This word was new to me today. I came across it because my brother said he'd seen a Jersey Tiger mother. This species was once rare in Britain, apart from the Channel Islands (hence the name), but in recent years has expanded its range, being seen in Southern England, then London and now in Bedfordshire, Hertfordshire, Cambridgeshire (where I live). See here for a map of its current distribution. There have been a couple of sightings in my garden but I didn't get decent photos, so here is one from Wikimedia.
Back to cathemeral (say cathy-mertal, with stress on the first syllable). According to the Butterfly Conservation website the Jersey Tiger is cathemeral, which means is active both in the day and at night. The etymology is from Ancient Greek kata-hemera, which means something like "throughout the daily cycle". The hemera element is also seen in ephemeral, "on the day" which applies to something fleeting, or which lives but for a day, like a mayfly. (A Greek daily newspaper is an "ephemeritha").
Animals are frequently classified into diurnal or nocturnal, but many don't fit neatly into either category and cathemerality seems to have become more recognised recently. Here's the ngram for cathemeral.
Picture credit: https://commons.wikimedia.org/wiki/File:Jersey_Tiger_Moth_(14903942490).jpg
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