External Factors of Sex Determination 377 



and females. The starved caterpillars gave no special excess of 

 males, although there were, it is true, a few more males; but 

 Riley pointed out that under natural conditions the females in 

 several species take a longer time than do the males to reach the 

 pupa stage, undergoing in some cases one more moult, so that, 

 if starved, the mortality would therefore probably be greater 

 amongst the females, and hence more male than female moths 

 might be found. Other observers, Bessels 1868, Briggs 1871, 

 Andrews 1873, Fletcher 1874, have also found that an excess or 

 diminution of the food does not alter the proportion of the sexes. 



The futility of many of these experiments has now become 

 apparent, since it has been shown that the sex of the caterpillar 

 is already determined when it leaves the egg. Under these cir- 

 cumstances it is not probable that feeding could produce a change 

 in the sex. It is much more probable that starvation or over- 

 feeding could only effect the proportion of males and females 

 by bringing about a greater mortality of the individuals of one 

 sex. 



Kellogg and Bell have studied food conditions in relation to 

 sex determination in silkworms (Bombyx mori). They have 

 guarded against the possibility of greater mortality of one sex. 

 The chief interest of their work is their examination of the pos- 

 sible effects of nourishment on the second generation. The 

 effects of feeding the caterpillars themselves different amounts 

 of food were as follows. Twenty larvae were present at first in 

 each lot. 



"Lot I. Fed optimum food ; no deaths before emergence of 

 moths; produced 8 males, 12 females. 



" Lot II. Fed optimum food ; 2 deaths before maturity ; 

 produced 7 males, 1 1 females. 



" Lot III. Fed one half (approximately) of optimum of food ; 

 4 deaths before maturity; produced 10 males, 6 females. 



" Lot IV. Fed living minimum of food ; 3 deaths before ma- 

 turity; produced 10 males, 7 females. 



" Lot V. Fed living minimum of food ; 6 deaths ; produced 9 

 males, 5 females." 



