Why do nocturnal and diurnal snakes have different eyes? Nocturnal snakes are active at night. Diurnal snakes are active during the day.

Snakes that are mainly nocturnal have eyes with vertical pupils (like the eyes of a cat). This includes the Puff Adder. 

Snakes that are mostly diurnal have eyes with round pupils (like the eyes of a human). Many snakes are diurnal.

Puff Adder (mostly nocturnal)

Snakes are not necessarily completely nocturnal (active at night) or completey diurnal (active during the day). They are usually categorized as mainly nocturnal or mainly diurnal.

Boomslang Snake (diurnal)

Photographer: Martina Nicolls

Martina Nicolls: SIMILAR BUT DIFFERENT IN THE ANIMAL KINGDOM