WaTCH OUT. Break no mirrors. Walk under no ladders. The emblem of death has been seen again.  The death's-head hawkmoth, Europe's most infamous insect, which bears the likeness of a skull upon its back, has turned up in England as part of the biggest influx of continental moths for many years. Several specimens have been spotted along the south coast.  Last weekend's extraordinary hot, caused by a mass of warm air surging up from southern Europe, brought with it hundreds of moths of numerous rare species from France, Spain and even the Mediterranean.  The most extraordinary of these is acherontia atropos, named in Latin after a river in Hades over which the souls of the dead were ferried and one of the three goddesses, or Fates, who spun the thread of human destiny.  With wings as big as a bat's, and a body the size of a shrew's, the death's-head hawkmoth has three distinctive characteristics: when disturbed it makes a noise, a mouse-like squeaking through its proboscis (no other moth does makes a sound in this way); it invades beehives with impunity and feeds on the honey; and it carries that amazing likeness of a human skull prominently upon its thorax.  For centuries, it has been viewed