18 UNITED STATES NATIONAL MUSEUM BULLETIN 276 



condition as the living host. In addition, tliere were times when the 

 native host was not available within a reasonable distance of the trailer, 

 so some related plant had to be substituted. These host transfers were 

 accomplished without difficulty in nearly every case, but there was 

 always the possibility of the larvae being affected in some way that was 

 not readily apparent. The last advantage in collecting nearly mature 

 larvae is the presence of a fully developed color pattern in contrast 

 with the reduced color pattern present in the earlier instars — especially 

 the first and second. Many of the western "species" were described 

 originally because of differences in the larval color patterns. It was 

 extremely important therefore, to be able to recognize these populations 

 in the field in order to study them in the areas where their distributions 

 meet, and to look for other diflferences or similarities that may be im- 

 possible to observe after they have been collected. 



An obvious disadvantage of collecting nearly mature larvae is that 

 one does not have any early instar larvae for preservation. Another 

 is the possibility of miscalculating larval development and arriving at a 

 potential collection site after all the larvae have pupated. The other 

 major disadvantage is the steadily increasing chance that older larvae 

 will become mixed with those from different egg masses, because older 

 larvae range farther in feeding and lose their gregarious habits as they 

 mature, especially during the later part of the last instar. Even if larvae 

 are collected in the first instar, however, it is not certain that only a 

 single egg mass is involved since larvae from two or more egg masses 

 often combine into a single colony. To be absolutely sure that all larvae 

 are from a single egg mass, one must collect the egg mass and hatch the 

 larvae. This, of course, is next to impossible, since egg masses are dif- 

 ficult to find except in areas where populations are extremely high. 



Disease 



In any study such as this which covers a large area, it is inevitable 

 that some diseased specimens will be collected, even though they may 

 appear to be healthy when collected. 



Once a disease has been introduced into the insectary, often it is 

 impossible to get rid of it completely. Tent caterpillars are quite 

 susceptible to diseases of various kinds. One of the most commonly 

 encountered is nuclear polyhedrosis. This disease has caused wide- 

 spread mortality on many occasions under field conditions, but during 

 this study it was of no importance in the insectary, and caused only an 

 occasional loss of a few caterpillars. The disease already mentioned, 

 however, and tentatively identified by Steinhaus (Steinhaus and Marsh, 



