64 UNITED STATES NATIONAL MUSEUM BULLETIN 2 76 



mass, however, and 85 to 95 percent of the eggs of both the F2 and F2 

 controls contained embryos which had developed at least to the point 

 where they were recognizable as caterpillars. This is somewhat less 

 than the approximately 95 percent of recognizable caterpillars present 

 in the 1960 Fi egg masses. It should be noted, however, that the F2 

 egg masses were held under conditions which obviously were un- 

 favorable at some time, since most of them were dead or dying when 

 they were examined. 



Summary of Experimental Crosses 



If the presentation of these data has emphasized the complete lack 

 of any absolute reproductive barriers between the various western 

 populations which build large tents, it has served its purpose. But a 

 comparison must be made between the results obtained in the labora- 

 tory and observations in the field. The best way to do this is to compare 

 the larval variability that was observed in the field with that which 

 was observed in the larvae reared from the crosses. 



It was possible to cross male M. americanum with females of other 

 western populations and obtain normal egg masses. Some of these eggs 

 went through their embryological development, but could not be 

 reared because of the difficulties already mentioned in holding the eggs. 

 Such crosses, however, probably never occur in the field since no 

 specimens have ever been seen which can be interpreted as hybrids 

 between americanum and other species. 



This is not the situation that was found in the field for the other 

 populations that have been discussed in the latter part of this section 

 (those occurring in California, Oregon, Washington, western Nevada, 

 Alberta, British Columbia, and bordering areas). Populations in these 

 areas have been described in the past as five different species (ambi- 

 simile, californicum, pluviale, fragile, and lutescens), but in every case 

 the larval variations that were found in the "hybrids" which were 

 reared in the laboratory were also found in field populations. The 

 variability of the "hybrid" adults also fell within the range of vari- 

 ability that was found in adults reared from field-collected larvae or 

 field-collected adults. 



The populations of Malacosoma which occur in the Great Basin, the 

 United States Rocky Mountains, and the Southwest were studied in 

 1961. Most of the populations occurring in these areas are less well 

 defined geographically, and show greater variability than many of 

 those studied in 1960. As is pointed out in Table 1, the success of 

 various attempted crosses was better in 1961 than in 1960. It was not 



