REVISION MALACOSOMA HUBNER IN NORTH AMERICA 59 



The larvae of all species and populations feed on a wide variety of 

 hosts, but this does not mean they develop equally well on all hosts. 

 Therefore, it would probably be necessary to rear them on several 

 different hosts to get reliable results. 



Each population probably is better adapted to the climatic con- 

 ditions in its native area than to conditions in other areas. There- 

 fore, careful consideration must also be given to the environmental 

 conditions that rearings are to be made under, or one population may 

 be favored by the selection of conditions that are closer to its optimum. 

 The use of controlled environment chambers that can be programmed 

 to duplicate the changing conditions what would be encountered in the 

 natural habitat is essential, and, ideally, each population should be 

 reared under the conditions that other populations which are being 

 compared occur under, as well as its own. Only when all of these 

 variables have been satisfactorily controlled, will it be possible to 

 obtain valid comparisons of developmental rates. 



When the developmental data from all of the 1960 Fj rearings are 

 segregated by sex, there appear to be slight differences in develop- 

 mental rates between males and females. The length of development 

 from hatching to adult emergence averaged close to 37 days for 

 1651 males and females from all rearings (excluding R38 and R39 

 which were reared during warmer weather). It averaged 36.73 days 

 for 959 males and 37.28 days for 692 females. Nearly always more 

 males than females emerged the first few days, and more females than 

 males emerged the last few days that emergence took place. These 

 differences are in agreement with observations made on field-collected, 

 reared adults, and indicate that the males tend to emerge slightly 

 earlier than the females. 



The sex ratios are interesting from the standpoint that in 29 of the 36 

 rearings (6 of 7 controls, and 23 of 29 hybrids) more males than females 

 were obtained. The sex ratios include preserved larvae, since there was 

 a definite tendency to select females for preservation in the last instar. 

 In the samples of last instars taken, 126 females and 48 males were pre- 

 served. The samples were taken before it was known how to sex the 

 caterpillars (see the section on larvae for the sexing technique, page 46), 

 so there was no conscious selection of one sex in preference to the other. 

 Mature larvae, however, were selected for preservation using the incor- 

 rect assumption that the largest larvae were the most mature, and since 

 it is known now that mature female larvae are on the average slightly 

 larger than mature male larvae, this undoubtedly accounts for the 

 higher number of females preserved in the last instar. 



The ratio of males to females, however, when all of the preserved 

 larvae are included is still 1.25 males to 1 female (42 percent females). 



279-280—68 5 



