Thursday, August 23, 2012

Black Widow Spider


There is more than one reason to have you wife do the weeding of the garden. In addition to being lazy, I am always looking for interesting creatures, and she has the knack of finding them. Just two days after finding the nursery web spider with its egg sack, she came upon a whole different type of arachnid mother.
Barb lifted a concrete block along the garden fence and found this black widow guarding its egg sac.  It became fairly agitated when I got within an inch or two with my camera for macro pictures, so I switched to video mode seen here.


There are 32 species of widow spiders  in the Latrodectus genus including the three "black widows" in the US, the northern, southern and western.  They are named for the habit of eating their mates, although other female spiders and some female insects have the same heartless habit.

The common black widow we think of is the southern black widow, Latrodectus mactans.  The females are jet black with  the famous bright red hourglass on the abdomen.  The males may brown bands on their legs and red and white body markings.

The female has a much more potent neurotoxic venom than the male, delivered by unusually large venom glands.  Their bite is usually painless with symptoms frequently developing later, a trait reflected in its name, Latrodectus, Greek translated as “biting in secret”.  Less that one percent of bitten humans die, a comforting thought until you read the symptoms.
"This spider's bite is much feared because its venom is reported to be 15 times stronger than a rattlesnake's. In humans, bites produce muscle aches, nausea, and a paralysis of the diaphragm that can make breathing difficult; however, contrary to popular belief, most people who are bitten suffer no serious damage—let alone death. But bites can be fatal—usually to small children, the elderly, or the infirm. Fortunately, fatalities are fairly rare; the spiders are nonaggressive and bite only in self-defense, such as when someone accidentally sits on them."  nationalgeographic.com
 Like other cobweb weaving spiders of the Theridiidae family, they have two rows with four eyes each.  Unlike others in that family, their lateral eyes are "completely separated".  Similar to the identifying scale pattern under the tail of our venomous snakes, this identifying feature is not of much value to amateur naturalists, because if you can seen their eyes, it is probably to late.

Black widows tend to hide during the day, coming out at night or when something vibrates their tangled, three dimensional web.  They frequently hang on to their web upside down as seen in our picture above.

An interesting cousin of the black widow is the brown widow, Latrodectus geometricus which was found in Africa and the US around the same time.  They are a lighter color and have an orange hourglass on their abdomen.  A unique identifying feature is the distinctive egg sac which is covered with little spiky points.


Brown widow egg sac- Wikimedia
There is considerable debate about the toxicity of L. geometricus  venom.  Apparently is tends to remain around the bite rather than spreading, and their venom doses are smaller.

A recent report quoted in Sciencedaily suggest that the brown recluse is outcompeting its black cousin in Southern California.  The first US documentation was in Florida in 1935, and they weren't found in California until 2003.  Now they are much more common, found in areas previously harboring black widows.  This is good news only if you are fated to be bitten by one of the widows.

Addendum- June 29, 2013
The latest black widow we found was carrying an egg sac.  This gave me the opportunity to dissect the sac to see the eggs.  I was surprised to see the eggs roll out freely like a bag of marbles onto the table.  Unfortunately it was the kitchen table.  Note to self: open egg sacs outside.
Black widow egg sac
Eggs in case


David Quammen's essay The Face of a Spider has interesting thoughts on the relative value of a venomous species.  

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