56 UNITED STATES NATIONAL MUSEUM BULLETIN 276 



The parents of all the 1960 Fj populations discussed here were collect- 

 ed in California, Oregon, Washington, British Columbia, Alberta, 

 and portions of states or provinces bordering them. All of them lay 

 the flat, clasping type of egg mass, build large, conspicuous tents, occur 

 on a wide variety of trees and shrubs, and are indistinguishable from 

 each other by any known structural characters of either the larvae 

 or adults. Some of the adults may be recognized by the color pattern, 

 but the populations which have been described as species in the past 

 have been separated mainly by differences in the larval color pattern. 



The reared 1960 Fj consisted of 36 egg masses (including controls), 

 totaling 5777 eggs, of which 4789 hatched, and 2042 were either pre- 

 served as larvae or reared to adults. The larvae which were preserved 

 are counted as adults in the mortality figures, although some of them 

 would not have survived to be adults. The full collection data for the 

 parents of each egg mass are given in Appendix 1 (page 253). The specific 

 names applied to each population are the names that were in use when 

 this study was begun. 



The 2,747 larvae which died of undetermined causes at any stage 

 from the first instar to the pupa make up 57 percent of those which 

 hatched. Desirable as it would have been to determine the cause of 

 death for every caterpillar which hatched, it simply was not possible. 

 No doubt many of them died of starvation in the early instars after 

 falling off" the foliage, since they are easily overlooked at this stage, 

 and many do not find their way back to the foliage. Others may have 

 refused to eat apple or may have succumbed to diseases, altliough 

 only a few obviously were diseased. No doubt at least a few were 

 genetic misfits, but they could not be distinguished from the others. 



One thing, however, is apparent when the percent mortality of 

 hatched caterpillars is examined. It is quite high in most cases, with 

 many of the controls showing a higher mortality than the hybrids. 

 No reason for the higher mortality of the controls is known, but in- 

 breeding could have reduced the viability of the controls or hybrid 

 vigor could have increased the viability of the hybrids, or it may be 

 due purely to chance since only a single egg mass of each was reared, 

 with two exceptions. 



These two exceptions (indicated by *) wei-e replacement egg masses 

 that were hatched after a poor hatch of tlie first egg mass or an early 

 dieoff" of the caterpillars that did hatch. In one case R39 replaced 

 R22 after all but two caterpillars had died in the second instar (possibly 

 it got too hot in the area of the greenhouse where they were). In the 

 replacement egg mass the hatch was poorer (76% instead of 91%), 

 but the mortality after hatching was less (82% instead of 100%). 

 In the second case R38 replaced R7, and both the hatch (52% instead 



