LUMINANCE IN PLANTS AND ANIMALS 83 
male insect has expansive wings and horny wing-covers, and 
makes his flight through the air ; the female is wingless, and 
crawls on the ground ; hence the English appellation glow-worm. 
The light of the former is comparatively feeble, that of the latter 
beautiful and brilliant. These insects are frequently met with 
in July and August in woods, meadows, and on banks beneath 
hedges. The utility of the light of the females is supposed to 
consist in attracting the attention of the males during the dark, 
when alone they are able to render themselves conspicuous — 
a circumstance to which Moore beautifully alludes — 
“ For well I knew the lustre shed 
From my rich wings, when proudliest spread. 
Was in its nature lambent, pure 
And innocent as is the light. 
The glow-worm hangs out to allure 
Her mate to her green bower at night.” 
This theory, though probably not correct, is not altogether 
fanciful, as was proved by Olivier, who frequently caught males 
by holding females in his hand. Besides, without some such 
apparatus, it is difficult to conceive how a crawling insect could 
attract the attention of its mate, whose principal medium of 
motion is the atmosphere. Be this as it may, the light un- 
doubtedly serves some important purpose in the economy of the 
glow-worm, and manifests itself even when the insect is in its 
larval state. 
If the luminous portion of the abdomen be removed, it 
retains its luminous property for some time, and when appa- 
rently extinct, it may be reproduced by softening the matter with 
water. Olivier, in his experiments, could only reproduce it 
within thirty-six hours after the death of the animal, and that 
only once, and by the direct application of heat. Darwin, who 
examined the Lampyridco of South America, found also that the 
light was most brilliant when the insects were irritated. “ The 
shining matter,” he says, “ 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 uninter- 
ruptedly bright, but not so brilliant as before : local irritation 
with a needle always increased the vividness of the light. 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 light is involuntary.” 
The brilliancy of the light is increased by plunging the insect 
into warm water, but cold water extinguishes it. If the insect is 
crushed, and the face or hands rubbed with it, they contract a 
luminous appearance similar to that produced from phosphorus. 
• Passing over several land insects — such as certain beetles, 
scolopendra, &c. — which exhibit more or less luminance, some 
of the marine animals presenting similar phenomena may next 
be referred to. 
