48 UNITED STATES NATIONAL MUSEUM BULLETIN 276 



Of course, successful interbreeding under laboratory conditions does 

 not prove that interbreeding takes place in the field, but it does prove 

 that it is possible for it to take place. Whether it does or not, and to 

 w^hat extent, must be determined by examining the populations in the 

 field. Conversely, a lack of interbreeding under laboratory conditions 

 does not prove reproductive isolation in the field. However, if inter- 

 breeding is successful within each test population in the laboratory, but 

 unsuccessful between the test populations, reproductive isolation due 

 either to a lack of attractiveness of one or both sexes (sexual, psycho- 

 logical, or ethological isolation), or because of physical noncorrespond- 

 ence (mechanical isolation) is indicated. In Malacosoma physical non- 

 correspondence does not appear to be important, since the terminalia 

 of all species are not greatly different in size and shape. Sexual isolation 

 under laboratory conditions seems to be quite pronounced in some 

 species, but w^eak or virtually absent in others. 



No attempt was made to observe mating or attempted mating. 

 Unmated females either lay no eggs or a few abnormally deposited ones. 

 This fact has been demonstrated by Williams (1939), and was cor- 

 roborated by this study. In every case where virgin females were placed 

 in a cage without males, no more than 30 eggs were laid (usually 1 or 

 less), and they were laid in an abnormal way. These eggs were held 

 under the same temperature and humidity conditions as the normal 

 ones, but none of them ever hatched, and subsequent examination of 

 them revealed that embryological development had not taken place. 



Cocoons were held in individual emergence cups, so there was no 

 possibility of any mating taking place. Males from one population were 

 placed with females of another population in the built-in cages in the 

 insectary- trailer, or in screened, 1 -gallon liquid food containers when 

 other cages were not available (most of the time). The reciprocal cross 

 was also attempted, as well as intrapopulational matings to serve as 

 controls. The number of males or females per cross varied from one to 

 ten or more, depending on how many were available on a given 

 evening. It was found, however, that the optimum number of moths 

 for obtaining the highest percent of normal egg masses was two to four of 

 each population for the 1 -gallon liquid food container cages. If larger 

 numbers of moths were used, they created such a disturbance by flying 

 about that the females were prevented from laying egg masses or were 

 forced to lay abnormal ones. Branches for oviposition were provided, 

 using the native host of the female whenever possible. 



Mating and oviposition usually took place the first night. Both males 

 and females appear to be sexually mature at the time of emergence from 

 the cocoon, since many of them mate soon after their wings are dry, and 

 the females lay Ggg masses the first night which are normal and viable in 

 every respect. Males may mate more than once. This is supported by 



