Piggybacking on yesterday’s wolf spider, I thought I would put up another sharp-eyed beauty and give a brief lesson about spider eyes and how their arrangement can aid in identifications. Most spiders have eight eyes to go with their eight legs. Conceptually, it’s probably easier to think of them as two rows of four eyes (even though they don’t always line up neatly) from a face-to-face perspective. In the Phidippus audax jumping spider shown above, we can distinctly see the anterior (i.e. “bottom”) eyes; the two eyes in the middle are the median eyes and the ones on the end are the lateral eyes. The posterior eyes make up the top row, also consisting of two median and two lateral eyes. Abbreviations such as “AME” (anterior median eyes) or “PLE” (posterior lateral eyes), etc., are used as shorthand.
Different kinds of spiders have differently-sized and -arranged eyes. The jumping spider’s large pair of anterior median eyes (AME) and fairly large ALE with the posterior eyes mostly or all out of view in a facing perspective clearly distinguish it from other kinds of spiders. Yesterday’s wolf spider also had two relatively large if not quite as impressive eyes, but if you just make a quick note of the rows, you can see they are the PME.
As with most arthropod questions, if they are of importance, it is best to consult an expert, but spider eye arrangements are a nice way of learning how to differentiate spiders (mostly to the Family level). Plus I think they have the benefit of making spiders a little less creepy (if you have that problem to begin with </wink>) — it helps to get to know your neighbors, right?! I highly recommend the very well done Spider Eye Arrangement page at BugGuide.net, which features diagrams of different kinds of arrangements alongside photos of example spiders.