414 



NATURE 



[December 9, 19 15 



In most of the luminous species the ej'es, particu- 

 larly of the males, are extraordinarily large and well 

 developed {e.g. Lampyris, Phptinus, Luciola, etc.), but 

 the antennae are simple. In the non-luminous 

 species, on the other hand, the eyes are of 

 a more normal size, but the antennas of 

 the male are frequently strongly plumose {e.g. 

 Cladodes, Lamprocera, etc.), a feature that 

 in this order as in the Lepidoptera is usually 

 regarded as indicative of a highly developed sense of 

 sex-perception in this sex, correlated with the posses- 

 sion of sluggish and retiring habits on the part of the 

 female. It is noteworthy that in the genus Phengodes 

 both the plumose antennae of the male and the powers 

 of luminosity of the female are unusually well 

 developed. 



Undoubtedly the chief function of the light is in 

 securing the mating of the sexes, but that this is a 

 secondary function only is evidenced by the different 

 degree to which the use of the light is developed. Its 

 wide occurrence within the family proves that the 

 power of emitting light must have arisen early in the 

 evolution of the family, though exactly how it 

 originated it is impossible to say. Possibly, it may have 

 served at first as an indication of unpalatability, com- 

 mon to both sexes, and then, coming under the influ- 

 ence of sexual selection as an evolutionary force, have 

 developed along the various lines we see indicated. 

 In connection with their unpalatable qualities, it may 

 be noted that the Lampyridae is a family that has 

 many mimics in other families of beetles, amongst 

 the Telephoridae, for example, and the longicorns. 

 Species of the longicorn genera Amphionycha and 

 Dadoychus even go so far as to have a ventral 

 pseudo-luminous patch resembling the luminous patch 

 of the fireflies, but in the allied genus Alampyris, where 

 the dorsal mimicry is quite as close, this patch is 

 lacking. 



Apart from its principal function in securing the 

 proper mating of the sexes, the light seems also 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 intermittent flashing type. It has 

 been noticed in various parts of the world that these 

 flashing males tend to congregate in large companies, 

 and that all the individuals of one of these gatherings 

 will flash in concert. All the fireflies around one tree 

 or group of trees, for instance, will flash together, 

 while those around a neighbouring tree will be pulsat- 

 ing to a different time. This feature has been ob- 

 served of a European species of Luciola (though Mr. 

 Main and myself were unable to detect anything of 

 the sort with L. italica at Lugano), of an Indian 

 lampyrid, genus not stated, and of the genus Aspido- 

 soma in South America. The American species of 

 Photinus and Photuris do not seem to possess the 

 habit. 



The exact reason of this flashing in concert, or 

 the method by which it is brought about, have not been 

 ascertained. It has been suggested that the light is 

 not really intermittent in character, but merely appears 

 so owing to its being alternately masked and ex- 

 hibited by movements of the creature's body, and that 

 a slight puff of wind might perhaps affect ail the mem- 

 bers of a companv and cause them all to conceal their 

 lights at once. Though this explanation of the inter- 

 mittent character of the light applies well enough to 

 Pyrophorus, an insect we shall shortly consider, it is 

 certainly not applicable to these Lampyridae. It is 

 true the light is not absolutely extinguished between 

 the flashes, but it is so diminished as to become prac- 

 tically dark ; moreover, the flashing in unison is too 

 regular to be caused by chance puffs of wind. A 



NO. 2406, VOL. 96] 



more probable explanation of the phenomenon is that 

 each flash exhausts the battery, as it were, and a 

 period of recuperation is required before another flash 

 can be emitted. It is then conceivable that the flash 

 of a leader might act as a stimulus to the discharge 

 of their flashes by the other members of the grou]), 

 and so bring about the flashing concert by the whole 

 company. 



The physical and chemical nature of the light of 

 these insects have been the subject of numerous inves- 

 tigations. Though often spoken of as "phosphor- 

 escent," the light has nothing whatever to do with 

 the oxidation of phosphorus. 



The most recent conclusions are those of Dubois, and 

 were summarised by "him in a communication to the 

 Zoological Congress at Monaco in 1913. He finds that 

 the mechanism for the production of light is the same 

 throughout for both plants and animals, and is the 

 result of the action of an oxidising zymase upon an 

 organic proteid product in the presence of water. 

 In the case of luminous insects the proteid, which ho 

 calls luciferine, is contained in the form of granules 

 in the photogenic organ, while the zymase, to which 

 he applies the name luciferase, is dissolved in the 

 blood. The light is given off by the action of the 

 luciferase on the luciferine as the blood passes through | 

 the luminous organs. This zymase can be replaced I 

 experimentally by a chemical oxidising agent such 

 as permanganate of potash, lead dioxide, hydrogen 

 dioxide, etc. 



The luminous organs in these insects are found on 

 dissection to be abundantly supplied with tracheae, 

 which open by means of very large spiracles. It is 

 probable that by opening or closing these spiracles the 

 insect is able to regulate the supply of oxygen to the 

 luminous organs, and so in some degree to control the 

 emission of light. 



Though in many species the flashing of the male is 

 so regular as to suggest its being due to reflex rather 

 than to voluntary action, yet in the female the light 

 appears to be more under the control of the insect. 

 In many species the light may be emitted after the 

 death of the insect, but in the case of males of the 

 flashing species the light is then constant instead of 

 intermittent and less intense than in life. While 

 searching for the females of Luciola italica, I was 

 several times deceived by the constant faint light of a 

 dead firefly caught in a spider's web near the ground. 



Another group of beetles the light-giving properties 

 of which have caused them to be very widely known — 

 at any rate by name — are the "fireflies" of tropical 

 America, Pyrophorus. These must not be confused 

 with the " fireflies " of Italy and southern Europe, 

 which, as I have said, are really winged glow-worms 

 allied to the lightning-bugs of the United States. The 

 fireflies of tropical America and the West Indies, the 

 creatures that the local belles wear in their hair, and 

 about their persons, as a kind of living jewelry, 

 known locally as Cucujos, belong to a very different 

 family, the Elateridae, or skip-jacks. They are con- 

 siderablv larger than the glow-worms, and their 

 light organs are differently situated. The most 

 obvious are a pair of large yellowish spots oh the 

 thorax, one near each of the posterior angles. If the 

 beetle is examined alive, these spots, the "eyes" as 

 they are called, will at first be quite dull and opaque; 

 but when the insect is handled they will soon be ob- 

 served to kindle, the glow increasing graduallv in 

 intensity until it reaches its maximum. This light is 

 onlv emitted by the firefly when stimulated by some 

 excitement, such as that caused bv handling, and 

 glows steadily so long as the excitement continues ; as 

 this wanes the light gradually dies away. 



When the insect is on the wing the light seen Is 



