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I think we should come clean on this one - only the male in the above photo (the smaller, orange moth with the feathered antennae) is a wild specimen, and it was attracted to a female that my son had hatched from a pupa. I took the female in to the Open University today and a few of us spent our tea break out by the river to see if a male would be attracted. In fact two males were, but the second one didn't hang around for long.
Although it is sometimes relatively easy to find caterpillars of the Emperor Moth, the adults are much more elusive, despite their size, bright colours and day-flying habit of the males. The practice of hatching out females and using them as lures to record the males is a time-honoured tradition among lepidopterists. It's sometimes possible to find males in quite unpromising habitats, and the moth is widespread than one might think.
Thanks to Mike for the photo and rather superb video footage (see link above).
Entomologist and biological recorder
Lat/Lng: 52.02751, -0.71276
OS grid ref: SP884373