Mid summer, Banchory, a still and warm late evening; the sun has set and in the afterglow, the moon is rising. Suddenly, a small, ghostly presence appears from the vegetation, for all the world like a fairy.
As I watch, it hovers as if suspended on an elastic thread, slowly rising and falling and swaying from side to side. Then three, four more come together and chase one another briefly before returning to hover as well-separated individuals.
This was the start of a fascinating few evenings observing the aptly-named Ghost Moth, of which I knew nothing previously other than that it is a ‘lekking’ species, that is to say, the males display communally and by doing so, attract females. I had previously studied Capercaillie and Black Grouse and was familiar with such behaviour, so I was intrigued to learn more about a lekking insect. I was also hoping to photograph it.
My problem was where to find them and what time they would start to display. I looked around in late June for rough grassland that might be suitable, and after a few frustrating evenings with no sightings eventually came across a patch of mixed tall sorrel, cow parsley and coarse grasses not far from home. To my relief a male rose from it just after half past ten and started hovering.
The male’s wingspan is about two inches and the upper wing surface appears to be a brilliant white though the lower surface is very dark. It was therefore extremely conspicuous in the fading light, which is not surprising since it has been shown that the individual scales on the upper wing have a complex internal structure which reflects the dimmest of light, rather like a cat’s eye.
When it was hovering, it was clear that the fore- and hind wings were parted and apparently beating semi-independently. The Ghost Moth is a very primitive species and has only a simple hook which links the fore and hind wings when at rest, but which easily unlocks them in flight. This allows for the curious wing action while hovering.
By quarter past eleven the display stopped as suddenly as it had begun. I returned the following evening with camera and flash in an attempt to take pictures – quite a challenge as it was not possible to focus manually because I could barely see the moth in the viewfinder! With the camera set to automatic, however, and pointing at the moving target, I did manage to obtain a reasonable picture. As is so often the case, it showed features that were quite impossible to see with the naked eye. This display has very seldom been photographed in such detail and I know of no other colour photo.
The body is held in an exaggerated posture at about sixty degrees and there is a small cluster of orange brushes attached to the male’s hind legs. It seems that when the moth is hovering these brushes are lowered and release a scent, or pheromone. The curious wing action now began to make more sense. The forewings probably keep the male aloft, but the hind wings may also act like paddles and waft the scent so that it can be better detected by a female, or perhaps envelope it and so serve as a marker to separate displaying males.
The adult moths have no mouth parts and soon die. All the feeding is done by the larvae while underground where for one or more years they eat into roots and tubers and, when in large numbers, can be an agricultural pest.
Although the males release a pheromone, females do not have sophisticated antennae to detect it at great distance. The reverse is the case with the Emperor Moth; in that moorland species it is the female that releases a pheromone which the male, with his very complex and sensitive antennae, may detect from two to three kilometres away.
The female Ghost Moth seems to be attracted to a male initially by sight. She is rather larger, yellow ochre in colour with dull brown markings. It has been suggested that female flies through the lek and knocks a desired male to the ground where mating takes place. This may be so, but I have not seen such behaviour. In my experience the female lands near one or more displaying males. If there is only one male, they pair up quickly, but if there is more than one there is quite a tussle for her.
Once coupled, the female clings vertically to a stem and the male hangs attached beneath her so preventing any other male from mating with her. They remain like this until after dark. The female does not lay eggs on the vegetation like most butterflies and moths, but releases them in flight. When the caterpillars hatch they burrow into the earth where they feed and eventually pupate.
This display raises a few questions, not the least of which is how does such a conspicuous species avoid being eaten by predators? The main method seems to be the very limited time during which they display. The males rise from the vegetation when the light reaches a critical low level at dusk, which may be just before most bats emerge. The display usually ends forty or so minutes later almost as suddenly as it begins.
A female would be able to see a displaying male from some distance and I have anecdotal evidence that they may actually fly around looking for displaying males. Once located, they may then be further stimulated by the pheromones dispersed from the males’ scent brushes. The lack of sophisticated antennae suggests that scent becomes important only at close range.
There is a final twist to the tale; the Ghost Moths on the Orkney Islands are indistinguishable from their mainland counterparts, but those further north in the Shetlands and Faroes are considered to be a separate race, the male usually being darker and more similar in colour to the female. It was once thought that this was an adaptation to the fact that the summer nights further north are never really dark, so there was no great advantage for a displaying male to be highly reflective. While this may be so, more recent studies indicate that the northern race can vary from rather dull ochre specimens to others almost as light as their southern relatives. Perhaps not surprisingly, it has been shown that the lightest forms in these northern islands are where there is least predation, usually by gulls.
So, next time you venture out late on a midsummer’s evening and see a ghostly presence fluttering among the vegetation, it is less likely to be a fairy than a moth with a very interesting life history.
Nicholas Picozzi was an ecologist, latterly with CEH in Banchory for 35 years. Since retiring he has continued to study birds in the North-East and, more recently, the local moths and butterflies. He enjoys wild places, hill-walking, travelling and photography.Tweet
This is an article from the November 2010 edition of Leopard Magazine. To read much more like this every month, subscribe to Leopard Magazine.