Structure of the male gonopodium.

In adult swordtail males the anal fin forms a gonopodium (bracket, A). A naturally developed gonopodium with typical terminal structures that form distal lepidotrich segments (B) and a testosterone-induced gonopodium (24 days of testosterone treatment) (C); caudal is to the top and ventral to the left (B, C). Arrows indicate terminal structures. Abbreviations: C, claw; RA, ramus; TB, terminal blade; S, spine; TH, terminal hook. Scale bars: 500 µm.