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POPULAR SCIENCE REVIEW. 
The sacs continue to shine, for a while, after removal from the 
body, and the epithelium-looking cells retain their luminosity, 
when smeared over a moist surface for a time, but drying destroys 
the power. Oil and water do not affect the sacs ; acids and 
alkalies arrest the light, and glycerine also, but the light returns 
on washing it off. The glow becomes extinct in vacuo , but 
returns on the admission of air, 
Mr. Darwin writes, “ All the fire-flies which I caught here (at 
Rio), belonged to the Lampyridse (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-instan- 
taneous 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 has 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 pro- 
perty 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. 
On the muddy and wet gravel walks I found the larvae of this 
Lampyris in great numbers. They resembled in general form 
the female of the English glowworm. These larvae possessed 
but feeble luminous powers ; and on the slightest touch they 
feigned death, and ceased to shine, nor did irritation excite any 
fresh display.” 
The Elater tribe furnish the commonest “ fire-flies ” of the 
tropics, and the light comes from a spot on either side of the 
front part of the body, where there is a yellow oval mass of cell- 
aggregates and tracheae. 
There is great scope for thought and speculation about all 
these facts, and it is evident that we do not yet know enough of the 
anatomy and physiology of the photogenic organs of many animals. 
But with our present knowledge it is possible to obtain some 
tolerably definite ideas on the subject of animal luminousness. 
Firstly, the spectroscope gives no satisfactory assistance. It 
tells us that the light is not produced by a gas, and still that 
there is something unusual about it, for the green part of the 
spectrum, in which it is found, glows as it were, in the least re- 
frangible part, or may be said to be more intense near the red 
than in the other part of the green. That the term phospho- 
rescence is of no scientific value is evident ; it only relates to the 
