LIFE-HISTORY OF THE BODY-LOUSE 23 



newly laid, but becomes more opaque later as the 

 young louse develops inside it. The empty shell 

 is hard and brittle and remains attached after 

 the louse has emerged ; the top fraying away, but 

 the base and cement remaining. On the bristles 

 of brushes the empty shells of the eggs of the 

 hog-louse (Haematopinus sui) are sometimes seen 

 in large numbers. They are of course harmless 

 in themselves, but are unsightly, and at any rate 

 afford evidence that the cleaning of the bristles 

 has been none too thorough. 



It is difficult to tell whether the egg is empty 

 or full without the aid of a lens. At the tempera- 

 ture which ordinarily exists between the skin and 

 the clothing the eggs hatch in from seven to ten 

 days, but if kept in a cooler atmosphere the 

 incubation period is lengthened. Thus when a 

 garment is put off at night hatching is retarded 

 in proportion to the coolness of the bedroom. 

 It is not essential to the hatching of the eggs that 

 the garments which hold them should be worn, 

 as the young will emerge if they are incubated at 

 any temperature above 72 F., and below that 

 which destroys them, provided that the air is not 

 too dry. At temperatures below 72 F. the 

 young will not emerge. Nuttall (1) describes the 

 hatching of the egg. The inside of the shell is 

 like that of a deep smooth chalice, and the young 

 louse completely fills it and has its legs folded 

 back along its body. The mechanism of its 

 emergence is remarkable. Air passes through 



