If you take one of these glow-worms home with you for examination, you will find that in shape it somewhat resembles a caterpillar, only that it is much more depressed; and you will observe that the light proceeds from a pale-coloured patch that terminates the underside of the abdomen. It is not, however, the larva of an insect, but the perfect female of a winged beetle, from which it is altogether so different, that nothing but actual observation could have inferred the fact of their being the sexes of the same insect. In the course of our inquiries you will find that sexual differences even more extraordinary exist in the insect world.
It has been supposed by many that the males of the different species of Lampyris do not possess the property of giving out any light; but it is now ascertained that this supposition is inaccurate, though their light is much less vivid than that of the female. Ray first pointed out this fact with respect to L. noctiluca[685]. Geoffroy also observed that the male of this species has four small luminous points, two on each of the two last segments of the belly[686]: and his observation has been recently confirmed by Müller. This last entomologist, indeed, saw only two shining spots; but from the insect's having the power of withdrawing them out of sight so that not the smallest trace of light remains, he thinks it is not improbable that at times two other points still smaller may be exhibited, as Geoffroy has described. In the males of L. Splendidula and of L. hemiptera the light is very distinct, and may be seen in the former while flying[687].—The females have the same faculty of extinguishing or concealing their light—a very necessary provision to guard them from the attacks of nocturnal birds: Mr. White even thinks that they regularly put it out between eleven and twelve every night[688]: and they have also the power of rendering it for a while more vivid than ordinary.
Authors who have noticed the luminous parts of the common female glow-worm, having usually contented themselves with stating that the light issues from the three last ventral segments of the abdomen[689]; I shall give you the result of some observations I once made upon this subject. One evening, in the beginning of July, meeting with two of these insects, I placed them on my hand. At first their light was exceedingly brilliant, so as to appear even at the junctions of the upper or dorsal segments of the abdomen. Soon after I had taken them, one withdrew its light altogether, but the other continued to shine. While it did this it was laid upon its back, the abdomen forming an angle with the rest of its body, and the last or anal segment being kept in constant motion. This segment was distinguished by two round and very vivid spots of light; which, in the specimen that had ceased to shine, were the last that disappeared, and they seem to be the first parts that become luminous when the animal is disposed to yield its light. The penultimate and antepenultimate segments each exhibited a middle transverse band of yellow radiance, terminated towards the trunk by an obtusely-dentated line; a greener and fainter light being emitted by the rest of the segment.
Though many of the females of the Lampyridæ are without wings and even elytra, (in which circumstance they differ from all other apterous Coleoptera,) this is not the case with all. The female of Pygolampis[690] italica, a species common in Italy, and which, if we may trust to the accuracy of the account given by Mr. Waller in the Philosophical Transactions for 1684, would seem to have been taken by him in Hertfordshire, is winged: and when a number of these moving stars are seen to dart through the air in a dark night, nothing can have a more beautiful effect. Sir J. E. Smith tells us that the beaus of Italy are accustomed in an evening to adorn the heads of the ladies with these artificial diamonds, by sticking them into their hair; and a similar custom, as I have before informed you[691], prevails amongst the ladies of India.
Besides the different species of the genus Lampyris, all of which are probably more or less luminous, another insect of the beetle tribe, Elater noctilucus, is endowed with the same property, and that in a much higher degree. This insect, which is called the fire-fly, and is an inch long, and about one-third of an inch broad, gives out its principal light from two transparent eye-like tubercles placed upon the thorax; but there are also two luminous patches concealed under the elytra, which are not visible except when the insect is flying, at which time it appears adorned with four brilliant gems of the most beautiful golden-blue lustre: in fact, the whole body is full of light, which shines out between the abdominal segments when stretched. The light emitted by the two thoracic tubercles alone is so considerable, that the smallest print may be read by moving one of these insects along the lines; and in the West India islands, particularly in St. Domingo, where they are very common, the natives were formerly accustomed to employ these living lamps, which they called Cucuij, instead of candles in performing their evening household occupations. In travelling at night they used to tie one to each great toe; and in fishing and hunting required no other flambeau[692].—Southey has happily introduced this insect in his "Madoc" as furnishing the lamp by which Coatel rescued the British hero from the hands of the Mexican priests.
"She beckon'd and descended, and drew out
From underneath her vest a cage, or net
It rather might be call'd, so fine the twigs
Which knit it, where, confined, two Fire-flies gave
Their lustre. By that light did Madoc first
Behold the features of his lovely guide."
Pietro Martire tells us that the Cucuij serve the natives of the Spanish West India islands not only instead of candles, but as extirpators of the gnats, which are a dreadful pest to the inhabitants of the low grounds. They introduce a few fire-flies, to which the gnats are a grateful food, into their houses, and by means of these "commodious hunters" are soon rid of the intruders. "How they are a remedy," says this author, "for so great a mischiefe it is a pleasant thing to hear. Hee who understandeth he hath those troublesome guestes (the gnattes) at home, diligently hunteth after the Cucuij. Whoso wanteth Cucuij goeth out of the house in the first twilight of the night, carrying a burning fire-brande in his hande, and ascendeth the next hillock that the Cucuij may see it, and hee swingeth the fire-brande about, calling Cucuius aloud, and beating the ayre with often calling out Cucuie, Cucuie." He goes on to observe, that the simple people believe the insect is attracted by their invitations; but that, for his part he is rather inclined to think that the fire is the magnet. Having obtained a sufficient number of Cucuij, the beetle-hunter returns home and lets them fly loose in the house, where they diligently seek the gnats about the beds and the faces of those asleep, and devour them[693].—These insects are also applied to purposes of decoration. On certain festival days in the month of June, they are collected in great numbers, and tied all over the garments of the young people, who gallop through the streets on horses similarly ornamented, producing on a dark evening the effect of a large moving body of light. On such occasions the lover displays his gallantry by decking his mistress with these living gems[694]. And according to P. Martire, "many wanton wilde fellowes" rub their faces with the flesh of a killed Cucuius, as boys with us use phosphorus, "with purpose to meet their neighbours with a flaming countenance," and derive amusement from their fright.
Besides Elater noctilucus, E. ignitus and several others of the same genus are luminous. Not fewer than twelve species of this family are described by Illiger in the Berlin Naturalist Society's Magazine[695].
The brilliant nocturnal spectacle presented by these insects to the inhabitants of the countries where they abound cannot be better described than in the language of the poet above referred to, who has thus related its first effect upon the British visitors of the new world:
"..............Sorrowing we beheld
The night come on; but soon did night display
More wonders than it veil'd: innumerous tribes
From the wood-cover swarm'd, and darkness made
Their beauties visible: one while they stream'd
A bright blue radiance upon flowers that closed
Their gorgeous colours from the eye of day;
Now motionless and dark, eluded search,
Self-shrouded; and anon, starring the sky,
Rose like a shower of fire."