Identifying North America's Widow Spiders

All methods covered will be regarding pattern/coloration.

When Latrodectus variolus, the northern black widow spider is born it is covered in a variety of white and black stripes/spots on the top of abdomen. Latrodectus mactans, the southern black widow is also born with these markings covering the abdomen. To the untrained eye it may seem as if they are a crazy assortment of spots and lines, but in fact a pattern exists! The second spot of the Northern Black Widow is wider than long while the second spot of the Southern Black Widow is actually longer than wide. You can see this in both of the photos linked above. Now, both of the species lose the white as the mature only Northern Black Widows keep the red spots with them as they mature. Southern Black Widows lose them as they mature. This brings us to our second method identification based on the markings on the bottom of their abdomen which looks similar to an hourglass. [Southern Black Widows have a complete hourglass]

(https://www.inaturalist.org/observations/120734645) while Northern Black Widows have a broken or incomplete hourglass. you can clearly see this in both sources linked.

Now that we have covered how to identify Northern and Southern Black Widows L. mactans & L. variolus we need to go over to make sure it isn't Latrodectus Hesperus - The Western Black Widow. L. Hesperus is so closely related to L. Variolus and L. Mactans it was considered a Western Subspecies of L. Mactans until recently. You can tell L. Hesperus apart from L. mactans & L. variolus by following this key:

1. Red Spot Just Above Hourglass (Generally visible from bottom of abdomen) it is L. Mactans not a Western Black Widow. Photo: https://www.inaturalist.org/observations/120734645
2. Red Spots on top of abdomen when fully mature? L. Variolus not a Western Black Widow. Photo: https://www.inaturalist.org/observations/479574

The next tricky matchup is Juvenile Black Widows VS. Brown Widow spiders. For decades Brown Widows only lived in Peninsular Florida, but since the early 21st century they have moved out all across the US crossing the range of other widow spiders making an identification guide necessary! The first and easiest way to identify them is by their egg sacks. Brown Widows have little silk "spikes" coming out of them. Other widow spiders have more smooth round egg sacks Without any egg sacs the best way is to identify them based off of the abdomen. Western and other widow spiders have brighter coloration on the abdomen. They also have red coloration on the abdomen which is something Brown Widows lack. Brown widows have a darker more drab abdomen generally, with no red at all.

This only relevant for adult females of all species.

Finally, we are at the last two widow species in America. Latrodectus Occidentalis and the Red Widow (Latrodectus Bishopi) both are amazingly colored and are unmistakable. The Red Widow is the only American widow species with red legs it is only native to select sand pine areas in Florida where the habitat is present that it needs to survive. Latrodectus Occidentalis a Mexican species has amazingly patterned stripes along the abdomen bordered with white.

Extra Key for ease of use:

1. Complete hourglass, second spot on top of abdomen longer than wide (When immature), Red dot just above hourglass (Only dot visible in maturity) - Southern Black Widow

-. Broken hourglass, second spot on top of abdomen wider than long - Northern Black Widow

-. Complete Hourglass, No other traits from other species when adult - Western Black Widow

-. Spikes on egg sacks, drabber abdomen with no red - Brown Widow

-. General "Red Appearance", red legs, dark black body w/ large red spots, unmistakable - Red Widow

-. Dorsal red stripes bordered by white - Latrodectus occidentalis

Important Notice: The hourglass method can be variable and in rare cases hourglass marking may be complete, split, partial, or absent on both species. The method for the spots may or may not be variable I haven't seen anything regarding it being variable so it likely may not be.

Important Notice: Widow spiders are frequently caught up in packages when moved so it is quite common for them to end up far out of their range. Be careful to not id any of them soley based on location! If the shots are to blurry to id don't identify it just keep it at Genus Latrodectus.

Credits: Nothing was copied from anywhere (besides singular words to insure correct spelling, etc.), but this wouldn't be possible without inat, and bugguide.net was a huge help!

Additional Sources: http://kaston.transy.edu/widow.html This page specifically provides some good general information about Widow identification and their "lifestyle".| https://cisr.ucr.edu/invasive-species/how-identify-brown-widow-spiders is a great page for Brown Widow identification less ↑