G. H. F. Nutt all 
89 
the present war, the incidence of lice on troops who could not perforce 
change their clothing except at long intervals, and who could not keep 
themselves clean, proves how important these factors are in relation 
to the incidence of pediculosis. Moreover, in winter, in normal times, 
people congregate more in houses, and the chances of infestation spread¬ 
ing are greater because of increased personal contact and the increased 
shedding of lice by the more heavily infested individuals. In summer 
the clothing is lighter, either fewer garments are worn or they are of 
lighter material, people huddle together less, clothing is more often 
changed, and there is more personal cleanliness. 
Apart from the factors above mentioned, which may influence the 
seasonal prevalence of lice on man, the climatic conditions prevailing in 
clothing deserve mention. Rubner 1 , who uses the word “Kleidungs- 
klima,” writes of it only in relation to man, but it is of equal importance 
to the louse. The temperature of the skin, when comfortably clothed, 
usually ranges, according to Rubner, between 30° and 32° C., the air 
temperature between succeeding layers of clothing falling toward the 
exterior, being lowest in the outside layer. The nearer the skin, the 
higher the temperature and the drier the air; the air is very dry near 
the body, and it is more charged with water in the outer layers of clothing 
in proportion as the air grows cooler. 
In the heat of summer, there is an increased amount of moisture 
given off from the skin, and with or without physical labour, there may 
be so much watery vapour given off as to render the innermost atmo¬ 
spheric layer almost or completely saturated, condensation taking place 
upon the clothing so that it becomes wet, with the consequence that 
the temperature of the skin rises. It has been observed in febrile 
conditions, that lice wander away from the host, and moreover, lice 
have been observed more frequently upon the outer garments in summer. 
Evidently the overheated body-surface and the excessive moisture are 
uncongenial to lice. Needless to say, active perspiration would lead 
to their being flooded out or driven away, especially from their usual 
haunts where the clothing lies most closely applied to the body-surface. 
It is these parts of the clothing that become wettest when active per¬ 
spiration sets in. It appears to me a fair hypothesis, therefore, that 
the climatic conditions prevailing in clothing in summer are on the 
whole less suited to the body-louse than in winter. This hypothesis 
is, moreover, borne out by the following evidence. 
That a few degrees higher temperature than that normally found 
1 Rubner, M. (1911), “Die Kleidung” in Handbuch der Hygiene, I. 600-606. 
