32 UNITED STATES NATIONAL MUSEUM BULLETIN 2 76 



being forced to leave. Nearly mature larvae which had been collected 

 near Morongo Valley, San Bernardino County, California (Coll. No. 

 27) and reared in the insectary at Albany, California, at prevailing 

 outdoor temperatures (quite cool) w^ere inactive and fed very little 

 before they were warmed with an infrared lamp. Larvae collected in 

 the San Francisco Bay area and kept in the adjacent cage were more 

 active than the desert larvae and grew faster, although development 

 still was slower than it would have been if they had been under field 

 conditions. The basic color of the larvae also seems to be correlated 

 with the climate to the extent that the Pacific Coast populations are 

 predominantly black and orange, which would tend to absorb the 

 sun's rays, while the Mojave Desert populations are predominantly 

 light blue and white, thus tending to reflect the sun's rays. It remains 

 to be seen how important such differences are in aflfecting the behavior 

 of various populations. 



If the tent does play an important part in preventing the desiccation 

 of tentbuilders, then there may be other physiological or structural 

 diff"erences between them and the nontentbuilders, since it seems un- 

 likely that disstria, constrictum, or tigris could prevent harmful desiccation 

 by merely clustering tightly together on branches, although water loss 

 undoubtedly is reduced by clustering. 



Another possible advantage that has been suggested for the tent- 

 buiders is the protection from the weather that the tent may provide. 

 A well-constructed tent sheds rain fairly well as long as it does not 

 rain too hard or too long, but after a good soaking rain many of the 

 caterpillars found inside tents will be just as wet as those without tents, 

 and they may be worse off if they must crawl over and through a soggy 

 mass of disease-laden debris to get outside to dry off. And who is to 

 say that a healthy caterpillar is worse off after getting soaked? The 

 protection afforded by a tent would doubtless prevent many caterpillars 

 from being blown or knocked off the host plant during a severe down- 

 pour or windstorm, but most nontentbuilders that are knocked oflf 

 the host are probably uninjured, and manage to find their way back 

 to a suitable host unless they are very small. Thus, the protection from 

 the weather that is provided by a tent is of questionable value. 



If the number and extent of outbreaks is used as a criterion of success 

 of all species of Malacosoma, it is apparent immediately that all species 

 are quite successful from time to time whether or not they build tents. 

 The most successful species undoubtedly is M. disstria, and it builds no 

 tent at all. The next most successful species would be M. americanum, 

 M. calif ornicum, and M. incurvum, all of which build large tents. And 

 the least successful species would be M. constrictum and M. tigris, 

 which build practically nothing in the way of a tent, yet even they are 

 capable of causing severe defoliation at times. In short, other factors 

 probably are more important in determining the success of a species 



