4.18 LUMINOUS INSECTS. 



and sometimes as hovering in the air. — Whatever be 

 the true nature of these meteors, of which so much is 

 said and so httle known, it is singular how few modern 

 instances of their having been observed are on record. Dr. 

 Darwin declares, that though in the course of a long life 

 he had been out in the night, and in the places where 

 they are said to appear, times without number, he had 

 never seen any thing of the kind : and from the silence 

 of other philosopliers of our own times, it should seem 

 that their experience is similar. 



With regard to the immediate source of the luminous 

 properties of these insects, Mr. Macartney, to whom 

 we are indebted for the most recent investigation on the 

 subject, has ascertained that in the common glow-worm, 

 and in Elater noctilucus and ignitus, the light proceeds 

 from masses of a substance not generally differing, ex- 

 cept in its yellow colour, from the interstitial substance 

 {corps graisscux) of the rest of the body, closely applied 

 underneath those transparent parts of the insects' skin 

 which afford the light. In the glow-worm, besides the 

 last-mentioned substance, which, when the season for 

 giving light is passed, is absorbed, and replaced by the 

 common interstitial substance, he observed on the inner 

 side of the last abdominal segment two minute oval sacs 

 formed of an elastic spirally-wound fibre similar to that 

 of the tracheae, containing a soft yellow substance of a 

 closer texture than that which lines the adjohiing region, 

 and affording a more permanent and brilliant li^ht. 

 This light he found to be less under the control of the 

 insect than that from the adjoining luminous substance, 

 which it has the power of voluntai'ily extinguishing, not 



