436 



perhaps others also), a innch modified species, inhabits galls on 

 leaves of several native species of Pelea. Three others have 

 been found on foliage of other native plants bnt their feeding 

 habits are not fully known, Oliia lehna appears to be, by far, 

 the most commonly attacked tree in these Islands. Several 

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

 surfaces. 



The genus Mdroskleros, according to Roek^ is represented 

 in the Hawaiian archipelago by five species, four of which are 

 endemic and one widely distributed over Polynesia, Xew Zea- 

 land, Tahiti and other Pacific islands. It is the cosmopolitan 

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

 Thus far no psyllid galls have been reported from the south 

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

 close way related to the Hawaiian species. 



It seems jjrobable that the four native species of Metrosi- 

 deros 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 blowai 

 to great distances by winds. At what time after the establish- 

 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 imprint 

 of a leaf of Metrosideros. V])(m this leaf imprint, it is said, 

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



1 "The Ohia Lehua Trees of Hawaii," hy Joseph F. Rock. Hawaii 

 Bd. of Agric. and Forestry, Bot. Bull. 4, 1917. 



