REVISION MALACOSOMA HUBNER IN NORTH AMERICA 117 



but there is no reason to believe there would be any great difference 

 in the other species. 



It is not known whether this narrow area of overlap in distribution 

 between c. lutescens and americanum is a stable situation which has existed 

 for many hundreds of years or longer, or whether it is a situation which 

 is changing in favor of one species. This would take many years of care- 

 ful observation to determine. Certainly, however, it is an ideal situation 

 for studying the competition between two very similar species under 

 field conditions, as they are easily located and readily identified yet 

 are closely similar in their requirements. It is possible that c. lutescens is 

 better adapted to the drier, more severe climatic conditions found in 

 the western part of the Great Plains, while americanum may be better 

 adapted to the more humid and less severe conditions found eastward 

 from the central Great Plains. A similar type of adaptation could ac- 

 count for the similar distributional pattern found between americanum 

 and M. calif or nicum pluviale at the northern edge of americanum'' % distri- 

 bution (fig. 2), and the same type of study could be carried out there. 



Specimens supposedly identified as americanum have been reported in 

 the literature as occurring west of the central Great Plains, but none 

 have been seen in any of the specimens borrowed from various institu- 

 tions except for three specimens that were probably mislabeled. Such 

 reports of its occurrence are probably misidentifications of certain in- 

 dividuals of M. calif ornicum and its sujjspecies (especially M. calif ornicum 

 lutescens) which may resemble americanum in general appearance. This 

 is particularly true of early instars which are difficult to identify for all 

 species except M. disstria. 



It is likely that climatic conditions are suitable for americanum in many 

 parts of the West, especially the Pacific Northwest, but whether it 

 would be able to compete successfully with the native populations of 

 M. calif ornicum is difficult to say. In southwestern Texas the distribution 

 of americanum seems to be limited by a lack of suitable food plants, 

 rather than by competition with other species, since c. lutescens was not 

 found in that area either. Suitable food plants occur in Big Bend Na- 

 tional Park and in the Davis Mountains, but no tent caterpillars of any 

 species were found there, and only a single museum specimen of M. 

 californicum from Big Bend National Park has been seen. 



Specimens Examined. — 2120 (Museum specimens — 906 males, 184 fe- 

 males, 860 larvae; reared specimens — 103 males, 67 females, plus numerous 

 larvae and egg masses). See Appendix I for data on collections made during 

 this study. Malacosoma americanum is generally distributed throughout the states 

 which are solidly checkered in figure 2. The following list includes only local- 

 ities from states or provinces which lie outside this area. 



United States. — Florida: Fernandina; Gainesville; Monticello; Orlando; 

 Quincy; Torrey State Park. Kansas: Douglas Co.; Riley Co. Nebraska: Dorsey 



