156 COI.ORATION IN I.£^PTIN0TARSA. 



caudalward, as shown in figs. 8 and 9, and in the line of the stripes found in 

 the lineata group, excepting that almost no color is ever formed in certain 

 parts of the stripes. In figs. 8 and 9 are shown the beginnings of the bands 

 of spots placed between the elytral stripes. In fig. 9 the entire color pattern 

 of the adult is outlined; and in figs. 10 to 12 the color changes are simply 

 those of intensification. The hypodermal or ground color (figs. 7 to 12) 

 presents a series of changes much like those found in the lineata group. That 

 which is of the greatest interest in the ontogeny of the elytral color pattern of 

 dilecta is the fact that the bands of spots develop after the markings belong- 

 ing to the intertracheal stripes have appeared and are fairly well marked out. 

 These centers, which develop over the trachese and in rows transverse to 

 them, represent possibly a younger system of color markings than that of 

 rows between the tracheae and parallel thereto. We can better interpret this 

 relationship after we have examined the condition of color ontogeny in other 

 species of this genus. 



The elytra of L. zetterstedti, shown on plate 19, figs. 13 to 18, present 

 several interesting points in their ontogeny. The initial stage (fig. 13) resem- 

 bles in general ground color the corresponding stage of other species; and 

 the order of the development of the cuticula colors follows much the same 

 course. These appear first in the intertracheal spaces at the proximal end of 

 the wing and soon move distally (figs. 13 and 14). Then there are developed 

 the transverse bands of spots which connect the intertracheal bands at certain 

 points (fig. 15), and finally complete the outline of the adult color pattern 

 (fig. 16). After the color pattern has been fully outlined the further develop- 

 ment consists in the deepening of the cuticula colors and the slight changes 

 observed in the hypodermal. 



All the species thus far considered have a pattern consisting of dark mark- 

 ings upon a lighter background of hypodermal color. There are some species 

 of Leptinotarsa, however, that have unicolorous elytra and an additional 

 development of chemico-physical colors. A study of the ontogeny of the elytral 

 colors of L. viol esc ens will serve to show the features of color development in 

 beetles of this class. The initial stage in violescens is shown on plate 19, 

 fig. 25, Vv^here the uniformly yellow- white elytron is faintly tinged with light 

 yellow-brown in the proximal portion of the wing, and especially in the inter- 

 tracheal spaces. This stage, which is extremely transient, lasting only ten or 

 fifteen minutes, is succeeded by the condition represented in fig. 26, when the 

 light yellow-brown has spread over almost the entire wing, but in the distal 

 portion is still darkest in the intertracheal spaces. In a short time the entire 

 wing becomes a uniform yellow-brown without any trace of bands, spots, or 

 stripes, but it is still darker at the base than at the tip (fig. 2y). The changes 

 represented in figs. 25 to 28 take place with great rapidity, occupying fre- 

 quently not more than fifteen or twenty minutes, and rarely over thirty ; but 



