Ixviii FAUNA HAWAIIENSIS 



partly or wholly back to their original food. If we look at the case of P. vitticollis 

 and its van longulus, we find a great change of habits concomitant with so little change 

 of the insect that the two are considered to be varieties of one species. The var. 

 longulus is the common form, and is attached to a forest tree [Bodea), and, as usual, 

 the larva burrows beneath the bark, but vitticollis bores out the stems of a species 

 of Riibus. Most of the individuals of the latter, so far as the evidence goes, are easily 

 distinguished from the var. longulus, so that the two may be considered on the verge 

 of becoming distinct species. It is quite possible that when a Plagithmysus first takes 

 to feeding on a new kind of tree, the females, though they may remain attached to 

 the new food-plant, would attract males that have bred on the original one, but when 

 one considers the entirely different nature of the food-plants of such varietal forms 

 as P. vitticollis and var. longulus, or again of such closely allied species as P. lamarcki- 

 anus and darwinianus, the foods of which are trees of quite different orders, with 

 different odour and taste, the one an acacia and the other Urticaceous, one cannot 

 overlook the possibility that this may aid in the isolation of the individuals on different 

 trees. It is a familiar fact how quickly these beetles scent out and arrive at a tree 

 that is their special food-plant, if it be by injury placed in a condition suitable for their 

 oviposition. 



In the case of Plagithmysus bishopi, of which quite identical examples have 

 been obtained and bred from Pelca the normal food, and Zanthoxylum as an excep- 

 tional occurrence, where these trees grow side by side, the food-plants are much more 

 nearly related and isolation might be less easily attained. Where isolation is complete, 

 in another district of Hawaii, P. bishopi becomes P. vicinus, though still feeding on 

 Pelea, the normal food-plant, but these two species have never been found in one 

 locality. Why certain individuals of a Plagithmystis occasionally, but apparently very 

 rarely, leave their proper food-plant to attack some quite different tree, is quite obscure. 

 But on all such matters much further observation in the field is needed. 



Sometimes we may find species as closely allied to one another as those above 

 referred to, living actually in company and apparently having similar habits, but it 

 is quite possible that special investigation will show important differences in habits. 

 In the case of the flightless species of Reduviolus in the Hemiptera, for example, of 

 which two slightly different forms, probably really distinct specifically, were found in 

 the same habitat, a decided difference in the habits of these forms was observed. 

 The case of the small crickets of the genus Paratrigonidium is more similar to that 

 of the beetles of the genus Plagithmyszis, referred to above, the distinction of habits 

 in these being, as one might say, far more striking than the specific differences. 

 I place the greatest importance on these changes of habits, leading to isolation, in 

 the production of new specific forms. 



The extreme variability of many of the insects and mollusca, whether in structure 

 or colour, the indefinite character of the species in many of the genera, e.g. in the 



