The Brown Recluse (Loxosceles Reclusa) or Violin Spider is one of the most venomous spiders in the world. They usually range from about 6-20mm (1/4-3/4 of an inch) long, but can grow larger, and have a wide range of colors from light brown to cream to blackish grey.
Though the violin patterning is iconic, a few other spiders share it. So the most efficient way to identify a recluse is to examine it's eyes. Most spiders have eight, the recluse has six. Arranged in pairs they posses one median pair and two lateral pairs. No other clearly unique markings exist as they have fairly common coloring on their abdomen and legs.
What else to look out for if you're living in Recluse Territory (Southeastern Nebraska through Iowa, Illinois, and Indiana or Central Texas to Western Georgia) are its asymmetrical webs in garages, cellars and wood piles. They are also quite fond of cardboard as it mimics their natural rotting tree bark habitat.If you've had the personal displeasure of being bitten by one of these, you know that the bite doesn't hurt at first then it grows itchy and bothersome and eventually spreads. It's no joke. Contact a doctor immediately as these little guys can be fatal, (but to be honest usually keep to themselves) and take a look at these pics so you know what to look out for.