Decembeb 1, 1894.] 



KNOWLEDGE 



271 



tude in proportion to the light displayed ; those species 

 that show but a feeble light have but small eyes, and rice 

 irrsa. The antennse also are developed in inverse ratio 

 to the light, and it is well known that a great development 

 of antennffi often exists in those insects that can perceive 

 their mates at great distances ; thus, when one power of 

 discovery is lessened, another is increased to supply the 

 deficiency. Thus the species may be divided into three 

 groups : first, those with plumose antenna; and small eyes, 

 and with light appearing in small spots only ; secondly, 

 those with simple antennae, larger eyes, and a considerable 

 amount of light, both sexes being winged ; and thirdly, 

 those with quite rudimentary antennte, extraordinarily 

 large eyes in the male, and most brilliant light, accom- 

 panied by an apterous condition in the female, or, at 

 most, with rudimentary wings in that sex. Evidently, 

 therefore, the light-giving power, eyes, antenna, and wings, 

 are all related in the progress of their development, and it 

 can scarcely be questioned that such a relation indicates 

 a causal connection. 



We may now sketch the .life-history of this curious 

 insect. Shortly after pairing, the female deposits her eggs. 

 They are of large size and pale in colour, and are eitlier 

 placed in the earth or ou moss or grass or other low plants, 

 adhering to the spot on which they are placed by means 

 of the viscid liquid with which they are wet. Curiously 

 enough, the eggs are luminous as well as the insect that 

 produced them. It might be thought that, considering 

 where they have come from, this is not to be wondered at, 

 since the appearance might be due to the moisture that 

 covers their surface. But this is not the case, for if 

 washed in pure water and then dried they still remain 

 luminous. According to Dubois, they may be kept in 

 water for an hour without losing their luminosity, though 

 after the lapse of that time the light begins to wane ; but 

 if they are taken out when this occurs, they soon recover 

 their brilliancy. Alcohol rapidly suppresses the luminosity, 

 and boiling water destroys it immediately. The luminosity 

 belongs to the contents of the egg, and not to the shell, so 

 that the light shines through the transparent egg-shell. 

 When the egg is hatched, such luminosity as there is 

 appears in the larva, but the empty egg-shell loses every 

 trace of it. Now, since this luminosity appears even in the 

 youngest eggs, before the segmentation of the yolk sets in, 

 it is evident that it is not, in this instance, whatever may 

 be the case with the adult insect, dependent upon nerves 

 or air-tubes, or indeed any anatomical element whatever, 

 though, of course, oxygen can pass through the thin egg- 

 shell without any special means of conveyance. The 

 luminosity of the eggs is, however, greatly dependent upon 

 moisture, and gradually disappears as the moss amongst 

 which they may be lying dries up ; it can be restored 

 again, if not too far gone, by the application of moisture. 



The larva is extremely like the perfect female, the 

 most noticeable difference being the smaller size of the 

 head shield. It may be found, of various sizes and ages, 

 during the winter and spring, and becomes full-grown about 

 April. The small size of the young larvie will serve at 

 once to mark their immaturity, since, of com-se, the adult 

 insect does not grow ; but the full-grown larva is less 

 easy to distinguish from the adult female. Besides the 

 shape of the fore parts, there are also minute differences in 

 the feet and antennae, which aid in their separation. Like 

 the perfect insect, the larva is carnivorous, feeding upon 

 snails, especially those of the genus Zonites or Helicclla, 

 small, flat, shining kinds, often seen under stones, moss, 

 Ac, and in damp places geueraUy. At the end of the body 

 it has some seven or eight short white rods, which are 

 usually kept retracted within the body, but can be protruded 



at will, and these it is said to use for cleansing the fore 

 parts of its body, should any of the snail's slime adhere to 

 it during the course of a meal. The coloration of the larva 

 is a little more distinct and variegated than that of the 

 adult female ; the centre part of the back is darker, and 

 each segment has a reddish-yellow patch at its exterior 

 angle. 



As the insect in its larval stage is so much like the fully- 

 developed female, we need not be surprised to learn that 

 the pupa stage is a very short one. Very Uttle metamor- 

 phosis, whether internal or external, has to take place, and 

 a very brief time is quite sufficient for this. In little 

 more than a fortnight after the larva has ceased to feed, 

 the perfect insect appears, and the pupa stage itself does 

 not occupy much more than half of this time. The pupa 

 of the female does not differ much in appearance from 

 either the larva or the adult ; it remains in a curved 

 position, with legs bent up, very much as the larva or 

 perfect insect would do if feigning death. In the male 

 pupa a difference can be seen ; the wings begin to appear, 

 but, of course, as usual, they are of very small size. When 

 the insect is ready to escape from its pupa-skin, the latter 

 slits in front, and the beetle wriggles its way out, a soft 

 and nerveless thing. But its skin soon gains strength and 

 consistency, and then it is prepared to atone for its fort- 

 night's fast, by renewing its attacks upon the snails. 



It has already been mentioned that the glow-worm is 

 not the only member of its family that is found in this 

 country. At Lewes and Hastings another and smaller 

 species has been observed, which is called Plwsphxmts 

 liemipterus. It is similar in shape to the glow-worm, but 

 has considerably longer antenms. The female is quite 

 apterous, but the male differs markedly from our common 

 species, in that wings are absent, and the elytra are very 

 short, not much more than a quarter as long as the body. 

 Moreover, they are pointed behind, and their inner edges 

 do not meet, but gape apart like those of the oil beetle 

 we described a short time ago. Curiously enough, in this 

 species it is the male that is usually found ; the female is 

 either much rarer, or manages to keep itself out of sight, 

 for it is very seldom met with. The male is usually found 

 crawUng about on walls. This insect is pretty widely 

 distributed on the Continent, but in this country it has 

 been observed only in the two localities above mentioned, 

 though there seems no reason why it should not occur 

 elsewhere. 



THE DISTANCE AND MASS OF THE BINARY 

 STARS. 



By J. E. Gore, F.R.A.S. 



IN a previous paper (Knowledge, November, 1891) I 

 considered the mass and relative brightness of those 

 binary stars for which a parallax had been found. 

 In the present paper I propose to discuss the probable 

 distance and mass of those binary stars for which a 

 parallax has not yet been determined. The method of 

 procedure I propose to adopt is to compute what is called 

 the " hypothetical parallax " of the binary star — that is, its 

 parallax on the assumption that its mass is equal to the 

 mass of the sun — and then to find the stellar magnitude to 

 which the san would be reduced if placed at the distance 

 indicated by this hypothetical parallax, assuming that the 

 sun's stellar magnitude at its present distance is —25-5. 

 Comparing, then, the star's magnitude, as measured with 

 the photometer, with the sun's reduced magnitude, it wiU 

 at once appear whether the binary is brighter or fainter 



