REVISION MALACOSOMA HUBNER IN NORTH AMERICA 31 



has shown that sluggish colonies had a higher mortality from disease 

 than the more active colonies, which constructed several new tents, 

 leaving behind the sluggish, dying, and dead individuals, and thereby 

 having less contact with diseased material. Other factors such as 

 starvation contribute to the higher mortality rate of sluggish colonies, 

 but it is apparent that an individual catei-pillar's chances of becoming 

 infected with disease are probably related to the amount of time that 

 is spent in contact with dead or dying individuals. Thus, it seems 

 reasonable to expect that species which construct and live in large 

 tents (whether the colonies are active or sluggish) would be more 

 likely to become diseased than those species which cluster on branches 

 and, thereby, avoid contact with dead or diseased individuals and 

 frass which fall to the ground. The species which do not live in tents, 

 however, cluster tightly together on branches, so they still would stand 

 a good chance of picking up any diseases that are present. 



According to Sullivan and Wellington (1953), a possible function 

 of the tent is the maintenance of a more humid microclimate which 

 helps to reduce desiccation. If this is true, it may explain a quite 

 noticeable difference in behavior between northern Pacific Coast 

 populations and desert populations of M. californicum. 



Along the Pacific Coast from San Francisco Bay northward where 

 the weather is usually rather dam.p and cool during the time when tent 

 caterpillars are present, they were often seen resting on the outside of 

 their tents and basking in the sun when it was shining. In contrast, 

 tent caterpillars found in the Mojave Desert rarely were seen resting 

 on the outside of their tents, but there are very few days in the desert 

 when the humidity is high. 



If these behavioral differences are due indeed to a simple response 

 to atmospheric humidity, then there may be other physiological 

 differences between these populations which permit the desert forms 

 to remain inside their tents even though the temperatures get quite 

 warm. Sullivan and Wellington (1953) have shown that the tempera- 

 ture inside the compartments of fifth instar tents of M. [californicum] 

 pluviale may vary widely, depending on the exposure to the sun and the 

 position of the compartment within the tent. They note that cater- 

 pillars move to cooler compartments as the one they are in heats up, 

 but they rapidly leave the tent and cluster on the outside as soon as 

 the temperature of the coolest compartment gets too hot. 



This same reaction m.ay take place with the desert larvae, but 

 casual observations made on caged larvae held near San Francisco 

 Bay indicate that the desert larvae may be adapted to warmer tem- 

 peratures than the northern Pacific Coast larvae, and therefore could 

 be expected to tolerate higher temperatures inside their tents before 



