410 



I'Ht AUKICULTUEAL NEWS. 



DEC£MBErJ29, lyll 



INSECT NOTES. 



THE PRODUCTION OF LIGHT IN CERTAIN 

 ANIMALS. 



The mechanism of light production i.s found in several 

 of the lower animals, and since the light given off by them 

 looks like that of phosphorus, the term phosphorescence is 

 often used to express this phenomenon. As will appear later, 

 this term is not really correct in view of recent researches, 

 and it has been suggested that the more general term lumin- 

 escence be employed. In the following notes it is proposed 

 first of all to refer to some of the luminous animals and their 

 habits, and secondly to give some account of the recent 

 inve.stigations as to the source of light in certain luminous 

 animals, particularly fire-flief. 



Much of the phosphorescence or luminescence of the sea 

 is due to minute marine protozoa belonging to the genera 

 Pyroeystis and Noctiluca. In the species Koctiluca miliaris 

 the luminescence glows as a rule only when the animal is 

 agitated, and a persistent stimulus causes a continuous but 

 weak light. The luminosity resides in minute points, mostly 

 crowded in the central mass, but aleo scattered all through 

 the protoplasm of this small one-celled animal. A slight 

 irritation only produces luminosity at the point touched; 

 a strong one causes the whole to flash, It is stated that any 

 form of irritation, whether of heat, touch, or agitation, 

 electricity or magnetism, induces the glow. (Camh. Xat. 

 i7jst., Vol. I, p. 134.) 



Light production also occurs in one of the marine 

 burrowing molluscs, Pholan, in an ostracod or shell-covered 

 crustracean Cypridiii'i. and in a few other marine forms. 



It is among certain members of the insect world that 

 we get the greatest development in the mechanism of light 

 production. The light producing insects in the American 

 tropics are commonly grouped under the general term 

 'fire-flies', while in Europe the term 'glowworm' is also used 

 in connexion with the wingless, worm-like females. 



Fire-flies belong to two different families of beetles 

 the Lampyridae and the Elateridae. Members of the 

 family Lampyridae occur in many parts of the world, and 

 include species of the genera Lampyru, Lxieiola, etc., in 

 Europe, and Japan, and Photiniif, I'luAuris, Lucidota, etc., 

 in the American tropics. These are the true fire-flies, and are 

 all comparatively small in.sects; some of them are quite 

 common in most of the West Indian islands The larger 

 kinds ot luminous beetles belong to the genus I'yrophorus 

 of the family Elateridae or click-beetles. These occur only 

 in the American tropics, including some of the West Indian 

 islands. The commonest species Pyrophorus noctilucus is 

 known in some islands as the 'cucuUo', and in others as 

 'La Belle'. 



In Southern Europe fire-flies belonging to the genus 

 Lvcioln swarm in large numbers on calm, warm nights. 

 Sharp slates that the.se are nearly all males, while the females 

 are rather rare. The light of the male is more brilliant than 

 that of the females, its wings and eyes are more strongly 

 developed, and it is thought that in this case the lights of 

 the male are displayed in rivalry. 



In the British forms the female is entirely wingless, and 



is commonly called a glow-worm, while the inale has wings, 

 and is able to fly about. The light of the female is more 

 conspicuous than that of the male, and in this case it seems 

 probable that the light of the female serves to attract the 

 males. (Camh. Nat. Hist., Vol. VI.) 



In the Eastern and Southern United States the comuioa 

 fireflies belong to the two genera Photlnus and Photuris. 

 An account of these luminous insects appeared in the Tournol 

 of Heredity for August 1917, from which drticle the follow- 

 ing notes are taken. 



Photinus, the commoner of the two, is rather deli- 

 cate, and flies near to the ground, flashine frequently, and 

 giving off an orange or yellow light. The male has a 

 larger light-producing apparatus than the female. 



Photurii. on the other hand, is strong and hardy, 

 carnivorous, flies high, and flashes only occasionally, giving 

 off a greenish-yellow light. The ligllting apparatus is 

 almost identical in the two sexes. 



It will be noted from the above thit there is a distinct 

 difference in the colour of the light gi^en off by the two 

 forms. B 'th produce a yellowish light, but while in one 

 genus the light has a reddish tinge, id the other the light 

 is distinctly greenish. It is also stated in this connexion 

 that the colour of the light is useful in distinguishing the 

 different species within the two genera, no two species 

 giving off' an identical shade. The giieen light has been 

 shown by experiment to be the brighter light of the two. 

 It must be remembered, however, thit such a light as is 

 given off by these insects probably has a different effect on 

 the compound eye of insects than it has on the simple eye 

 of man. ( 



There have been various suegestiops put forward as to 

 the uses of the light produced by these insects. Some have 

 thought that it serves to attract prey, others that it blinds 

 the prey, and prevents its escape, others again that it blinds 

 enemies, and prevents attacks. It has also been suggested 

 that the light is simply an adornment, or that it serves to 

 illuminate the path of the insect. 



The most plausible view, however, seems to be that the 

 light is used as a signal between male and female of the 

 same species. ' 



In this connexion the following account may be of 

 interest. It is taken from a paper bj- K. G. Blair on 

 'Luminous Insects', a report of which appeared in a past 

 number of Nature: — 



'Apart from its principal function in securing the 

 mating of the sexes, thu light seems to be largely 

 used, at any rate by the males, for purposes of display. 

 Where the powers of luminosity are largely developed 

 in this sex, the emission of the light is usually of an inter- 

 mittent type. It has been noticed in virious parts of the 

 world that these flashing males tend to congregate in 

 large companies, and that the individuals of one of these 

 gatherings will flash in concert. All the fite-flies around one 

 tree or group of tree.", for instance, will flash together, while 

 those around a neighbouring tree will be pulsating to 

 a different time.' 



Mr. Blair goes on to say that the exact reason of 

 this flashing in concert, or the method by which it is 

 brought about, has not been ascertained. He suggests, 

 as an explanation of the phenomenon, that each flash exhausM 

 the battery, as it were, and a period^of recuperiition is required 

 before another flash can be emitted. It is then conceivable, 

 he continues, that the flash of a leader might act as a stimulus 

 to the discharge of their flashes by the other members of the 

 group, and so bring about the flashing concert by the w^hole 

 company. 



J.C.H. 



