486 



perhaps others also), a unieh luodified species, inhabits galls on 

 leaves of several native species of Pelea. Three others have 

 been found on foliage of other native plants but their feeding 

 habits are not fully known. Ohia lehua appears to be, by far, 

 the most conmionly attacked tree in these Islands. Several 

 make galls on ohia leaves while others live free on the leaf 

 surfaces. 



The genus Metrosuleros, according to Rock^ is represented 

 in the Hawaiian archipelago l)y live species, four of which are 

 endemic and one widely (listril)uted over Polynesia, New Zea- 

 land, Tahiti and other Pacific islands. It is the cosmo|)olittin 

 species which is the chief food plant of the ITawaiian psyllids. 

 Thus far no psyllid galls have been rejiorted from the south 

 Pacific or elsewhere the insects making which seem to be in any 

 close way related to the Hawaiian species. 



It seems probable that the four native species of Metrosi- 

 dcros have not sprung from the cosmopolitan species, but that 

 the latter has been introduced more recently. That its intro- 

 duction was very ancient, however, is evidenced by its position 

 in the forests and its relations in these forests with the more 

 recent trees. It is quite probable that these trees were first 

 introduced by the very small and light seeds which are l)lown 

 to great distances by winds. At what time after the estal)lisli- 

 ment of the Ohia lehua here the gall psyllids came in is im- 

 possible to say, because of the absence of fossils. 



Because of the volcanic nature of the rocks of these Isl- 

 ands, fossil remains of plants or animals are almost never 

 found. A few have been discovered in the stratum overlying-^ 

 the volcanic rock — obviously of comparatively recent date, for 

 the lava flows followed a long, previous existence of the Isl- 

 ands. One such fossil, found by J. C. Bridwell on the Island 

 of Oaliu but very unfortunately lost, was evidently an im])rint 

 of a leaf of Mcfrosidcfos. Ppon this leaf im])rint, it is said, 

 .were galls which beyond doubt were of psyllid origin. This is 



1 "The Oliia Lehua Trees of Hawaii," by Joseph F. Rock. Hawaii 

 Bd. of Agric. and Forestry, Bot. Bull. 4, 191 7. 



