INTRODUCTION xciii 



much rarer in some years than others, and further they appear at irregular times, so that a 

 species may be abundant one year in March for instance, and another year in August, and 

 in some years may hardly be seen at all. Consequently during any year unless one is con- 

 stantly collecting at all seasons in a fixed locality one may easily miss the period of greatest 

 abundance of any species or fail to find it at all. With experience and close attention in 

 the field it is fairly easy to discriminate between species that are exactly alike superficially, 

 owing to indescribable differences in appearance, often due to mode of flight and posture. 



Only in exceptional cases do the Hawaiian Eumenidae exhibit important variation, 

 and in very few cases is this more than of a trifling character, affecting the colour. 

 There is, however, great variation in the size of individuals of a species, and this is due 

 usually to the amount of food supplied to the larva, but sometimes to the fact of two 

 eggs being laid in a single cell, so that the food supplied normally to one, has to nourish 

 two larvae. To this constancy of structure is partly due the ease with which the many 

 species can be discriminated. A common variation, which occurs again and again and 

 in the most diverse species, is the occasional assumption of a feeble yellow band or 

 traces of such a band on the first or first two abdominal segments in species which 

 typically have an entirely black body. Examples of this are Nesodynenis rudolphi, 

 Odynerus venator, O. heieroc/u-otmis, to instance only species very widely separated in 

 structure. Sometimes the yellow band appears only on the ventral surface. The 

 phenomena are precisely identical with those observed in the Crabronidae, and I think 

 are explicable in the same way. The blackness of so many Hawaiian Eumenids has 

 been produced in the islands, and the abnormal individuals are reversions to a former 

 general condition in colouring. These too, like the Crabronids, have retained in some 

 species the original yellow-banded colouration. 



The general tendency to blackness of the Hawaiian Aculeata as a whole is one 

 of their most remarkable features. The blackness of these insects is increased by the 

 dark colour of their wings, which in a large number of the species exhibits striking blue 

 or purple reflections. The result of this is to produce a great superficial resemblance 

 between many of the bees, wasps and fossorial wasps. Though similar phenomena 

 appear in the Aculeates of other countries, I know no case quite comparable in extent 

 with that observable in this fauna. We are not able to suggest any satisfactory 

 explanation as to the cause of this widespread melanism. Were it universal it might 

 certainly be considered protective, for the result is conspicuousness, but the allied 

 species that lack the blackness thrive as well as those that possess it. 



If we now return to the consideration of the Eumenidae by themselves, we again 

 find most interesting facts in connection with their colouration. It may be said in brief 

 that the Hawaiian Odynerus (s.l.) have become divided up into a number of colour 

 groups, and that these are entirely different from groups based on structure and real 

 affinity. As these colour groups occur on each island, and some of them indeed are 

 only found on one of the islands, we may review the species on each island separately. 



