Photo of the Week: Eye See You
This week I was lucky enough to take pictures inside North Carolina Museum of Natural Science's Living Conservatory - a room full of butterflies, and one the the most peaceful and beautiful rooms I've even been in! Using my Canon Rebel T3i, 100mm Macro lens and external flash with Honl light modifier, I captured up-close shots of many of the butterflies in the conservatory.
The shot above shows the 'eye' on the wing of an Owl Butterfly, of the species Caligo memnon. This species, can be found in the rainforests and secondary forests of Mexico down to the Amazon rainforest, is particularly large for a butterfly, with a wingspan up to 150mm.
The Owl butterfly likes to feed on rotting fruit - which is why the butterfly can be found resting on rotting bananas in dishes in the Museum's Conservatory.
But what is up with that large 'eye' on the wing of the Owl Butterfly, for whence this beautiful create earned its name? One could certainly see how the 'eye' would be a deterrent to predators, a threatening feature disguising the fragile butterfly.
Unfortunately, that is about all that I could find out about Caligo memnon from a typical Google search. For more, I had to go to my trusty Google Scholar:
Caligo memnon thrives in shaded forest habitats. As an adult butterfly, it seems to be most active at night, or rather early morning, in order to escape day predators. It often feeds on fruits, such as bananas, that have already been pierced by beetles.
Caligo memnon's apposition compound eyes are more light sensitive (about 4x more sensitive) than the eyes of its close relative the daytime blue morpho Morpho peleides, shown below. The facets on the compound eye of Caligo memnon are larger than the facets on the compound eyes of other butterflies. Larger facets, like larger apertures on an SLR camera, allow more light to pass through than smaller facets do. Caligo memnon also has has a larger eye in general that its cousin butterfly - almost twice as large. The Owl Butterfly has thus evolved to have larger eyes, and larger eye facets, than butterflies that are active mainly during the day.
According to a paper in The Journal of Experimental Biology:
"The crepuscular C. memnon has evolved large eyes with large facets but has retained a reasonably high spatial and temporal resolution. Nonetheless, its vision is 3–4 more sensitive than that in the diurnal M. peleides. The major visual adaptations contributing to this sensitivity difference are the enlarged facets (2 increase), the larger acceptance angles (1.7 increase) and the longer integration time (1.2 increase).
We conclude that the visual systems of crepuscular insects have evolved adaptations that improve visual reliability in dim light. The most important adaptations found in the species we studied are the enlarged facets allowed by a greater total eye size. Our data strongly suggest that the visual systems of insects are sufficiently flexible to evolve to be matched and optimized for a particular intensity window..."
For more on butterflies, visit my recent NC Museum of Natural Sciences blog post, "What butterflies have in common with straws!"