BOOK-LICE 33 



active, seldom stray far from the water whence they 

 came. About twenty-four British species are known, 

 but as they are all dull-coloured, they fail to attract the 

 notice of the amateur naturalist. Perhaps the best- 

 known species is Perla bicaudata, which is esteemed 

 by anglers as a good bait for trout. 



The fifth order of insects, the Corrodentia, includes 

 a number of small species which are especially inter- 

 esting because they are probably more nearly related 

 to the termites, or " white ants," of the tropics than to 

 any other insects. Mandibles are well developed, but 

 the other mouth-parts have sustained considerable 

 modification. When present, the delicate membran- 

 ous wings have few veins ; but many species (all in 

 the second sub-order) are wingless. In all cases 

 metamorphosis is slight. Probably the most familiar 

 representative of the Corrodentia is the so-called 

 " book-louse," 1 which is common in many houses. 

 In the house at present occupied by the writer this 

 insect is very abundant, and is especially attached to 

 a particular tiled washstand, in the crevices of which 

 it harbours. Although very minute just visible to 

 the naked eye it has a pair of stout little mandibles, 

 and is said to work havoc in libraries by gnawing the 

 surface of paper, whence its name. It is certainly 

 destructive to collections of dried plants and insects 

 unless measures for its suppression are adopted. Its 

 maxillae and labium are remarkable, the latter being 

 thick and fleshy, while each of the former carries a 

 hard, elongate process called a "pick," but what end 

 these peculiar structures serve is not known. The 

 " book-louse " shares with certain beetles the title of 

 " death-watch," because it is supposed to produce a 



1 A tropos divinatoria. 

 3 



