Colour-groups of the Hawaiian Wasps, etc. 689 



described a species, gaily marked with yellow as N.bidecor- 

 atus, adding a remark to the effect that " In spite of its 

 extremely distinct appearance I suspect it may prove to 

 be a variety of the following," viz. N. rubrocaudatus, Blackb., 

 and Cam. (" Fauna Hawaiiensis," vol. i, Pt. I, Hymenopt. 

 Acul., p. 27, 1899). This now proves to be the case, inter- 

 mediate varieties having been secured. The variation in 

 this case is even more extreme than in the other, since 

 typical rubrocaudatus is an entirely black-bodied insect, 

 whereas the variety notost ictus of distinctus has at least 

 yellow thoracic markings. It is interesting to observe 

 that the markings of the most highly coloured N. rubro- 

 caudatus (var. bidecoratus) almost entirely resemble those 

 of Xenocrabro distinctus. Looking at the localities where 

 these highly marked varieties of Nesocrabro occur, we find 

 that, far from living in the hot and dry places, they are 

 found in the wet woods near Kilauea (4,000 ft.), in the still 

 wetter^ district of Olaa, and other localities of Windward 

 Hawaii. I think that these highly coloured varieties are 

 " reversions " to an ancestral style of coloration, and I 

 believe this is borne out by an examination of the varieties 

 of other Hawaiian species. In these there is a general 

 tendency to blackness of coloration, some few retaining 

 conspicuous yellow markings, while most have these re- 

 duced to inconspicuousness or they are entirely absent. 

 Xenocrabro hawaiiensis and fv.lvicrus, Oreocrabro abnormis, 

 Blackb. and Cam., and Hylocrabro tumicloventris, species 

 with normally black abdomen, all become spotted as 

 exceptional and sometimes very rare varieties. Species 

 like Nesocrabro stygius, Kirb., and daemonius, with immac- 

 ulate abdomen above, frequently retain yellow pigment 

 spots beneath, where they are concealed from view. 

 Generally speaking yellow markings, especially thoracic, 

 are less easily lost in the female than in the male. The 

 general blackness of the Hawaiian Crabronids, as now 

 manifested, has I think been produced within the islands, 

 and while some still retain more or less the colour of their 

 ancestors the majority have greatly departed therefrom, 

 though many of them in exceptional individuals reproduce 

 that coloration to a greater or less extent. Further, a 

 study of the case cited of Nesocrabro rubrocaudatus and 

 Xenocrabro distinctus lends strong confirmation to the 

 community of descent that is suggested by the considera- 

 tion of their structural characters. At least I find it diffi- 

 TRANS. ENT. SOC. LOND. 1912. — PART IV. (FEB.) 3 A 



