OCR Text |
Show 1896.] METALLIC COLOUES OF BIKOS. 287 gradual to show that the stalk of the metallic barbule is the rudimentary lamina, and the club-shaped body a modification of the distal region of an ordinary barbule. As was observed by Gadow, the cilia are totally suppressed. To contrast with this feather, w e may take one from the brilliant green gorget of the Humming-bird Basilinna leucotis (PI. X I. fig. 7). Such a feather is comparatively short, and the especial brilliancy is confined to a darkly pigmented apical band. The apex of the feather is very abruptly rounded and the barbs are closely connected so as to give a plate-like or scaly appearance. The surface is strongly marked with furrows, which when examined with a lens are seen to correspond to the barbs, each of which lies as it were at the bottom of a trough. The sides of this trough are formed by the two rows of barbules, which are inserted at a distinct angle. The sides of the trough have an unequal slope, as the proximal barbules are inserted at a larger angle than the distal. A further point of interest is that the naked barbs are prolonged beyond the apex of the feather, producing the appearance of a very delicate fringe. This prolongation of the naked barb was noticed many years ago (see the Introduction to Gould's Monograph of the Humming-birds), and its meaning will be explained later on. Examined microscopically, the downy barbules of this feather show nothing particularly worthy of notice. The remaining barbules have all a well-developed lamina, and a distal region usually well provided with cilia. Those nearest the apex of the barb, that is those which are very brilliantly metallic, are very darkly pigmented (PI. X I . fig. 9). Those near the base of the upper barbs are much less strongly pigmented, and show very little metallic colour. Between the two extremes there are also other marked differences (PI. XI. figs. 8 and 9). Thus in the basal barbules there is no marked angle between the proximal flattened region and the distal cilia-bearing region. In the apical barbules the proximal region is widened and has a well-marked infolding, while the distal region is inclined to it at such an angle as to be almost invisible in surface view (PI. X L fig. 10). Microscopically, the naked tip of the barb is seen to be furnished with rudiments of barbules. As this type of metallic feather occurs in all the Hummingbirds of which the feathers were examined, it is obvious that Gadow's statement that all metallic barbules are devoid of cilia is too universal. It is certainly true for a great number of cases (Sun-birds, Peacock, & c ) , because in these it is the region which ordinarily bears the cilia-the distal or filamentous region-which is modified into the colour-producing structure, and in the course of the modification the cilia are lost. In the Humming-birds, on the other hand, it is the proximal region of the barbule which is the colour-producing structure, and in consequence the filamentous region is not affected and m a y bear cilia as usual. In the feather described above, the cilia are exceedingly well-developed in the metallic barbules. Though this paper is not concerned with the physical cause of |