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Word of the Week: Rhabdom

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Welcome to Word of the Week! Stay tuned for a new word each Friday to amp up your nature vocabulary!

Rhabdom [rhab-dom] (noun): a transparent, rodlike structure found in the eyes of arthropods.

Arthropods (insects, crustaceans, and millipedes) have compound eyes, which contain units called ommatidia. The rhabdom is located in the ommatidia, beneath the cornea, and is similar in function to the retina in human eyes. In the eyes of butterflies, there are multiple ommatidia, and each contains nine photoreceptor cells and pigment cells.

Red admiral butterflies lack a filtering pigment surrounding the rhabdom, which differentiates their colour visions from other butterfly species. This means that red admirals have a limited range of the visible spectrum, which limits their vision to lower range colours such as indigo, blue, green, and yellow. This means that they cannot differentiate between colours in the higher range of the spectrum such as orange and red.

The uniqueness of vision between butterfly species hints that red admirals likely evolved before other butterflies with a wider range of colour vision.

Look out for red admirals on your next visit to High Park!