Mr. Rennie on the Cleanliness of Animals. 21 



male glow-worm is intended as a converging reflector of the 

 light of the female, ' always beneath him on the earth.' * As 

 we commonly,' he adds, ' and with advantage, place our hand 

 over the brow, to obstruct the rays of light falling from above, 

 which enables us to see clearer an object on the ground, so 

 must the projecting hood of this creature converge the visual 

 rays to a point beneath */ 



Unfortunately for this theory, the grubs which, being in a 

 state of infancy, are therefore incapable of propagating exhibit 

 a no less brilliant light than the perfect insect. De Geer says 

 the Jight of the grub was paler, but in the one which I had it 

 was not so. He also remarked the same light in the nymph 

 state, which he describes as * very lively and brilliant ;' and, in 

 this stage of existence, it is still less capable of propagating 

 than in that of larva. * Of what use then,' he asks, ' is the 

 light displayed by the glow-worm ? It must serve some pur- 

 pose yet unknown. The authors who have spoken of the male 

 glow-worms say positively that they shine in the dark as well 

 as the females {.' These plain facts appear completely to ex- 

 tinguish the poetical theory. But to return to our immediate 

 subject. 



A very remarkable instrument, which recent observations 

 seem to prove to be intended for a similar purpose to that of 

 the caudal apparatus of the glow-worm, just described, occurs 

 in the fern-owl, or night-jar (Caprimulgus Europaeus), popu- 

 larly called the goat- sucker, from an erroneous notion that it 

 sucks goats a thing, which the structure of its bill renders 

 impossible as that of cats sucking the breath of infants, as is 

 also popularly believed. The bird alluded to has the middle 

 claw cut into serratures, like a saw or a short-toothed comb ; 

 the use of which structure seems to have been misunderstood 

 by White of Selborne. 



Foot of the European night-jar, shewing the pectinated clav 



Journal of a Naturalist, p. 292, first edition. 

 f De Geer^ Mem. iv. 44. 



