Colour-groups of the Hawaiian Wasps, etc. 679 



is sufficient to allow of some species becoming established 

 where previously they could not exist. 



" Practically speaking, the cattle open up the dense 

 forest, letting in sunlight and making it much drier. It 

 is remarkable that no species of Odynerus should have 

 been able to enter our densest and wettest virgin forests, 

 because it would have found there such a vast store of 

 (Lepidopterous) food, without other species to compete 

 with it. Some of the bees have occupied such forests, in 

 spite of the sun-loving habits of the group." Nov. 13th, 

 1911. 



The prey of Hawaiian Eumenidae, so far as is known, 

 consists entirely of caterpillars. On the whole it may 

 be said that Pyralid and Microlepidopterous caterpillars 

 are the favourite prey and that Geometridae are rarely 

 utilised. It is most remarkable, seeing that the latter are 

 occasionally taken (e. g. by 0. montanus, Sm., eucharis, etc.), 

 that this should occur so rarely, for the Geometrid cater- 

 pillars are so very numerous that they could be often 

 obtained in any quantity. 



In many localities at favourable seasons the number of 

 individuals that are seen is extraordinary. On one occasion 

 I visited a mountain gulch on Molokai nearly every day 

 for three weeks, and I estimated that in a length of a 

 couple of miles (below the line of forest) the population of 

 adult wasps was at least one million. Five or six species 

 were represented, but two or three were much more 

 numerous than the others. I have noticed an almost 

 similar abundance in other localities. It is probable that 

 very few of the large number of species are really rare. 



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, 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. 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 

 and second abdominal segments in species which typically 

 have an entirely black body. Examples of this are 

 Nesodynerus rudolphi, Dalla Torre, Odynerus venator, and 

 0. heterochromus, to instance only species very widely 



