REVISION MALACOSOMA HUBNER IN NORTH AMERICA 57 



of 4%) and the mortality after hatching (76% instead of 100%) 

 were better, although still not good. Nevertheless, these two cases 

 indicate that considerable variability in hatching and mortality could 

 have been expected if it had been possible to rear more egg masses. 



The egg masses are grouped into three sections: those crosses with 

 reciprocals, those without reciprocals, and controls. Considering the 

 fact that only a single egg mass was hatched, the data are fairly 

 uniform for all egg masses, with few exceptions. In general the con- 

 trols show greater variability than either of the other groups. 



The total number of eggs per mass ranged from 55 to 252, with a 

 mean size of 160. The mean size of the controls was 156 and of the 

 hybrids 162. All egg masses were removed from the temperature 

 cabinet on April 27th. Date of hatching was taken as that day when 

 most of the caterpillars had hatched. Usually most of them hatched the 

 same day, but occasionally a few hatched a day eai'lier, or if the hatch 

 was poor, those that were successful in hatching sometimes straggled 

 out for several days. This may account for the apparently later hatch 

 of the controls (approximately 1 day later on the average) than the 

 hybrids, since the hatch of the controls was poorer in most cases. 



The number of eggs hatching was quite high for most egg masses. 

 The two outstanding exceptions are R4 and R7 (both controls) in which 

 the hatch was very low. Nearly all of these eggs had gone through 

 their embryological development successfully, but for some reason 

 they failed to hatch. No reason is known for this failure, but the 

 caterpillars in R7 had eaten away about half as much of the chorion 

 as would have been necessary for them to escape from the eggs, indi- 

 cating that the eggs were fertile, but the chorion was too tough, or 

 the viability was reduced for some reason. The hatch of the replacement 

 egg mass for R7 (R38) was about 50 percent, which is considerably 

 better, but still much below that of most other egg masses. 



Among the other egg masses R2 showed only about 50 percent 

 hatching. The male parent of this egg mass was the same as the parents 

 of R4 (which had a very low hatch, 4%), but four other egg masses 

 with one R4 parent (Rl, R5, R6, R21) showed very good hatching, 

 indicating that these variations in hatching were probably due to chance 

 and of no significance. 



The mean time for development from hatching to emergence as 

 adults (both sexes) varied from 33.3 to 39.2 days for the controls, 

 and from 35.1 to 40.7 days for the hybrids under greenhouse conditions 

 at prevailing temperatures in May and June. This is a spread of 5.9 

 days for the controls and 5.6 days for the hybrids, but the mean time 

 of development for nearly all of the controls and hybrids is somewhere 

 between 35 and 39 days. These difTerences could be due to factors 



