172 UNITED STATES NATIONAL MUSEUM BULLETIN 276 



occur. Larvae of disstria, tigris, and americanum can be separated by the 

 diagnostic characters given for them. Larvae of M. constrictum have 

 apparently been confused with larvae of M. incurvum incurvum in the 

 past since constrictum has erroneously been reported from Arizona. See 

 the larval diagnosis for M. incurvum incurvum for the best ways to 

 separate these two (page 175), 



EGG MASSES (figs. 114, 117).— The eggs are laid as a basically 

 flat, clasping mass, covered with relatively tough spumaline which 

 contains many small, fairly uniformly sized bubbles. The spumaline 

 may be nearly white in some populations, but in others it is dark brown, 

 or some intermediate color. No reason is known for this great difference, 

 but it should be investigated to determine whether it is hereditary, 

 host influenced, environmentally influenced, or due to some other 

 factor. 



Egg masses of M. californicum fragile always can be separated from 

 those of M. incurvum discoloratum by the small specks in the spumaline 

 (fig. 118) which are absent in i. discoloratum. Egg masses of other 

 populations of M. californicum cannot be positively separated, but they 

 are generally covered with spumaline which is more medium brown or 

 grayish, instead of white or dark brown, and the bubbles are of various 

 sizes (those of incurvum are nearly uniform in size). M. americanum does 

 not occur in the same area as incurvum, but the spumaline on its egg 

 masses is dark brown, and the bubbles are of various sizes. 



TENTS. — They are essentially the same as those built by M. ameri- 

 canum and M. californicum, but quite often larvae from several tgg masses 

 combine their efforts and construct very large tents, especially on 

 Cottonwood. This also happens with americanum and californicum, but 

 their tents are rarely as large. 



COCOONS. — Cocoons are fairly tightly constructed, usually with- 

 out an outer envelope of silk, and dusted with a whitish powder which 

 may occasionally have a pinkish tint. 



FOOD PLANTS. — The favorite host appears to be Populus fremontii, 

 the southwestern cottonwood, in Arizona and Utah, but other trees 

 are also attacked. These are listed for each subspecies. 



DISTRIBUTION (fig. 4). — M. incurvum with its subspecies ranges 

 from the Grand Junction area of western Colorado, south through 

 Utah, Arizona, and Mexico to the vicinity of Mexico City. It probably 

 occurs in extreme western New Mexico. No tents could be located 

 along the Rio Grande River in New Mexico at Albuquerque, or from 

 Truth or Consequences, New Mexico, south to Fort Hancock, Texas, 

 or in Big Bend National Park, although cottonwood is abundant along 

 the river. Adults definitely referable to incurvum have not been seen 



