Because animals are built so differently, figuring out which has the biggest eyes can be tricky, Sönke Johnsen, a biologist at Duke University, says via email.
Big Eyes in the Ocean
But some tiny marine animals, such as some hyperiid amphipods, clearly come out ahead.
These include Cystisoma, a genus of transparent crustaceans whose giant compound eyes face upward on a head that takes up about a third of its about 18-centimetre long body, Johnsen says.
Paraphromina, another clear amphipod, has unique compound eyes, which look like rows of runway lights that face both upward and sideways. These peepers take up 45 percent of its clear, tiny body, which is only one to two centimetres long.
Clear bodies help these animals to avoid detection, and these enormous upward-facing eyes likely help spot predators swimming above.
At about 2.6 centimetres long, giant ostracods dwarf their smaller cousins, which can be just 0.1 millimetres long. The retinas of these clear, globe-shaped crustaceans sit in front of a mirrored plate that focuses light, causing some to compare their large eyes to car headlights.mages
This giant ostracod lives at depths of up to roughly 2,000 metres in the North Atlantic Ocean [Image: David Shale, Minden Pictures]
Vampire squid also have a high eye-to-body ratio. The deep-dwelling cephalopods grow up to 0.3 metres long, with eyes about 2.6 centimetres wide. These big eyes are helpful at ocean depths of up to 2,500 metres.
Arachnids, Insects, and Tarsiers
Michael F. Land, co-author of the book Animal Eyes, agrees that since so many animals have large eyes, finding the biggest relative to size is a tall order.
But some nocturnal spiders, such as ogre-faced spiders, "come pretty high on the list," Land, a neurobiologist at the University of Sussex, says via email.
Of their eight eyes, the two largest, front-facing ones are enormous, helping the arachnids in their night-time quest for prey.
Among insects, dragonfly eyes "are probably the biggest, with 30,000 lenses per eye" that collect overlapping images from every angle, says Katy Prudic, an entomologist at the University of Arizona.
The rufous net-casting spider of Australia uses its two giant eyes to find prey at night [Image: Kazuo Unno, Minden Pictures]
And yes, there are some big-eyed animals of the cute-and-cuddly variety.
Tarsiers, tiny nocturnal primates from Southeast Asia, are often cited as having huge eyes for their body size, with a body length of 10 centimetres and an eye width of 16 millimetres.
“One eye is larger than their brain,” says Rafe Brown, a herpetologist at the University of Kansas Biodiversity Institute, says via email.
Unlike humans, their eyes are fixed in their sockets, so “they swivel their heads all the way around to look behind them” 180 degrees, like a rain forest version of The Exorcist.
If we were this cute, we'd be taking selfies all day.