THE PEA APHIS WITH RELATION TO FORAGE CROPS. 



43 



Figure 1 1 shows graphically the lines of generations carried through 

 in 1912 and the different individual experiments in each generation, 

 including the cage number and dates of birth above the line and the 

 date of death or termination of the experiment below the line, for each 

 cage record. Figure 12 shows the length of each generation of the 

 same series (1912). 



Thus it will be noticed that the first generation in the series (prob- 

 ably the second generation from the egg) was the shortest, while the 

 ninth generation was the longest, extending over a period of 156 days. 

 Likewise it will be seen that on June 1 two generations coexisted; 

 on July 1, four generations, from the second to the third; on August 1, 

 seven generations, from the third to the ninth, inclusive, and on Sep- 

 tember 1, eight generations, from the fifth to the twelfth. 



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 * This is the first generation found in the fisld and is probably about the third from the egg. 



HATCHING OF THE EGG. 



At La Fayette, Ind., the eggs of M. pisi hatch the latter part of 

 March; in the cases recorded in 1913 they hatched March 31. Folsom 

 (1909) records the hatching of eggs at Urbana, 111., March 23, in 1905. 



MOLTING. 



According to our experiments this plant louse, like others of this 

 family of insects, has five instars and never molts more than four 

 times. In 1905 Mr. R. L. Webster, then an assistant of Dr. S. A. 

 Forbes, State entomologist of Illinois, observed 10 individuals, all of 

 which molted four times, although Mr. J. P. Gilbert, at the same 

 laboratory, claims to have observed an individual molt five times 

 (Folsom, 1909). Table IV gives our detailed records. 



