ENTOMOLOGY

308 SANITARY ENTOMOLOGYyoung eggs, may evaporate without acting directly on the embryo andleave the air canals open by the time the embryo has reached the stageof pumping movements. In control experiments results have been obtainedwhich indicate tliis, the mature eggs being destroyed and theyounger eggs emerging some days later, showing that the chemical hadnot affected the contents of the egg, but killed the older eggs by occlusionof the air canals, and passed off' in time to permit the younger ones tohatch. .Another point of importance is that proper temperature conditionsmust be provided in such experiments, to permit normal emergence aswell as normal incubation. If the temperatures are too lmv the processof emergence is slow and the vitelline membrane will dry before the larvahas 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 endof the body and the second and third pairs of legs stick to the driedmembrane, or it may be that the larva will die without bursting the membrane.In some cases lanre have been found with all but one leg freefrom the membrane, but this so firmly stuck fast as to prevent escape. Itis important, theref~re, to bear in mind that the eff'ect of low temperaturesmay entirely outweigh the effect of the control measure under trial. Effectivetemperature is higher than for most other insects.The egg shell is very tough and resistant to chemicals as is alsothe cement by which it is fastened and there is no known way of removingthem without first destroying the fibers or hair to which they are attached.Hase describes how the Russian prisoners tried to reduce infestation byhanging shirts on a wire and beating with sticks, and Legendre recommendsvigorous brushing with a stiff brush. Rase is doubtless correctin pointing out that beating fails to dislodge many of the lice or to crushany of the eggs and that brushing may tear loose some fibers with attachedeggs, but actually destroys very few. On the contrary, it iopointed out that this may be the means of spreading the infestation toother men rather than affecting any reduction. Rase carried out experimentsshowing that lice can crawl up to the surface after burial in severalinches of dry sand or earth. If shaken or beaten out of the clothingto the ground and pressed into the sand under the heel they will crawlto the surface and attach to the firs't host near them, which they haveabundant 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 brushed from clothing will hatch if temperatures arc favorable, andthe issuing larvre reach new hosts in the same way.Many experiments have been carried out by Hase and Nuttall with aview to determining what kind of materials lice prefer for oviposition.They agree in showing that rough materials such as felt, wool, and flannel