From HomoExcelsior

n. 1. A sclerotized plate, a special copulatory organ, on the ventral (lower) surface of the abdomen of a spider, in front of the epigastric furrow and between the book lungs. 2. The external cover of the internal reproductive organs, much larger in the mature female; the external foldings of its surface conceal the entrance to the seminal receptacles of the female, and are formed in such a way to fit the complex palpal intromittent structures of the male of the same species. 3. A structure frequently used to classify a female spider to species.

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