IT must be well known that the lantern of a firefly, teased by being held with the fingers, emits irregular flashes from minute point-sources in the lantern which follow the normal flash, the light of which fills the entire lantern. I am wondering, however, if the very astonishing spectacle presented by a fly which has been bitten by a spider has been described. Within a few minutes after a few ‘nips’ have been made by the spider, if the fly is removed from the web, the lantern will be observed to glow with a faint green light which shows the same ‘shimmering’ quality as the radium paint on the dial of a watch. Examination with an ordinary hand magnifying glass, of moderate power, shows the lantern filled with bright points of light, which at first sight appear to be in rapid vibratory motion, and at the end of half an hour we have a seething cauldron of hundreds of bright points, dancing and flashing, and giving an almost perfect picture of a screen of zinc sulphide under a terrific bombardment of ‘alpha’ particles (spinthariscope). The phenomenon persists with undiminished intensity for forty-eight hours but finally dies out, the fly apparently having returned to normal. The spinthariscope effect is frequently accompanied by an occasional normal flash, which ceases if the fly is decapitated, though the other effect continues. The normal flash of the lantern is obviously due to the simultaneous excitation of all the luminous centres by nerve impulses from the brain, but the scintillations persist even after the lantern has been severed from the body. Both types of illumination cease if the fly is placed in a narrow glass tube through which a stream of carbon dioxide is flowing. I have produced the spinthariscope effect by innoculations with 1:1000 solution of snake venom, but in this case the fly did not recover.