PARASITIC WEAVERBIRDS 29 



some sphinx moths or some deep sea crustaceans. In total darkness 

 when the room was tightly shut, however, they produced no visible 

 gleam. On letting a little light into the room the glow was again 

 visible. Thus, the "light organs" of the 3^oung bh'ds appeared to be 

 not true luminescent structures, but merely highly effective reflecting 

 devices. 



Chun then proceeded to study the microstructure of these tuber- 

 cles, and found them to be essentially hemispheres filled with two 

 laj'^ers of cellular padding, but ringed basally with black pigment, 

 deep in the epidermis, which itself becomes relatively thin in the area 

 of the curved edges of the tubercles. The pigment underlying the 

 tapetallike configuration of the tubercles acts much as the amalgam 

 or metallic backing of a mirror and enables the tubercles, which have 

 a finely reticulated surface, as was first described by Butler, to act as 

 a combination of a mirror and a diffraction prism. 



The above description is based on one of the Australian estrildine 

 species, Poephila gouldiae, and my illustration (fig. 2) is of a New 

 Caledonian species, Erythrura psittacea.^ The two agree very 

 closely, and this agreement suggests that there is a general basic pat- 

 tern common to the various estrildines. A similar study should be 

 made of a species of Vidua to determine whether the structure is 

 actually essentially the same in the two groups, or if it is to some 

 extent a matter of superficial convergence. The former alternative 

 certainly seems far more probable than the latter, but until such a 

 study is made, the question remains unanswered. 



Nestling plumage: The similarity in juvenal plumage of the 

 viduines to that of their most frequent hosts might seem to be criti- 

 cally significant, as the young of these parasites grow up together 

 with those of the hosts, whereas in other parasitic birds the young 

 of the foster parents frequently are eliminated before the develop- 

 ment of plumage. Here again, as is true of mouth markings, com- 

 munity of descent is sufficient to account for the similarities in 

 plumage. Kather than assume adaptive evolution of such resem- 

 blances, all that needs to be invoked is moderate selective reaction 

 by the host birds. This reaction could serve to restrict the successful 

 parasitism of individual species of Vidua normally to only a few 

 species of hosts. 



Hoesch (1939) pointed out that of all the supposed adaptations of 

 the young viduines, the least satisfactory case may be made out for 

 the mimetic resemblance of the juvenal plumage because it runs 

 counter to the general picture of the breeding biology of birds. In 

 most other parasitic birds, we find no such approximation to the 



" I'TingHla psittacea Qmelln, Carol! a IAaa6 . . . systems naturae, cd. 13, vol. 1, 1879, p. 903 (Now 

 Calodonia). 



