Brown recluse spider
Male and female brown recluses can be identified once the spider has molted several times. Spiders have a set of appendages on their faces called pedipalps. The pedipalps on the male have much larger ends than the ones on females. These fattened pedipalps serve as the sex organs for the male. The female has her sex organs on the underside of her body. This is perhaps the only way to easily tell a male from a female. Scientists who study brown recluses and their relatives can even identify relatives of the brown recluse by studying the shapes of the sex organs.