88 
Biology of Pediculus humanus 
1909-1916, and the accompanying chart, which I have drawn, comprises 
the records for the whole period 1909-1916, the percentage of infested 
beds being shown in monthly columns. Dr Hamer informs me that 
he attributes the remarkable fall in the curve since 1909 to the undoubted 
improvement in the condition of the beds consequent upon the increased 
attention directed to them as the result of his investigation. 
Hamer (1910, p. 7) refers the greater prevalence of body-lice in winter 
in London to the clothes being less frequently changed and washed at 
this season. Sergent and Foley (vi. 1910, pp. 337-373) observed a 
similar increase of lice during winter in Algeria, when the indigenous 
population wear their clothes continuously for weeks, few lice being 
found on their persons in summer when light clothes are worn, the 
children wearing hardly more than a shirt. Muller (1915, p. 53), in 
Austria, refers the increase of lice in winter to woollen clothes being 
more suited-to lice than lighter fabrics. Klemperer and Zinn (ii. 1915, 
p. 324), in Germany, attribute the subsidence of typhus in summer to 
the effect of heat in killing off the lice, probably basing their statement 
upon the observations of Anderson and Goldberger in Mexico (see 
p. 90). Ha-se (xi. 1915, pp. 160-163) attributes the much lessened 
prevalence of lice in the German army, during the preceding summer, 
to three causes having been in operation: (a) the heat, ( b ) increased 
personal cleanliness, and (c) greater experience in dealing with lice; 
he states (p. 158) that lice are more frequently encountered on the 
outer garments in warm weather. An increase of pediculosis capitis 
in winter has been observed in children by Pinkus (1915, pp. 239-241) 
owing to the hair being allowed to grow and its not being washed 
because of the cold (see the section on lice and disease, p. 68). 
From the foregoing evidence, we may regard it as established that 
there is an increase in the louse population in the winter months and a 
decrease in summer. Kisskalt’s figures (p. 87), showing that there is 
no special difference in the number of verminous persons admitted to 
Berlin refuges in winter and summer, do not disprove this general 
conclusion, since they fail to take into account the probably increased 
louse population per man and relate but to a very limited class of 
persons. 
Leeuwenhoek (ed. 1807, p. 163) already noted that lice particularly 
trouble those who cannot change their apparel frequently; such 
persons are necessarily unclean. It is a matter of common knowledge 
that the poorer classes are more prone to wear their clothing continuously 
and that they are more unclean in winter than in summer. During 
