COLORATION IN POLISTES. 45 



PHYSICAL AND CHEMICAL NATURE OF THE PIGMENT. 



Reference has already been made to certain differences existing 

 between the yellow and the darker pigments of the exoskeleton of 

 Polistes. The latter are incorporated in the outer layers of the chiti- 

 nous integument of the insect, the underlying or embryonic cuticle 

 remaining uupigmented. The former is deposited in and among the 

 hypodermal cells. (See text figs. 5 and 6.) 



This yellow is not evenly distributed all over the inner surface of 

 the chitin, but is confined to the areas where the chitin has remained 

 in any degree transparent. There is also a relation between the thick- 

 ness of the yellow hypodermal layer and the depth of pigmentation of 

 the chitin overlying it ; the more transparent the chitin the thicker 

 the deposit of hypodermal yellow. Thus in the modal condition of 

 P. variatus, where the lateral abdominal spot is yellow with a ferrugi- 

 nous margin, we find the hypodermal yellow thickest under the yellow 

 area of the spot. The ferruginous effect is due to a slight deposit of 

 the dark pigment in the chitin, leaving it more or less translucent and 

 allowing a thin layer of the underlying yellow to show through. At 

 the margin of the ferruginous area the yellow gradually thins out, and, 

 finally, where the cuticular pigment is dense, disappears entirely. 



The more deeply pigmented areas of the chitin have a distinct rela- 

 tion to underlying structures ; they represent places of attachment of 

 the muscles to the integument. This is better expressed by saying 

 that the areas where the pigment first appears in the pupa represent 

 the position and attachment of developing muscles in the manner 

 already described by Tower (23) for the Coleoptera. 



In the thorax of the pupa there are three great bundles of muscles. 

 One is median and longitudinal in its direction, and on either side of 

 this there is a large lateral bundle which lies in a dorso-ventral direc- 

 tion. The first is inserted anteriorly in the naesothorax just below the 

 median pigmented region, the other two at either side below the lateral 

 pigmented areas. The posterior end of the median longitudinal muscle 

 is attached below the three primitively pigmented areas of the meta- 

 thorax ; some fibers are also inserted in the scutellum. 



In the abdomen the powerful muscles are attached along the anterior 

 edge of the respective segments. An extra large and powerful muscle, 

 which is used in the movement of the abdomen as a whole, is inserted 

 below the triangular area which so constantlj^ appears on the surface 

 of the second abdominal segment. The segmental dots also probably 

 represent the position of the spiracular muscles. 



