EVOI.UTION OF COLORATION. 22$ 



neighboring hypodermis, and which has a single distinct trachea distributed 

 to it from the main tracheal trunk. In the early embryo the homodynamous 

 thickenings occur upon all of the segments, in the position of the spiracular 

 spots, and from these develop the tracheal invaginations and the pigment- 

 enzyme cells of the spiracula spots. The entire set of homodynamous spirac- 

 ula and wing spots receive from the same source in the embryo some cells 

 which are later specialized for the purpose of producing pigment enzymes, 

 others for the development of the tracheae, and still others for the general 

 hypodermis. The existence of these specialized cells has been demonstrated 

 cytologically in the embryo, and experimentally in the larva. 



In the development of the wing the fundament takes out of the tissue be- 

 neath the wing spot ordinary hypodermal and color-producing cells. As the 

 wing fundament forms a connection is retained at the point where the trachea 

 touched the fundament, and it becomes the apex or margin of the wing. The 

 wing grows largely from the apex, the base remaining relatively fixed. As the 

 apex grows outward the various areas of the wing are laid down, and into the 

 spaces formed the permanent tracheae enter later. As the wing grows outward 

 from its base the outer end (apex or margin) forms behind it the structure of 

 the adult wing, and from the color mother cells there are left in rows, bands, 

 or spots the cells which are to produce the color pattern. In this growth of the 

 wing the base is always ahead in development, and is completed first, the tip 

 last. The essential fact in wing development, as far as coloration is concerned, 

 is that the wing is derived from one of the systems of the homodynamous color 

 areas on the body wall of the insect, and that from this area it receives both 

 hypodermal and pigment-forming cells, the latter of v/hich give rise to the 

 cells which form the adult coloration. This arrangement, which is character- 

 istic of all Coleoptera at least, and I believe of insects in general, is the basis 

 upon which we must explain color patterns and color evolution. It can be 

 traced back to the homodynamous centers, and from there to the highly prob- 

 able but not yet demonstrated mother cells of the body centers of coloration ; 

 then still farther back to the original blastomere, then to the formative sub- 

 stance of the ovum, and finally, in our imagination, to the more remote and 

 less tangible elements of organic structure and activity. From this we may 

 gain some little appreciation of how fundamental and how early provided for 

 in development is the coloration of an animal. Here in the general phyletic 

 constitution of insects we see a fundamental system provided out of which is 

 to be shaped a great variety of color patterns by many forces acting in diverse 

 directions. 



In the phyletic development of color patterns of genera and species this 

 fundamental plan of coloration is variously modified, and this furnishes a 

 basis for the determination of color pattern homologies in insects. We are 

 not able to explain why in Leptinotarsa the mother color cells leave behind 

 rows of cells to form stripes in some species and bands or spots in others. We 



