G. H. F. Nutt all 
153 
Therefore out of 1158 eggs maintained at 30° C., 806 or 69-6% 
hatched. In two observations, the figures are low, especially in that 
where the eggs were kept at 37°. Under constant temperature con¬ 
ditions in the thermostat at 30° C., we may therefore expect roundly 
70 % of all eggs to hatch when laid by females to which males have 
access. Of the eggs that fail to hatch, some are sterile and others are 
fertile. In the latter case development may be arrested, or the larva 
may fail to issue. Hatching alone is not therefore a true test of fertility 
(vide infra and pp. 160, 161). 
Peacock (1916, p. 41) records that of 291 eggs taken from a shirt, 
82 % hatched. Swellengrebel (1916, p. 4) saw but 47 % of the eggs 
hatch under experimental conditions which he fails, however, to 
specify. The importance of the conditions under which the parents 
and eggs are maintained in relation to the proportion of eggs that hatch, 
is well brought out by the following records relating to capitis. 
The number of eggs of head-lice that hatch under ordinary experi¬ 
mental conditions is usually very low. At most a very few eggs will 
hatch in the thermostat at 30° C. under conditions where corporis 
yield an average hatching of 70 %. After repeated trials and failures 
to raise capitis satisfactorily in the thermostat with two daily feedings, 
etc., as applied to corporis, I concluded that the conditions were so 
obviously unfavourable as to render further efforts futile. The wristlet 
method (see p. 107) was therefore devised and remarkably successful 
results were at once obtained, merely because the conditions were as 
nearly normal as it was practically possible to obtain. The following 
three experiments demonstrate the difference between a good and a 
bad method in studying the biology of lice. 
Expt. 1 . Some $9 were placed with dd in a wristlet, and laid 
211 eggs on 2-4. vi. 17; 204 eggs hatched, two larvae showed arrested 
development and five eggs shrivelled up at an early stage, this indicating 
that they were sterile. Therefore at least 97-6 % of these eggs were fertile 
and 96-6 % hatched. 
Expt. 2. Seven 99 under the same conditions, the progeny of the 
foregoing, laid exactly 500 eggs on 19-29. vi. 17; of these six were 
sterile (ca. 1 %), whilst four larvae failed to complete their development. 
Therefore 99 % of the eggs were fertile and 490 or 98 % hatched. 
Expt. 3. Three 9? with two dd, of the same generation as the last, 
were placed under the same conditions, and laid a total of 275 eggs 
