308 SANITARY ENTOMOLOGY 



young eggs, may evaporate without acting directly on the embryo and 

 leave the air canals open by the time the embryo has reached the stage 

 of pumping movements. In control experiments results have been ob- 

 tained which indicate tliis, the mature eggs being destroyed and the 

 younger eggs emerging some days later, showing that the chemical had 

 not affected the contents of the egg, but killed the older eggs by occlusion 

 of the air canals, and passed off in time to permit the younger ones to 

 hatch. 



Another point of importance is that proper temperature conditions 

 must be provided in such experiments, to permit normal emergence as 

 well as normal incubation. If the temperatures are too low the process 

 of emergence is slow and the vitelline membrane will dry before the larva 

 has freed itself. As a result the larva dies with the head and first pair 

 of legs and part of the thorax outside the shell, but the posterior end 

 of the body and the second and third pairs of legs stick to the dried 

 membrane, or it may be that the larva will die without bursting the mem- 

 brane. In some cases larvae have been found with all but one leg free 

 from the membrane, but this so firmly stuck fast as to prevent escape. It 

 is important, therefore, to bear in mind that the effect of low temperatures 

 may entirely outweigh the effect of the control measure under trial. Ef- 

 fective temperature is higher than for most other insects. 



The egg shell is very tough and resistant to chemicals as is also 

 the cement by which it is fastened and there is no known way of removing 

 them without first destroying the fibers or hair to which they are attached. 

 Hasc describes how the Russian prisoners tried to reduce infestation by 

 hanging shirts on a wire and beating with sticks, and Legendre recom- 

 mends vigorous brushing with a stiff brush. Hase is doubtless correct 

 in pointing out that beating fails to dislodge many of the lice or to crush 

 any of the eggs and that brushing may tear loose some fibers with at- 

 tached eggs, but actually destroys very few. On the contrary, it ic 

 pointed out that this may be the means of spreading the infestation to 

 other men rather than affecting any reduction. Hase carried out experi- 

 ments showing that lice can crawl up to the surface after burial in sev- 

 eral inches of dry sand or earth. If shaken or beaten out of the clothing 

 to the ground and pressed into the sand under the heel they will crawl 

 to the surface and attach to the first host near them, which they have 

 abundant opportunity to do in a crowded prison or prison camp, espe- 

 cially when the weather permits the prisoners to lie down on the ground. 

 Eggs binished from clothing will hatch if temperatures are favorable, and 

 the issuing larva' reach new hosts in the same way. 



Many experiments have been carried out by Hase and Nuttall with a 

 view to determining what kind of materials lice prefer for oviposition. 

 They agree in showing that rough materials such as felt, wool, and flannel 



