Raw and Uncut 2: The Flashes of Fireflies
All the fireflies, which I caught here, belonged to the Lampyridae (in which family the English glowworm is included), and the greater number of specimens were of Lampyris occidentalis. I found that this insect emitted the most brilliant flashes when irritated: in the intervals, the abdominal rings were obscured. The flash was almost co-instantaneous in the two rings, but it was just perceptible first in the anterior one.
The shining matter was fluid and very adhesive: little spots, where the skin had been torn, continued bright with a slight scintillation, whilst the uninjured parts were obscured. When the insect was decapitated the rings remained uninterruptedly bright, but not so brilliant as before: local irritation with a needle always increased the vividness of the light. The rings in one instance retained their luminous property nearly twenty-four hours after the death of the insect.
From these facts it would appear probable, that the animal has only the power of concealing or extinguishing the light for short intervals, and that at other times the display is involuntary.
Charles Darwin in THE VOYAGE OF THE BEAGLE, CHAPTER II RIO DE JANEIRO