REVISION MALACOSOMA HUBNER EST NORTH AMERICA 47 



without magnification, but younger ones can be difficult even with the 

 aid of a microscope. 



The same characters were found to be useful in sexing the larvae of 

 some species belonging to other families of Lepidoptera, but additional 

 species in the same families could not be sexed by this method. 



Experimental Grosses 

 Introduction 



One method to determine whether interbreeding between two popula- 

 tions takes place under field conditions is to search for intermediate 

 populations in the area where the two populations in question meet. A 

 problem that occurs with tlois method is the difficulty of knowing how an 

 intermediate should look. Should it be midway between both popula- 

 tions or will some characters tend more toward one population than 

 the other? One way to determine this is to try to cross the populations 

 in the laboratory and observe the variation that shows up in the progeny 

 (if any). The observed variation can then be compared with what is 

 found in the field. Of course, it is desirable to rear such crosses through 

 the F2 in order to obtain a more complete picture of the possible 

 variations and evidence of any hybrid breakdown if it exists, but this 

 is not always possible owing to such factors as disease, length of life 

 cycle, difficulty of holding certain life stages, etc. Such crossbreeding 

 was attempted for many of the populations of Malacosoma in question, 

 but it was only partially successful because of a combination of diffi- 

 culties, the most important of which was disease. 



The data presented in the following section are incomplete in many 

 respects, but they are the best that could be obtained under the pre- 

 vailing conditions, and they are presented for some of the possibilities 

 that they suggest. 



Methods 



Crosses between the described species, forms, etc., were attempted 

 whenever it was possible to synchronize the emergence of the adults. 

 Some of the crosses attempted can never take place in the field under 

 present conditions since the populations do not meet, yet such crosses 

 may indicate the possible origin of populations that occur in the 

 intervening areas. 



