58 THE NATUEAL HISTORY 



Thus, e'er night's veil had half obscur'd the sky, 

 Th' impatient damsel hung her lamp on high : 

 True to the signal, by love's meteor led, 

 Leander hasten'd to his Hero's bed.^ ' J 



I am, &c. 



LETTER XXV. 



TO THE SAME. 



Selborne, Aug. 30, 1769. 

 Dear Sir, 



It gives me satisfaction to find that my account of the ousel 

 migration pleases you. You put a very shrewd question when you 

 ask me how I know that their autumnal m^igration is southward ? 

 Was not candour and openness the very life of natural history, I 

 should pass over this query just as a sly commentator does over a 

 crabbed passage in a classic ; but common ingenuousness obliges 

 me to confess, not without some degree- of shame, that I only 

 reasoned in that case from analogy. For as all other autumnal 

 birds migrate from the northward to us, to partake of our milder 

 winters, and return to the northward again when the rigorous 

 cold abates, so I concluded that the ring-ousels did the same, as 

 well as their congeners the fieldfares ; and especially as ring- 

 developed in both sexes, and there is little difference in the eyes. This fact seems 

 to indicate that the perception of fhe light by the male is of importance to the 

 species. 



The light is emitted from the ventral surface of the 6th, 7th and 8th ventral seg- 

 ments, more particularly from the 6th and 7th. It proceeds from a yellowish fatty 

 substance, which shines through the transparent skin. Moderate stimulation in- 

 creases the emission of light, but anything which' alarms the insect diminishes or 

 extinguishes it, and captured glow-worms often cease to shine. The light is inter- 

 mittent, flashes succeeding one another rapidly (80-100 in the minute). In the 

 Observations (p. 344) White says that they put out their lamps between eleven and 

 twelve, and shine no more that night. This is not alwajrs the case ; we have seen 

 them shine long after midnight. The light in both sexes is quenched after pairing, 

 but the female glows again during egg-laying. The luminous segments glow when 

 removed from the body ; in oxygen they are said to become more luminous, and to 

 go out in carbonic acid, hydrogen, sulphurous acid or in a vacuum. The lumini- 

 ferous substance is fatty, and contains no phosphorus. 



Glow-worms usually keep close by day, but climb up stems or low shrubs at 

 night. The larvae are carnivorous, feeding upon small molluscs, dead or alive. 

 The adults feed upon plants, or, as appears to be the case with the males, not at 

 all. 



In the fire-flies (Luciola) of Southern Europe it is the gregarious males that 

 shine ; they possess large eyes, while the non-luminous female has small ones. The 

 fire-flies of tropical America belong to a different family of beetles (Elaters).'] 



1 See the story of Hero and Leander. 



