Yesterday, I wrote about my stumbling efforts to identify a spider, and about the help I'd gotten along the way. Within hours, more help arrived, with enough info on spider anatomy to warrant another post.
Rod Crawford had identified my spider as Steatoda bipunctata. I asked about defining marks; how had he identified her, and did the name refer to the dimples on the upper abdomen? Lynette Schimming forwarded my question to him; he wrote,
There aren't any (defining marks). I identified it from the epigynum. But yes, it's probably named after the "dimples" (which actually most spiders have - they're the apodemes or attachment points for the heart muscles).Time for some definitions:
Epigynum (or epigyne): the female genital opening in spiders. (Wikipedia) Often used to distinguish species (as in this case). Christopher Taylor says (in the comments),
If you look at the front end of the underside of the abdomen, in front of the markings, you can see a dark sclerotised structure.
The epigyne is the black thing up near her waist.
PZ has a nice diagram and an explanation of spider sex, here: Spider Kama Sutra.
Sclerotised: hardened or toughened tissue. (Csiro)
it doesn't get sclerotised like that until they reach maturity (though a non-sclerotised epigyne may be visible in the second-to-last instar).Question for Christopher: how do you know it's hardened from a photo? Does the colour change?
Instar: a developmental stage of arthropods, such as insects, between each moult (ecdysis), until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or assume a new form. Differences between instars can often be seen in altered body proportions or changes in the number of body segments. (Wikipedia)
Apodeme: Ridge-like ingrowth of the exoskeleton of an arthropod that supports internal organs and provides attachment points for muscles. (WordWeb) In this case, it's the heart muscles.
I found it hard to imagine this, but Visual Dictionary Online has a good diagram:
From Visual Dictionary Online.
You can see the points where the heart* (red) attaches to the exoskeleton.
And here are the dimples, on another, very tiny S. bipunctata, I found this summer. (Maybe it's Brownie, as a baby.)
And thanks, all, for your help!
*Next Valentine's Day, I'm going to make my hearts this shape; long and skinny, with spikes.