100 A TOUR AMONG THE ISLANDS 



features of interest in the insect fauna : a D. chrysippus 

 was seen crossing the channel betAveen Ngamba andNsadzi, 

 and nests of the interesting ant (Ecophylla smaragdina, 

 and the fierce 'little black ant called by the natives 

 " Obusaji-saji," were found on leaves of trees ; they are 

 not by any means universally distributed among the 

 islands. 



The tubular cases of larvae of some species of Psychid 

 moth were seen on the tree trunks in great numbers, and 

 afforded the first example of the fact that a small circum- 

 scribed area may support only a few species, but that some 

 of these may flourish much more abundantly than in larger 

 areas where they are exposed to greater competition. 

 During the course of this tour many other examples were 

 noted, and will be mentioned as instances of " insularity " 

 in the fauna or flora. 



Glossina was scarce on Ngamba, the average catch being 

 9" 9 per boy-hour. We remained at Nsadzi camp until 

 February 5th, but spent only a little time over the island 

 itself, since it was too big to be thoroughly investigated 

 on a short visit. It is about three miles long, but very 

 narrow : four-fifths consist of a steep grassy ridge almost 

 bare of trees but with innumerable termite hills, closer 

 together than I ever saw anywhere else. At the centre 

 of the south side of the island cliffs descend steeply to a 

 sandy bay, with a strip of forest at the foot of the cliffs. 

 There is another patch of forest on the north side, but 

 elsewhere the shore has only a very narrow fringe of bush. 

 The eastern fifth of the island is low and flat, and sharply 

 marked off by the escarpment of the end of the main 

 ridge ; it is covered with forest, full of birds. On Nsadzi 

 we found most interesting evidence of insularity. The 

 grass on the uplands had been burnt off by Fiske on a 

 previous visit some weeks before, and the new green blades 

 were growing up among the burnt black stems of the 

 tussocks. Over an area of a great many square yards 



