Spiders usually have eight eyes, each with a single lens rather than multiple units as in the compound eyes of insects. The specific arrangement of the eyes is one of the features used in classifying different species. Most species of the Haplogynae have six eyes, although some have eight (Plectreuridae), four (e.g., Tetrablemma) or even two (most Caponiidae). Sometimes one pair of eyes is better developed than the rest, or even, in some cave species, there are no eyes at all. Several families of hunting spiders, such as jumping spiders and wolf spiders, have fair to excellent vision. The main pair of eyes in jumping spiders even sees in color.