The postorbital bar is a bone which runs around the eyesocket of strepsirrhine primate

Primate

A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

s. This is in contrast to the higher primates, haplorrhine, which have evolved fully enclosed sockets to protect their eyes.

One theory for this evolutionary difference is the relative importance of vision to both orders. As haplorrhines (tarsier

Tarsier

Tarsiers are haplorrhine primates of the genus Tarsius, a genus in the family Tarsiidae, which is itself the lone extant family within the infraorder Tarsiiformes...

s and simian

Simian

The simians are the "higher primates" familiar to most people: the Old World monkeys and apes, including humans, , and the New World monkeys or platyrrhines. Simians tend to be larger than the "lower primates" or prosimians.- Classification and evolution :The simians are split into three groups...

s) tend to be diurnal

Diurnal animal

Diurnality is a plant or animal behavior characterized by activity during the day and sleeping at night.-In animals:Animals that are not diurnal might be nocturnal or crepuscular . Many animal species are diurnal, including many mammals, insects, reptiles and birds...

, and rely heavily on visual input, strepsirrhines are mainly nocturnal and have a decreased reliance on visual input.