Moth of the Moment – the royal succession

Once or twice a year, sometime in March and then again in early April, I put a little rubber bung in one of those little gauze laundry detergent bags and hang it a sunny spot in the garden. The rubber bung arrived from a lepidopterist supplies company  in March 2019 and is covered in a molecule that male Emperor moths (Saturnia pavonia) find irresistible: the female sex pheromone, (6Z,11Z)-hexadeca-6,11-dien-1-yl acetate.

The female moths exude the chemical during the day to attract a mate, and the diurnal males fly from as far away as 10 miles or so to find the nocturnal females. Using the EMP lure gives us amateur lepidopterists a chance to see and to monitor the males in our neighbourhood.

If it’s a warm, sunny day, at least one male turns up within about 20-30 minutes of put out the lure, attracted to this (sadly) dead-end destination as the pheromone diffuses on the Fenland breeze. The males are frantic, zigzagging back and forth and usually abandoning their efforts and flying off within a minute or two. I usually net one so that I can photograph it for my records before putting the lure away for another year, and freeing the moth to find a genuine mate.

It’s a highlight of my scientific moth monitoring, but it always leaves me with one question: in this sea of intensive farmland, where exactly are they coming from? Conventionally, this species lives on the heathery uplands of the country. But, while the females in such terrain will lay eggs on heather for their hatched larvae to eat, in this part of the world, they’re more likely to consider brambles as a foodplant, and we have plenty of those around here.

While my EMP lure can pull them in from 10 miles away, the real “engine rooms” are likely right on our doorstep. The old airfield sites at Oakington and Waterbeach are hidden sanctuaries. They are being rapidly developed, but there are patches of low-nutrient soil and cracked concrete that will bec colonised by scrubby Bramble, Blackthorn, and Sallow, creating great habitat for a lot of insects, including Emperor larvae. These sites, along with the banks of the Old West River and the fenland drains could be acting as local strongholds for this species.

Note in my photo the exquisite detail of the eye spots, or ocelli, scaly markings on the moths wings that have evolved to resemble mammalian eyes. Indeed, the detail of those eye spots is quite remarkable, showing a pupil, an iris, and even a catchlight so that any predator approaching the moth might be perturbed and do a quick U-turn for fear of being eaten themselves by this creature suddenly staring back at them. Of course, with its hindwings hidden, the moth has only two “eyes”, but disturbed it will flash a second pair, just to make sure the predator is frightened away!

A few additional thoughts about the Emperor’s eyes. Those spots don’t just suggest eyes, they look weirdly like human eyes, specifically the ones on the forewings. It’s the oval shape and the areas suggestive of the whites of our eyes, the exposed sclera. But, moths like this were around millennia before humanity and most predator don’t routinely expose their sclera like we do. This nuance is odd, to say the least…