50 UNITED STATES NATIONAL MUSEUM BULLETIN 2 76 



populations laid normal egg masses under caged conditions, but only 

 about 20 percent of those from Alberta and Manitoba populations laid 

 normal egg masses. No reason is known for this difference. 



Only a limited number of attempts at crossing M. americanum with 

 the western populations of other species could be attempted, but these 

 were partially successful in one direction. No eggs were obtained 

 from several attempted crosses totaling 13 female americanum from 

 Texas which were caged with a total of 1 9 males from various western 

 populations. However, eight normal egg masses and one abnormal egg 

 mass were obtained from crosses totaling 39 females from various 

 western populations which were caged with a total of 36 male ameri- 

 canum from Texas. Unfortunately, owing to the difficulties in holding the 

 eggs, it was not possible to rear these eggs, but a check of the eggs 

 showed that some of them had gone through embryological develop- 

 ment. However, no field-collected specimens have ever been seen which 

 could be interpreted as being hybrids between americanum and any 

 other species, so it must be concluded that reproductive isolation is 

 complete under field conditions, but breaks down partially under 

 laboratory conditions. This is not unusual in Malacosoma, as is shown 

 by numerous reports in the European literature concerning the pro- 

 duction of hybrids under caged conditions between the European 

 species M. neustrium, M. castrensis, and M. franconicum, three appar- 

 ently distinct species which are probably well isolated under field 

 conditions. See Collier, 1 936, for a partial list of references to these. 



Excluding M. disstria, M. tigris, M. constrictum, and M. americanum, 

 the rest of this section on experimental crosses deals with those "species," 

 subspecies, forms, etc., which have been described from western United 

 States and Canada. Attempts at crossing these were made whenever 

 possible. Table 1 shows the combined results for controls and "hy- 

 brids,"^ comparing the numbers of normal and abnormal egg masses 

 laid for the 1960 and 1961 field seasons. A normal egg mass is defined 

 as one in which the eggs are deposited in the way that is typical for the 

 species, and which is fully covered with spumaline if spumaline is 

 normally present. (See the section on egg masses, figs. 105-118, and 

 the description of the egg mass for each species.) Abnormal egg masses 

 are those which differ in some respect from a typical one. Most com- 

 monly abnormal &gg masses were laid irregularly (fig. 108) or they 

 were covered only partially with spumaline. 



The lower percent of normal egg masses and the higher percent of 

 abnormal egg masses laid in 1960 compared with 1961 by both con- 

 trol and hybridized females most likely is due to larger numbers of 



* The term "hybrid" is used here in the sense that it is an organism which is the 

 result of a mating between individuals from two different populations, whether 

 or not these populations are regarded as different species. 



