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Mainly the prominent furrow - white lips + shape. It's not an exact science though. Most but not all species get gradually easier to ventrally sex when they are over 3"+. At the end of the day, ventral sexing gives you a good idea or a reasonably educated suspicion of the sex, which is confirmed without doubt when you have a moult that you can check.
From my own perspective, to begin with I found ventral sexing completely bewildering. I started practicing by looking at literally 100's of photos etc, until eventually it started to make sense. I'm still very much learning as some genera and species can look very different to others ventrally. Some I find harder than others!
The “mustache” above the epigastric furrow. Between the anterior book lungs there is a “line” that runs across, that’s the epigastric furrow. When you look midway and you see a John Bolton looking mustache, it’s female. However if you see a “dot”, male. It takes practice but once you practice enough, it gets easier. I use a 40x lighted magnifier, you can get them on eBay for $3-4.
The “mustache” above the epigastric furrow. Between the anterior book lungs there is a “line” that runs across, that’s the epigastric furrow. When you look midway and you see a John Bolton looking mustache, it’s female. However if you see a “dot”, male. It takes practice but once you practice enough, it gets easier. I use a 40x lighted magnifier, you can get them on eBay for $3-4.