132 



CHAP. XI. 



THE GLOW-WORM. OPINIONS ON THE LUMINOUS 



MATTER OF INSECTS. THAT OF THE GLOW-WORM, 

 AND CAUSE OF ITS EXHIBITION. PHENOMENA ON 

 ITS INTRODUCTION, AND THAT OF THE INSECT, INTO 

 VARIOUS MEDIA. OUR EXPERIMENTS AND CONCLU- 

 SIONS. DR. CARUS'S DISCOVERY. TO WHAT PUR- 

 POSE SUBSERVIENT. CONCLUSION. 



NUMEROUS opinions have obtained on the proximate 

 cause or source of this curious illumination. Mr. 

 Macartney, in the Transactions of the Royal Society, 

 has given an interesting detail of experiments and ob- 

 servations on several luminous insects. In the lam- 

 pyris noctiluca, and in the elater noctilucus and igni- 

 tus, the light, he observes, proceeds from a substance 

 not distinguishable from the interstitial substance 

 except in its colour, which is yellow. The light- 

 yielding matter reposes under the transparent parts 

 of the skin, through which casement it is seen; 

 and he infers that the luminous matter in the glow- 

 worm is absorbed, being replaced by the interstitial 

 matter when the season for emitting light is gone by. 

 He observed two minute elliptical sacs formed of an 

 elastic fibre, wound spirally, and similar to that of the 

 tracheae, which contained a yellow substance, soft in 

 consistency, and closer in texture than that lining the 

 adjoining region, and affording a more brilliant and 

 permanent light. This light he concluded to be less 

 under the control of the insect than the luminous sub- 

 stance in its vicinity, which he infers it has the pro- 



