42 UNITED STATES NATIONAL MUSEUM BULLETIN 2 76 



and certainly is not a good specific character, although it is useful in 

 separating subspecies. 



In some respects the color pattern is more constant than setal color, 

 since certain color patterns may be restricted to definite geographic 

 areas, while setal color varies more randomly. Four basic colors make 

 up the color pattern of Malacosoma larvae. They are black, blue, white, 

 yellow, and various shades of each. The localization and relationships 

 of these colors in making up the color pattern (fig. 11) are described 

 since so much reliance has been placed on them in the past (and still 

 is) in determining the species. 



Blacks are due to pigmented microtrichia. The most intense blacks 

 are the result of heavily pigmented, tightly packed microtrichia. In 

 areas where the black is less intense, the microtrichia are either less 

 heavily pigmented or more widely spaced, often both. Microtrichia 

 are present over nearly the whole body, even in some areas that appear 

 white or yellow. In these areas they may be colorless or sometimes 

 lightly pigmented, but relatively widely spaced, permitting the yellow 

 or white to show through from beneath. Small amounts of pigments, 

 especially yellow and orange, may be present in the cuticle, but most 

 yellows and whites are due to pigments located in the peripheral fat 

 body immediately beneath the basement membrane. Some white also 

 is due to the white visceral fat body. External whites (sometimes with 

 a bluish tinge) are produced in ways similar to those described below 

 for the bluish colors. 



The intensity of the pigments in the fat body varies considerably, 

 but as a general rule the most intensely pigmented areas occur beneath 

 the most transparent cuticle, and the less strongly pigmented areas 

 occur beneath the more opaque cuticle. This is not a simple effect of 

 the more opaque cuticle filtering out more of the color. Removal of the 

 more opaque cuticle shows the fat body beneath it to be less intensely 

 pigmented than the fat body of neighboring areas located beneath 

 more transparent cuticle. This suggests the possibility of a direct 

 relationship between the amount of light that penetrates the cuticle 

 and the intensity of the pigment that is formed. 



The blue, blue-green, blue-gray, and blue-tinged white areas are 

 produced by an amorphous layer of material deposited externally to 

 the microtrichia. When these areas are viewed with an "ultrapak" 

 vertical illuminator, the color of the material is white, in contrast to 

 the bluish color seen in diffuse light. It follows that the bluish color 

 must be due to a Tyndall effect rather than a pigment. Nearly all of 

 the blue also disappears if the integument is wetted, while the other 

 colors are unaffected. This layer is possibly equivalent to the "cement 

 layer" of Wigglesworth (1947) or the "tectocuticle" of Richards (1951). 



