This is a pedipalp. More commonly referred to as a spider penis. This is a rare image of the erection of a orb weaving spider. The transparent bulbs are similar to hydrolic pumps. The structure extends and swells inside the female and delivers sperm into her epigynum. To charge his pedipalp, the male spider must first ejaculate onto a special web and suck it up into this organ that is attached to his front legs. Males have two of these, which is convenient if he must break it off and run for his life to avoid being consumed by the female who is usually three times his size.
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