146 UNITED STATES NATIONAL MUSEUM BULLETIN 276 



trees in the area. Only a single egg mass was found on the branch 

 bearing the tent, making it reasonably certain that all caterpillars 

 in the tent had hatched from the same egg mass. The caterpillars 

 appeared to be a mixture of some with white and some with orange 

 lateral setae, so they were reared to the last instar and then segregated 

 into separate cages on the basis of setal color. Three groups resulted : 

 one group with orange lateral setae, another with white lateral setae, 

 and a third group which had yellowish setae that were not clearly 

 white or orange. All three groups contained larvae with varying 

 amounts of lateral blue. The adult males and females which emerged 

 from all three groups were quite similar in every respect and could 

 not be segregated into groups. 



The similarities between c. ambisimile and c. californicum are many 

 and the differences are few. The only differences that have been 

 observed are those in the larval color pattern, possible differences 

 in native host preferences, and an indication that c. ambisimile may 

 sometimes have one more larval instar than c. californicum. Measure- 

 ments of head capsules of some collections indicate that c. ambisimile 

 may go through six instars, and dates of adult emergence for at least 

 one collection which contained larvae of both types indicate that 

 larvae with orange lateral setae may emerge on the average slightly 

 earlier than those with white lateral setae. Other similar collections 

 gave no such indication, however, and the number of larval instars 

 is known to be influenced by many different factors, so carefully 

 controlled rearings would have to be carried out with different local 

 populations before any positive statements concerning the number 

 of instars could be made. Even if such a difference exists, it does not 

 appear to be great enough to isolate effectively the two populations, 

 as is demonstrated by the occurrence of numerous intermediates where 

 the two populations meet. 



The apparent difference in native host preference between c. ambi- 

 simile and c. californicum is also of interest. It appears that c. ambisimile 

 originally preferred Ceanothus spp, while c. californicum preferred Quercus 

 agrifolia. Both of them occur on Salix spp. and both have readily ac- 

 cepted fruit trees of various kinds. Many of the older references to 

 c. californicum indicate it was abundant on Q^. agrifolia, but Langston 

 (1957) found it (including populations oi M. californicum found farther 

 north) on oaks in only three localities, despite extensive surveys of oaks. 

 During this study c. californicum was collected on Q_. agrifolia more often 

 than on any other host. Malacosoma c. ambisimile was collected on 

 Q. agrifolia only once, but was commonly found on Ceanothus spp., 

 willows, and fruit trees. Malacosoma c. californicum was not collected on 

 Ceanothus at all, although this may be due to the lack of the right species 



