182 
Biology of Pediculus humanus 
ordeal and subsequently batch if brought under favourable circum¬ 
stances. Incidentally the experiments do not lend support to the 
statement by Nocht and Halberkann (1915, p. 626) that young nits 
are more resistant than old ones. e 
Cold. As a matter of practical importance it is well to know 
that nits are capable of surviving for a considerable time at a low 
temperature which checks their development. The active stages are 
less resistant. 
Nits. Widmann (18. viii. 1915) saw nits survive 13 days at 10° C., 
whilst Heymann (18. viii. 1915, p. 305) states they survive 2-3 weeks 
at 9° C., and several days’ exposure to frost. Peacock (1916, p. 41) 
reports that some nits exposed at ca. 0° C. on a shirt for 8 days, subse¬ 
quently hatched in 4-5 days when carried on the body; after 13 days’ 
exposure they failed to hatch although he kept them under observation 
for 28 days before he concluded that they were dead. Kisskalt (n. 
1915, p. 154) found that nits survived exposure to — 5° C. overnight. 
Active stages. The authors cited below do not state upon what 
stages they made their observations; we may assume that some of the 
lice at any rate were adults. 
Time lice 
survived 
24 hrs 
24 hrs 
48 hrs 
4 days 
5 successive 
nights 
7 days 
Temperature ° C. Remarks 
Authority 
- 0 - 2 ° ... — 
0° ... on wet cloth that became frozen 
-2-3 to 1-1° revived when warmed 
- 2 to 0° ... on cloth 
- 8° ... in winter, revived when warmed 
-12° ... in dry snow (N.B.; they must thaw 
slowly or they die) 
Wulker, 1915, p. 630 
Hase, 1915, p. 70 
Bacot, n. 1917. p. 235 
Hase, loc. cit. 
Heymann. 18. viii. 
1915, p. 309 
Hase, loc. cit. 
Heat. The resistance of lice to heat will be dealt with in the next 
paper in connection with louse destruction. 
Vacuum. According to Heymann (18. viii. 1915, p. 310) lice and 
nits survive exposure to a vacuum of 770 mm. for 15 minutes, the 
insects being lively after the ordeal. 
Mechanical pressure. Galli-Valerio (1916, p. 41) records that a 
louse subjected to pressure from a weight of 13 g. survived for ca. 24 
hours, the insect being confined between a slide and coverglass. 
