INTRODUCTION cxvii 



on the scLitellum. The more slender-legged species run with extraordinary rapidity 

 on the trunks and branches of the forest trees, and they often perform curious 

 evolutions, running in short curves or zigzags, and at times almost leaping. Some- 

 times the hind femora are notably spread outwards and kept well away from the 

 elytra, as if to avoid their rubbing together. Individuals of many of the species 

 exhibit great variability. Variation in size is very striking in many species, and 

 there is very noticeable variation in structure of some, apart from size, e.g. in the 

 form and development of the pronotal crest. The most conspicuous variability occurs 

 in the colour of the legs of some of the species, which in this respect are dimorphic 

 or trimorphic. (Plate XV, figs. 5 — 11.) 



Thus in P. aequalis the hind femora are either red or black, and of 181 examined 

 70 had red legs. 



In /*. blackburni of 68, five only had red legs. 



In P. lamarckianus black and red-legged forms are both common, the red femora 

 of the latter usually being black or dark apically. 



Of 98 P. variatis 41 had black legs, 53 had black or dark legs with the apical 

 part conspicuously red, while four were of a nearly uniform reddish colour. 



P. varians, P. lamarckiatius, P. blackburni and P. dariviniamis are excessively 

 closely allied species, and but for the fact that they have been carefully studied in 

 the field, many might doubt their claim to be distinct. P. c/afivitiianus is very 

 constant in its uniformly red femora, which are merely of a darker shade in some 

 than in others. 



It is interesting to note the very different nature of the variation in these 

 species. Thus da7-i.mnia7ius is constantly red-legged, blackburni is black-legged, rarely 

 with red legs ; lamarckiamis has commonly either red or black femora, varians has 

 a very common form quite unlike any of the others, a very scarce red-legged form 

 like dartvinianus or some lamarckianus, and a common black-legged form like 

 blackburni and some lamarckianus. Other species, like P. giffardi and P. bishopi, 

 with red femora black or dark apically are extremely constant and no noticeable 

 variety has been found. 



As with Clytarlus, the two native acacias. Acacia koa and Sophora chrysophylla, 

 are favourite foods of Plagithmysus, for six species are found on the former and 

 three on the latter tree. 



The remaining 25 species are attached to a very varied list of food-plants 

 (with the rarest exception each being confined to one). Thus Urticaceae, Myrtaceae, 

 Myrsinaceae, Rubiaceae, Rosaceae, Rutaceae, etc., are attacked. In this connection 

 it is interesting to refer to the closely allied species of the P. darwinianus group 

 above mentioned. All these are common species and have been much observed in 

 the field, and in some spots P. darwinianus, lamarckianus and varians have been 

 found in numbers within a few yards of one another. Yet in spite of this and the 



