absent) with either posterior or anterior half prominent or reduced, or hourglass is completely absent. Same problem as mentioned with L. mactans, still unsure to whether they can have complete hourglasses. Adult males show same hourglass variations as adult females.

The rest of the species can be somewhat easily identified

Latrodectus geometricus - Hourglass variation in females, with the exception of color, ranging from red to orange. Male hourglasses do show variation in color and shape, but hourglass unnecessary for aid with ID. Egg sacks are "spikey" unlike all other Latrodectus species. May have variations of the color orange on the abdomen, but never dark red on the longitudinal stripe.

Latrodectus bishopi - Hourglass primarily partial or absent, sometimes split*, rarely complete. Adult males show same hourglass variations as adult females. As the common name suggests (red widow) the legs and head are light red.

Latrodectus Occidentalis - Hourglass primarily complete, but can be split. May be partial or non-present as well, but the certainty on that is unknown. Sides of abdomen have red stripes bordered with white, characteristic of this species.

Factors of identification are pretty much uncertain with varying degrees of reliability. I have compiled a guide with all of the known methods and possible methods of identification located here: https://www.inaturalist.org/guides/18700?view=card

Important Reminder: Even professionals are unsure about what actually are marks for identification as contradicting information is constantly brought in. If multiple marks coincide than it is probably that species, but if you only have one mark and it seems off it could easily be wrong. Use each and every mark in a mind-set of uncertainty until a large amount of the marks lead towards one species.

Important Reminder No. 2: This guide is not an infallible resource. Please remember to check the sources cited and do other background research about Widow spiders!

Sources Cited:

Kaston, B. J. (1970). Comparative biology of American black widow spiders. Transactions of the San Diego Society of Natural History

Genus Latrodectus - Widow Spiders, BugGuide.net

When adding observations of tracks are signs of animals always include measurements of the tracks.

Once you've added a photo of the track (hind and front if possible) with a measurement is their anything else left you should add? Yes, measurements of the stride and the particular gait that the animal was using (a photo of multiple strides). Next, a description of the habitat and what you think the animal was doing is always helpful. Lastly, with tracks sometimes you need to resort to guesswork or perhaps a better way to put would be a description of what the animal was likely doing based on the evidence present (and a little bit of filling in the gaps). Anyone can do this, beginner or experienced. For example, if you saw some smaller bird tracks and they were spaced, but it didn't look like they were walking, they were probably hopping, then you look at the surroundings and you see the ground was picked apart with bill impressions you would come to the conclusion that a smaller bird came looking for food within the ground. Now, you might ask why is that important? Well, behavior is a major part in differentiating tracks as some species can have almost identical tracks, but completely different behaviors. One somewhat common bird that is important to include behavior for is a Tri-colored Heron. Their "jog" is a dead give-away.

I have added an observation with the most basic things you should include.

You can always add more!

Quick FAQ:

Question 3: I have an observation of a species with at least limited data, but I'm not sure if nothing has been research about it?

That's ok! As long as the species has little to no data it is fine. A good scale for a species can only be created with a large data pool!

Question 2: How do I know what species to add?

After checking various guides, websites, and discussing with other trackers about the species and you still can't find any data/measurements on it then you can probably assume that's the case. If not it almost surely at least has a small data pool of measurements.

Question 3: I don't know how to find animals to add to the projects. What ones should I look out for?

As mentioned in the answer of question two you can get a good idea of what animals don't have much research on their tracks. Check some of the common species in your area! Especially reptiles and amphibians. It very unlikely that you won't find anything, but if it so happens that you do take a look through the other observations in the project. In the near future I hope to keep the project updated w/ species specifically to look out for!