270 



KNOWLEDGE. 



[December 1, 1894. "1 



Fig. 2. — Male Glow-worm, maguiiied 

 two diameters. 



the same distribution of colour, and the same short legs and 

 antermne. It is the wings and elytra that make the male 

 look so different. The 

 elytra, or wing-covers, 

 are long, narrow and 

 paraOel-sided, entirely 

 concealing the body, and 

 indeed projecting beyond 

 it both at the sides and 

 behind. They are of 

 a greyish-brown colour, 

 covered all over with 

 minute pit-like depres- 

 sions, and bounded by a 

 raised rim all round ; there are also two or three slightly 

 raised parallel ridges on their surface, and the same 

 clothing of minute and scarcely visible silky hairs covers 

 them as is to be found over the rest of the insect. If we 

 raise these wing-covers, we see a pair of smoky wings 

 beneath them about twice as broad as the elytra, and 

 slightly longer, so that they overlap one another when 

 closed, and have to be slightly folded at the tip to get them 

 beneath their covers. The nervures stand out distinctly as 

 dark smoky lines, and at the extreme tip of the wing they 

 meet to form a polygonal area which is destitute of 

 nervures, so as to facilitate the slight folding that has to 

 take place. The insect is a good flier, spreading its elytra, 

 but using only its membranous wings for the purpose of 

 flight. If placed on its back, it is said to right itself by 

 slightly opening its wing-covers and thus getting its wings 

 free, and so struggling over by their means. It is 

 nocturnal in habits, and is therefore not often seen unless 

 attracted by light. In places where they are common, an 

 open window with a strong light burning inside will prove 

 an irresistible attraction, and they may be easily caught as 

 they fly towards the lamp. 



In all the characteristics we have already described, the 

 male, though so unlike his consort, in no respect differs 

 from the ordinary beetle type. The only point which is at 

 all exceptional is the large size and projecting form of the 

 thoracic shield, but even in this respect the insect is 

 not quite singular ; other Coleoptera, such as the tortoise- 

 beetles, show exactly the same arrangement. But there 

 is a far more peculiar feature yet to be noticed, and that 

 is the enoi-mous size of the eyes. Of course, these caimot 

 be seen from above, but if the insect be laid on its back, 

 two large, round, black knobs will be seen almost touching 

 one another, just in front of the first pair of legs and, 



therefore, under the thoracic 

 shield. (Fig. 3.) A lens shows 

 that they are covered with an 

 immense number of hexagonal 

 facets, and they are thus seen 

 to be the compound eyes. They 

 occupy almost the whole of the 

 head, the mouth organs and 

 antennae being squeezed into a 

 very small compass between 

 them in front. As the insect 

 lies in this position, another 

 peculiarity is easily observable. 

 A horny flap from the disc-Uke 

 thoracic shield bends down on each side of the head, to 

 which it fits so closely that the idea is irresistibly suggested 

 of a broad-brimmed hat placed on the back of the crown 

 and tied beneath the neck by a very wide ribbon. A similar 

 arrangement is made in the female, but as the eyes are not 

 nearly so large, the efi'ect is not so striking. 



Luminosity is not the heritage of the female only ; the 



Fig. 3.— Head of Male Glow 

 worm, from beneatli. 



male possesses the power to a slight extent, though his 

 brilliancy is far inferior to that of his mate, and is chiefly 

 confined to the two bright points on the last abdominal 

 segment. Hence the pale area of the abdomen is not 

 nearly so extensive in the male as in the female, and an 

 inspection of the dead insect even would be sufficient to 

 lead to the conclusion of its far inferior brilliancy. In 

 neither sex is the luminosity an external feature. The 

 seat of the activity which gives rise to the light is internal, 

 and the light merely shines through the transparent skin 

 beneath. The light-producing organs are situated in the 

 last three segments of the body, and consist of two layers 

 of yellowish-white, rounded cells, abundantly supplied with 

 a network of air-tubes from the spiracles. The two layers 

 of cells, though similar in form, appear to difi'er in consti- 

 tution and in function, those of one layer only becoming 

 luminous. 



It cannot be said that anything very satisfactory has yet 

 been determined with regard to the real cause or nature of 

 the luminosity. According to some careful investigators, 

 it results from a process of oxidation of some of the 

 contents of the luminous cells, produced in them as the 

 result of nervous stimulus. The oxygen required for this 

 irarpose, it is maintained, is supplied from the outer air, 

 which, introduced at the very conspicuous spiracles placed 

 at intervals along the sides, finds its way to the light 

 organs through the air-tubes. Thus the intensity of the 

 light depends upon the vigour of respiration, supplemented 

 by nervous activity. It is, however, difficult to understand 

 how so bright a light should be produced in this way 

 without any appreciable rise in temperature. The light 

 appears to be, to some extent, under the insect's control, 

 and the advocates of the oxidation theory contend that the 

 regular intermittence of light which is observable in some 

 species is produced by an alternate opening and closing 

 of the spiracles, or at least synchronizes with acts of 

 inspiration and expiration. Other observers, again, while 

 maintaining the oxidation theory, deny that nervous 

 stimulus has any connection with it, apd stOl others deny 

 the oxidation theory altogether. 



It is not difficult to determine one function of the light. 

 When one considers the apterous condition, the brilliant 

 light, and the ordinary eyes of the female glow-worm, and 

 contrasts with these the winged condition, the feeble light 

 and the enormous eyes of her partner, it is evident that 

 these are complementary features in the two sexes, and 

 there can be little doubt, therefore, that one function of 

 the luminosity is to serve as a sexual attraction, and 

 guide the roving male to his destined spouse. But there 

 is no reason why it should not have other functions as 

 well, and that such is the case would seem to be implied in 

 the fact that the insect is luminous in all its stages, and 

 therefore long before it is sexually mature. The suggestion 

 has been made that the light is protective in function, 

 being a sort of warning signal, like the brilUant colours 

 of certain caterpillars. Considering the softness of the 

 female's skin, and its sluggish habits, it would seem to 

 need more protection than usual, and the suggested use of 

 the luminosity is therefore, at the least, plausible. It is 

 not imreasonable to suppose that a would-be captor would 

 think twice before seizing on so dangerous-looking a 

 morsel. On the other hand, the insect is carnivorous, 

 and furnished with a tolerably efl'ective pair of jaws, so 

 that it may perhaps be able to give a better account of 

 itself in a struggle than its soft skin would lead one to 

 suppose. 



In confirmation of the sexual function ascribed to the 

 luminosity, the Eev. H. S. Gorham has pointed out that 

 in the family Lampyridm the eyes are developed in magni- 



