58 UNITED STATES NATIONAL MUSEUM BULLETIN 276 



such as random variation, location in the greenhouse, food quaUty, 

 and disease. The differences betAA'een the controls do not appear to be 

 correlated with any otlier known differences between these populations. 

 The hybrids are more uniform than the controls, but this is what one 

 would expect since any differences between the parents would tend to 

 be moderated in the Fj. 



The developmental period for the two replacement egg masses (R38 

 and R39) was 31.4 days and 33.4 days respectively. This shorter 

 developmental period was caused, no doubt, by warmer temperatures, 

 since hatching took place approximately 3 weeks later in the spring 

 than for the others. 



In the past there have been suggestions made that developmental 

 times may be useful in separating some of the closely related species 

 of Malacosoma (Langston, 1957). It is certainly true that there are 

 differences in developmental times between contrasting localities such 

 as alpine and valley, but much of this variation can be attributed to 

 temperature differences (especially night temperatures). This does not 

 mean that differences in developmental rates do not exist between 

 some populations, but such observed variations cannot be considered 

 to be reliable unless the two populations in question occur at the same 

 locality, and therefore have been subjected to the same basic weather 

 conditions. (See the introductoiy discussion to M. californicum and M. 

 incurvum on page 1 18 for an example of a meaningful difference.) If the 

 populations in question do not occur in the same locality (as will 

 usually be the case) then they must be reared from disease-free eggs 

 under very carefully controlled conditions before any valid comparisons 

 can be made concerning developmental rates. This has not been done, 

 and was not done in this study because of a lack of disease-free stock 

 and no temperature control in the insectary-trailer. 



In any rearings that are undertaken consideration also must be 

 given to such factors as individual variation, host preferences, and 

 adaptive differences between populations occurring in different climatic 

 areas. Under laboratory conditions Wellington (1957) found differences 

 as great as 7 days between the developmental rates of active (Type I) 

 larvae and sluggish (Type II) larvae of M. {californicum] pluviale. 

 These differences were even greater under field conditions where 

 colonies of sluggish larvae didn't move to their food as often as active 

 colonies, and resulted in some active colonies completing development 

 as much as 3 weeks before sluggish colonies. Obviously, if valid com- 

 parisons are to be made, the larvae must be segregated by activity 

 types as soon as possible after hatching. 



