92 FAUNA HAWAIIENSIS 



of a species actually existing are very few ; the species having not only a very small 

 geographical distribution, but also being found in but few stations in the area. 



The fact that species are sometimes found in considerable numbers at some of 

 these stations renders it doubtful whether there is diminished fertility of the individuals, 

 as I formerly supposed. At present there is but little evidence on this point. 



Whether the Insects are unusually specialised in their modes of life is also doubtful. 

 Most of them are either forest species, or are of alpine or subalpine habits. 



Many forest Insects in other parts of the world are notorious as being but rarely 

 met with. In the New Forest, in the south of England, this is certainly the case, and 

 it is probably due to the fact that trees are only attacked when they are in certain 

 stages of decay, or offer some special kind of shelter, or are accompanied by some 

 particular kinds of fungoid growths. 



Variation. In the case of two or three of the species dealt with in this paper 

 I have had at my disposal some hundreds of specimens for examination. It therefore 

 seems necessary that I should make some remarks on their variation, though on 

 considering this I find that the data are very inadequate for a complete discussion. 

 Nevertheless it will be well to present certain facts with regard to the genus Plagithmy- 

 sus which appears to be one of the most suitable genera for the study of variation in 

 Hawaiian Insects. 



Twenty-nine species of this genus are now known. But about nineteen of these 

 have each been found only in a single station, and of the remaining ten several are so 

 rare that no opinion can be pronounced as to their variation. We have little or no 

 information as to the distribution of the species beyond the fact that no species has yet 

 been found on more than one island. It would scarcely be possible to find species more 

 closely allied than are some of these Plagithniysus and yet, so far as the specimens yet 

 procured entitle us to judge, though found in the same locality and at the same season 

 they appear to be really distinct. P. varians, P. darwinianus and P. lamarckianus are 

 examples of this, all of them having been procured at Kilauea in August ; they are as 

 closely allied as any three species can be. 



Varieties of the same species may be found together, and copulate indiscriminately. 

 The variation is not indiscriminate. It appears probable that each species has its own 

 set of variations and in some cases closely allied species could be defined by their 

 variation. As an example I may refer to the colour of the hind legs. In Z'. aeqiialis 

 these are either black or red, there are no intermediates although the two kinds copulate 

 indiscriminately. The species is as regards this character dimorphic. In the three 

 closely allied species — P. varians, P. darwinianus, P. lamarckianus — the facts are 

 different. Here also there is variation in the redness or blackness of the legs, but the 

 varieties of one species are not completely discontinuous. Moreover the species 

 differ from one another in their variation in this respect. P. va^dans is very variable as 

 regards the character in question. A highly remarkable variety of it has each leg half 



