50 
BULLETIN 276^ U. S. DEPARTMENT OF AGRICULTUEE. 
to be about 7 young per day, or even more. In oiu' experiments indi- 
vidual females bore as many as 124 young, and Dr. J. W. Folsom 
(1909) reports a case where a single female produced 147 yomig. In 
Mr. K. L. Webster's experiment conducted at Urbana, 111., in 1905 
(Folsom, 1909) the average number of young borne by females of 
16 consecutive generations was 46, while the average number of 
young produced by individual females in all our experiments con- 
ducted at La Fayette, Ind. (53 examples), excepting a few which 
were accidentally or prematurely killed, was 68.3 + . . 
From these figures it is not difficult to see why this msect becomes 
so remarkably abundant, apparently within a few days, on its 
various hosts, and why it is able, collectively, to ravage and com- 
pletely destroy crops almost before they become apparent to the 
casual observer. 
SEXUAL FORMS. 
Sexual forms may occur in the fall of the year, but there seems to 
be no uniformity in their production as is the case of certain other 
plant-lice. For instance, oviparous females may be produced by 
either wingless or winged females and the same female .may produce 
both viviparous and sexual forms alternately'; for example, in one of 
the experiment cages of 1912 a ^vmgless female gave birth to her first 
young on October 10 and these proved to be oviparous females; 
later she gave birth to young which became viviparous females, and 
still later agam bore oviparous females. A number of instances 
where females gave birth to viviparous and sexual forms alternately 
were observed in 1912 and 1913. Our earliest record of the birth 
of individuals of the sexual generation was October 10 m 1912 and 
October 14 m 1913. Dr. Folsom (1909) found the males in the field 
as early as October 10 in Illinois, and in one instance, in an experi- 
ment cage, an oviparous female was born as early as September 22. 
At Funkstown, Md., Mr. J. A. Hyslop observed the sexes of this species 
swarmmg on an alfalfa field November 12, 1912, the males and ovi- 
parous females predominating, although some viviparous females 
and young were observed. This observation was repeated at the 
same place by Mr. C. M. Packard October 28, 1913. 
From these observations it is impossible to attribute the pro- 
duction of sexes to any particular cause. Certain apliids, notably 
ApJiis maidi-radids , Sipha Jlava, Callipterus trifolii, Cliaitopliorus 
negundinis, Eulachnus rileiji, etc., invariably produce the egg-lapng 
forms toward ^\dnter in this latitude, and this may be attributed 
largely to the weather conditions; but in the case of pisi, both Tixi- 
parous and oviparous forms are commonly borne of the same mother, 
and in the same line of generations, conducted under exactly identical 
conditions, reproduction may continue viviparously throughout the 
winter, while parts of one or more of these generations may become 
sexual forms. It is noted, however, that sexes are never produced 
