LUMINOUS IXSECTS. 513 



With respect to the remote cause of the luminous property of insects, 

 philosophers are considerably divided in opinion. The disciples of modern 

 chemistry have in general, with Dr. Darwin, referred it to the slow com- 

 bustion of some combination of phosphorus secreted from their fluids by an 

 appropriate organization, and entering into combination with the oxygen 

 supplied in respiration. This opinion is very plausibly built upon the ascer- 

 tained existence of phosphoric acid as an animal secretion ; the great re- 

 semblance between the light of phosphorus in slow combustion and animal 

 light ; the remarkably large spiracula in glow-worms, and the decided con- 

 nexion of their light with respiration ; and upon the statement, that the 

 light of the glow-worm is rendered more brilliant by the application of heat 

 and oxygen gas, and is extinguished by cold and by hydrogen and carbonic 

 acid gases. From these last facts Spallanzani was led to regard the luminous 

 matter as a compound of hydrogen and carburetted hydrogen gas. Carradori 

 having found that the luminous portion of the belly of the Italian glow- 

 worm (Pygolampis Italica) shone in vacuo, in oil, in water, and when under 

 other circumstances where the presence of oxygen gas was precluded, with 

 Brugnatelli, ascribed the property in question to the imbibition of light 

 separated from the food or air taken into the body, and afterwards secreted 

 in a sensible form. 1 Mr. Macartney having ascertained by experiment that 

 the light of a glow-worm is not diminished by immersion in water, or in- 

 creased by the application of heat ; that the substance affording it, though 

 poetically employed for lighting the fairies' tapers 2 , is incapable of inflam- 

 mation if applied to the flame of a candle or red-hot iron ; and when 

 separated from the body exhibits no sensible heat on the thermometer's 

 being applied to it rejects the preceding hypothesis as unsatisfactory, but 

 without substituting any other explanation ; suggesting, however, that the 

 facts he observed are more favourable to the supposition of light being a 

 quality of matter than a substance. 3 Lastly, Dr. Todd finding that the 

 luminous substance of Lampyris continues to shine when detached, some- 

 times for a longer and at others a shorter period, but never exceeding 

 twenty minutes, and that under mercury, various gases, water, and in vacuo, 

 considers it solely as an effect of vitality. 4 



clear. He probably means that the insect will not shine in a dark place in the day- 

 time, unless previously exposed to the solar light ; for it is often seen to shine at 

 night wnen it could have had no recent exposure to the sun. 

 i Annal di Chimica, xiii. 1797. Phil. Mag. ii. 80. 



3 " And for night-tapers crop their waxen thighs. 



And light them at the fiery glow-worms' eyes." 



5 Some experiments made by my friend the Rev. R. Sheppard on the glow-worm 

 are worthy of being recorded. One of the receptacles being extracted with a pen- 

 knife, continued luminous : but on being immersed in camphorated spirit of wine, 

 became immediately extinct. The animal with one of its receptacles uninjured, 

 being plunged into the same spirit, became apparently lifeless in less than a minute ; 

 but the receptacle continued luminous for five minutes', the light gradually disappear- 

 ing. Having extracted the luminous matter from the receptacles, in two days they 

 were healed, and filled with luminous matter as before. He found this matter to lose 

 its luminous property, and become dry and glossy like gum, in about two minutes ; 

 but it recovered it again on being moistened with saliva, and again lost it when 

 dried. When the matter was extracted from two or three glow-worms, and covered 

 with liquid gum-arabic, it continued luminous for upwards of a quarter of an hour. 



* Phil. Trans. 1824. 



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