@boina well, not exactly. When I look at male vs. female undersides, all I can see is that the males have more of a horseshoe shape right about where you circled, but I don't know what I'm *looking* for. I thought that was just the epigastric furrow, what I'd like to know is what makes it that shape in males, and what the fusillae look like, where they are, and how to recognise them. Does that make sense? I don't think I know enough about this stuff to ask specific enough questions, so thank you so much for even giving me the time of day here
Ok, inside that circle is the field of epiandrous fusillae. They are microscopic spinnerets that the male uses to make a pouch in the sperm web where he places his sperm. They are interspaced with dense setae. The actual fusillae are microscopic, meaning you can't see them, you only see the setae around them. The 'horseshoe shape' is actually formed by the surrounding normal setae.
The furrow is the line under this patch. It's the opening of the sexual organs and it's wider in females (obviously, because the eggs need to fit through there in females)