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Summary.

1. The darkening of the puparium in Drosophila probably involves a tanning reaction similar to that described for Calliphora. It is reduced in ebony, black, and straw-3, and changed in nature, so as to give a yellower tint, in yellow and yellow-2. It is somewhat increased in speck, particularly in the region of the anus, where there may be a sharply marked dark band.

2. The pigmentation of the wild-type adult is due to two processes, a browning which is probably a tanning, and a blackening which is probably a deposition of melanin. The browning is more apparent in the earliest stages of pigmentation, and first affects the chsetse, the darkening of the cuticle becoming stronger at a later stage, chiefly after emergence.

3. In yellow the browning is changed into a yellowing, and the blackening much reduced. The change in the character of the tanning occurs earlier in yellow than in yellow-2, so that in the latter the bristles are pigmented normally.

4. Straw-3 reduces both phases of pigmentation, while ebony, and to a less extent speck, increases both phases. Black intensifies the blackening more strongly than the browning.

5. In compounds of black and ebony with straw, the early stages of pigmentation (of chætæ) proceed as in straw, while the later ones (of cuticle) proceed as in black or ebony. It is argued that this indicates that black and ebony act earlier in the reaction sequence than does straw.

6. It is clear that ebony affects not only the quantities of pigments formed, but also the time at which they make their appearance, since it reduces the tanning of the puparium and increases that of the adult. It is suggested that this may provide an explanation of the behaviour of ebony as an antimorph.