THE LIFE HISTORY OF HUMAN LICE 307 



hatch may be due to low temperature rather than to the chemical or other 

 agent tested. 



In interpreting results from control experiments, it is important to 

 bear in mind that lice will lay infertile eggs. Isolated females to which 

 males have not had access will lay eggs at about the normal rate. Such 

 eggs are all sterile and show no development. There is no evidence of 

 any parthenogenesis. But even females to which males have had access 

 will lay some infertile eggs especially near the beginning and end of their 

 lives. Nuttall says that at constant temperature of 30° C. we may 

 expect about 70 per cent of a given lot of eggs to hatch. Of those which 

 fail to hatch, some are fertile, some undergo partial development, but 

 for some unexplained reason fail to complete development. He points out 

 that "hatching alone is not therefore a true test of fertility." For accur- 

 ate work, eggs of known age should be used, preferably after they have 

 reached the stage when the eyes appear as faint brown spots on each 

 side of the head of the embryo. By examination with the binocular, the 

 presence of these eye-spots will indicate the number which are fertile, and 

 their absence, the number sterile. 



The freshly laid egg is almost transparent, but as the embryo de- 

 velops, the egg assumes a yellowish color and the eyes first appear as 

 pinkish spots, gradually turning red or brown in color, finally becoming 

 black. After the limbs become clearly defined and the claws and eyes 

 darken, there are slight movements of the limbs, and of particles within 

 the body of the embryo, and periodic pumping movements of the pharynx 

 begin to appear. These pumping movements become more frequent as 

 time for emergence approaches. Sikora and Nuttall were the first to 

 grasp the meaning of these pumping movements and show that they 

 are intimately concerned with the act of emergence. When the larva is 

 ready to emerge, the air is pumped in rapidly through the so-called air 

 canals of the operculum. The air is accumulated in the anterior end of 

 the shell, the body of the embryo completely filling the remainder. As 

 pumping continues, the air is passed on through the gut, "the bubbles 

 being distinctly seen through the transparent glassy shell as they pass 

 backward" and are expelled through the anus and accumulate in the pos- 

 terior end, thus pushing the embryo up against the operculum. "This 

 pressure of the air cushion finally overcomes the resistance of the oper- 

 culum and the latter springs open." The head of the larva is thus forced 

 out and assumes a normal position. Soon the first pair of legs is with- 

 drawn. These are quickly brought into action and with their aid the 

 remainder of the body is soon withdrawn. This highly interesting 

 process is important in its relation to control measures. In the first 

 place, if oily or greasy substances are used they occlude the air canals 

 of the operculum and the larva dies. Some substances when applied to 



