| OCR Text |
Show 1896.] METALLIC COLOURS OF BIBBS. 289 well-developed lamina; therefore in this case metallic barbules occur only towards the middle of the barb in contour-feathers, for this is the only place where the lamina is well-developed. In other words, in contour-feathers the metallic band cannot be, or is not primitively, terminal. Where it seems to be terminal, as in the feather described above, this is accomplished by a total or partial suppression of the terminal barbules, the naked barb persisting and forming the dehcate fringe already noticed. This does not, however, occur in quill-feathers to the same extent, because these as a rule are characterized throughout by having barbules with very well-developed laminae. Thus it is by no means uncommon to find terminal bands of metallic colour in the tail-quills of Hummingbirds, or we may find the whole surface metalhc : there never seems to be a longitudinal edging of metallic colour. Conviction as to the truth of the statement here made, that metallic bands on the contour-feathers of Humming-birds are not primitively terminal, is best attained by considering a serie3 of cases. In Eustephanus galeritus, female, the breast is covered with greyish feathers which have near their centre a spot of brownish pigment which shows a faint metallic-green sheen. In the breast-feathers of E.fernandensis, female, undoubtedly a more speciahzed species, the spots are not much larger but the pigmentation is darker, and the metallic colour is much more brilliant. This kind of metalhc colouring is very frequent, especially among the females of many species or genera (cf. the species of the genus Oreotrochilus). If the barb of such a feather is examined microscopically, it is seen that all the barbules with well-developed laminae contain pigment and are metallic. Towards the base of the barb the barbules are colourless and downy, towards its apex they are colourless and short, the lamina is rudimentary, the filamentous portion is somewhat expanded and bears only rudimentary cilia (PI. X L fig. 11). Between this type and that of the gorget-feathers of Basilinna leucotis (PL XI. fig. 12) there are all stages in the suppression of these apical barbules. Thus in Eustephanus fernandensis, female, itself the feathers of the posterior region of the back are brilliant green, but are quite distinctly tipped with white. In the specialized crest which this female is almost pecuhar in possessing this white edging is much reduced, but the colourless apical barbules are still quite visible with a lens. In general, we may say that while the feathers of the specially brilliant patches have barbs with naked tips, the ordinary metallic contour-features have barbs which bear at their tips a series of non-metallic barbules. Thus the green feathers on the back even of Basilinna leucotis have a distinct border of pale rufous colour. This is of some interest from the point of view of the development of the metallic colours of Humming-birds. In Phaethornis eurynome, one of the so-called ' hermit' forms without any brilliancy of colour, the feathers of the back are greyish black edged with a broad band of yellowish colour; the dark region exhibits a very faint greenish sheen. If we begin with a type like this, the evolu- PKOC ZOOL. Soc-1896, No. XIX. 19 |
