REVISION MALACOSOMA HITBNER IN NORTH AMERICA 51 



Table 1. — Normal and abnormal egg masses obtained from experimental 

 crosses and controls in 1960 and 1961 



1960 Fa 



1961 Fi 



disease-weakened moths reared in 1960, and to the excessive disturb- 

 ance produced by the higher numbers of moths confined together in 

 1960. As previously mentioned, when too many moths were confined 

 together they created such a disturbance by flying about that the 

 ovipositing females often were disturbed and prevented from laying 

 egg masses or forced to lay abnormal ones. There are no other differ- 

 ences between the controls and hybrids or between years that appear 

 to be of any importance. 



The following terms are used in discussing the results: 

 1960 Fi Egg masses obtained during 1950 from crosses of field- 

 collected specimens and reared during the spring of 1 96 1 . 

 Egg masses obtained from the 1960 Fj. 



Egg masses obtained during 1961 from crosses of field- 

 collected specimens. 

 Originally it was planned to rear selected Fj egg masses to adults, 

 obtain F2 &g^ masses and rear them to adults in order to obtain esti- 

 mates of hybrid viability, sterility, or inferiority, or the lack of these. 

 No major difficulties were encountered in holding the 1960 Fj eggs 

 through the v/inter or in rearing them the following spring, but diffi- 

 culties in assigning causes of mortality, and in holding the 1960 F2 

 and the 1961 Fj egg masses through the winter necessitated abandon- 

 ment of the plan to rear them through the F2. The difficulty in holding 

 the egg masses over the winter affected all of the egg masses regardless 

 of whether they were 1960 F2S, 1961 FjS, or controls, so the holding 

 technique was probably at fault, although it had worked satisfactorily 

 on the 1960 FjS. The exact cause of the difficulty is not known, but it 

 resulted in little or no hatch the following spring. 



Presentation of all the data concerning all of these crosses that were 

 attempted in 1960 and 1961 is not deemed necessary since essentially 

 what is shown is that any isolating mechanisms between some of these 

 populations which may be present under field conditions break down 

 under laboratory conditions. However, some of the data for the 1960 

 Fi rearings are presented in Table 2 because they are of interest in 

 other respects. 



