REVISION MALACOSOMA HUBNER EST NORTH AMERICA 135 



of incurvum is also tougher and contains smaller bubbles than that of 

 the A's. Larvae of both species are highly variable, and some larvae of 

 incurvum discoloratum wliich have conspicuous black spots surrounding 

 setal group Dl (fig. 395) could easily be confused with larvae of the 

 A's that are similarly marked (figs. 388-389). On the other hand, 

 incurvum larvae never have a vertical black bar like some of the A's, 

 and larvae similar to incurvum larvae such as those in figures 390-394 

 have never been seen among the A's. The preferred host of the A's is 

 aspen and that of incurvum is Cottonwood, so they occur on related hosts, 

 but it is not known if they would do well on the other's host. Both of 

 them also occur on other hosts, and both build conspicuous tents. Most 

 adult males (figs. 257-259) and females (fig. 260) of the aspen popula- 

 tions are basically soft reddish-brown, and differ from most male and 

 female incurvum which are gray-brown or yellow-brown (figs. 291-308), 

 but they cannot be separated by any known morphological characters. 



The differences in development between incurvum discoloratum and the 

 populations of californicum and californicum fragile which occur at the 

 same localities have already been discussed in the comparison of cali- 

 fornicum and incurvum in the southwestern United States (page 1 18). These 

 differences are sim,ilar to the differences in development between the A's 

 and BS's which were just described, but tlie A's cannot be considered to 

 be conspecific with incurvum because of the differences between them 

 given above. This does not mean, however, that connecting populations 

 of some kind do not exist because a few collections were made at inter- 

 mediate elevations which showed combinations of characters that may 

 be the result of gene exchange between incurvum, the A's and californicum 

 on other hosts at intermediate elevations. Two examples of such collec- 

 tions are Nos. 252 and 263 (figs. 261 and 262), but they have been clas- 

 sified as californicum because they resembled californicum more than 

 irKurvum. 



It is obviously impossible for direct gene exchange to take place 

 between populations occurring at elevations which differ by several 

 thousand feet, since the low elevation population may develop and 

 emerge weeks or even months earlier. Nevertheless, it would be possible 

 for genes to be carried from one population to another over a period of 

 years, since some mated females could fly to higher or lower elevations 

 before laying their eggs. During the next year these egg masses would 

 then be subjected to the same climatic conditions as those of the native 

 population. They would hatch, therefore, at the same time and develop 

 at the same rate as the native colonies if there were no inherent differ- 

 ences. Adults would then emerge at the same time and some would 

 mate with the native population if there were no other isolating mecha- 

 nisms. Theoretically, in the absence of other forces such as natural 



