306 SANITARY ENTOMOLOGY 



this temperature shortens their lives. Infected persons who remove their 

 clothing at night consequently become less heavily infested than those 

 who wear their clothing continuously. The periodic cooling of the cloth- 

 ing and the lice therein leads to their progeny being materially re- 

 duced. 



Nuttall and Bacot are both agreed that capitis prefers to lay its 

 eggs on hairs, but they do not agree as to whether corporis prefers cloth 

 to hairs for oviposition. 



The length of the egg period varies from 5 to 16 days under normal 

 conditions and may be retarded to the 35th day, or possibly later, by 

 cold periods. Under European conditions of humidity, apparently 30° 

 C. (86° F.) gives about the optimum condition for hatching, although 

 the shortest period experienced was at 37° to 38° C. (99-101° F.). In 

 experiments at New Orleans at 37° C, hatching occurred in four to 

 eight days ; while the eggs hatched in six to seven days at temperatures 

 of 33° to 34° C, and in eight days at 30° C. The effective zone for 

 the egg stage is from slightly under 20° to 40° C. (68-108° F.). At 

 temperatures of 40° to 45° C. the embryo dies. In this connection, 

 Nuttall has made some very unfortunate remarks. He discredits the 

 seven-day record with two individuals at 20° C. made by Widmann, the 

 ten to twelve-day records at the same temperature made by Heymann, 

 and Legroux's statement that they rarely hatch at 16-18° C, because 

 Sikora and Hase recorded failures at 20° and Nuttall and Hindle failed 

 to hatch eggs at 22° C. It is quite possible at 20° C. at one humidity 

 to obtain death ; at another humidity, 12-day development ; at another, 7- 

 day development ; and at still a different humidity, possibly a very long 

 developmental period. All of Nuttall's remarks on temperature effects 

 must be more or less discounted because of his ignoring the important 

 humidity factor. In fact, he states that there is no evidence that eggs 

 maintained at 22° C. or under are capable of hatching, but he quotes 

 quite a series of retarded development records in which the eggs were 

 maintained for more or less long periods at low temperatures. For in- 

 stance, Widmann kept eggs for 24 hours at 10° C, and then transferred 

 them to 26-30° C, and they hatched in 17 days. After keeping eggs at 

 9° C. for two or three weeks, Heymann transferred them to a favorable 

 temperature and they developed in 15 days. The length of time the eggs 

 can stand a given low temperature will depend to a large measure on the 

 humidity. At a given temperature it appeared that dryness may retard 

 development two or three days or more. Thus it may be seen that there 

 is still work to be done on the effect of humidity on the incubation period. 

 In testing various insecticides for their effect on the eggs, it is neces- 

 sary to provide the proper temperature conditions; otherwise, failure to 



