120 UNITED STATES NATIONAL MUSEUM BULLETIN 2 76 



tions confirm this difference. All adults of both populations emerged 

 during May, and if the emergence dates for each are averaged, the 

 average emergence date for c. fragile is the 1 1 th, and for i. discoloratum 

 the 20th. The last few c. fragile to emerge overlapped the first emergence 

 of i. discoloratum, so there is a limited possibility of some gene exchange 

 between these populations. 



These difi"erences in development are believed to be due to more 

 than just diff"erences in microclimate. It could be argued that the 

 c. fragile egg masses and larvae are closer to the ground than the 

 /. discoloratum egg masses and larvae, and they may, therefore, develop 

 faster under the warmer conditions that could be expected near the 

 ground. Collections of both c. fragile and /. discoloratum, however, made 

 on the same apricot tree near Leeds, Washington County, Utah, 

 suggest that there may be other factors responsible for the differences. 

 Several colonies of c. fragile were found on this apricot tree, and all 

 contained nearly mature larvae. A single colony of i. discoloratum found 

 on this tree contained mostly penultimate instar larvae, and nearby 

 Cottonwood trees contained many colonies of i. discoloratum in the same 

 stage of development. The colony of i. discoloratum on the apricot tree 

 was reared, as was one colony of c. fragile. The average emergence dates 

 of adults from these two colonies show the same diff"erences of approxi- 

 mately 10 days as the Shivwits populations, even though both colonies 

 must have been subjected to similar environmental conditions since 

 they were located within a few feet of each other. This suggests that 

 there may be inherent differences in hatching times or developmental 

 rates between c. fragile and i. discoloratum. 



Comparable diff'erences were found at other sites along the Santa 

 Clara and Virgin Rivers. No matter what the real reason is for the 

 difference of about 10 days in developmental rates, it means that the 

 vast majority of the c. fragile adults will emerge, mate, and die before 

 the majority of i. discoloratum adults emerge, resulting in seasonal 

 isolation which has undoubtedly restricted gene exchange between 

 these two populations to a very low level. 



Large populations of i. discoloratum were observed on cottonwoods in 

 Buckhorn Wash upstream from the bridge across the San Rafael River 

 as far as cottonwoods grew. These caterpillars were middle to mature 

 last instars. No californicum colonies were found in the immediate 

 vicinity, but typical californicum colonies were found several miles up 

 the wash. They were on Cercocarpus ledifolius and Amelanchier utahensis, 

 but only a few stragglers and cocoons could be found, indicating that 

 californicum was considerably ahead of i. discoloratum in development 

 even though they were located at a somewhat higher elevation. West 

 of Huntington the situation was similar; the i. discoloratum larvae feeding 



