74 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxx. 



Development of the Color Pattern in the Larv^ of 

 Spelerpes Macut^ic audits. 



By Waldo L. McAtee. 



The youngest larvae of the species are nearly uniformly pigmented, 

 while the adults are marked with separate, sharply defined spots. 

 The development of the color pattern, therefore, is the change from 

 even distri])ution of pigment cells on the smaller animal to their con- 

 centration in restricted areas on the larger. 



At 17.5 mm, (fig. 1, Plates VIII and IX) the pigment"* cells are abun- 

 dant and about evenly distributed over the whole body, with the 

 exception of certain invariably pigmentless areas which form the most 

 conspicnou.s and characteristic feature of the larval coloration. The 

 least important of these (that is, the one which for all practical pur- 

 poses is identical in adult and larva) is the ventral area and its histor}' 

 ma}' a.s well be completed at once. This ventral area extends from the 

 gular fold or from slightly in front of it to the cloaca, often being 

 prolonged in a narrow line on the tail, and includes the inner surfaces 

 of the limbs with which it may or may not be connected. It is gen- 

 erally l)ridged by a band of pigment cells over the pubic region and 

 in cases where it extends anterior to the gular fold, by a narrower 

 band along the gill slits. In 3"ounger larvae the latter condition exists, 

 as the lower surface of the head is not so completely pigmented as in 

 older larvae and adults. In the later larval stages the anterior bridge of 

 pigment vavij separate in the median line or it ma}^ persist, leaving 

 thus a small separate pigmentless area. In the adult, however, the 

 head is entirelj^ pigmented on the underside back to the gular fold. 



In larvie above 30 mm. in length the ventral area is liable to be 

 encroached upon by pigment in various places, but the middle line 

 at least remains spotless until transformation. In the adult, while 

 the lower side of the tail loses the marbling which it generalh' pos- 

 sesses in the larva, it retains some pigment; in fact there is no con- 

 siderable area on any part of the body free from pigment dots. The 

 dots can be made out with close scrutiny by the unaided eye, but are 

 more easily studied with a lens. 



Thus while the presence of pigment cells over the whole ventral 

 surfac(^ is exclusively an adult character of the species, and demands 

 consideration in a discussion of the development of the color pattern, 

 this area, with the exception of the lower surface of the head, ma}', for 

 the purpose of ordinary description, as before mentioned, be con- 

 sidered innnaculate in the adult as it actually is in the larva. 



The other conspicuous pigmentless areas of the larva are arranged 

 in three longitudinal series on each side, and together with the pigment 

 around them play the major part in the evolution of the color pattern. 



« The term pigment as used here refers to the apparently black color spots only. 



