Colour-grouips of the Hawaiian Wasps, etc. 691 



except rare varieties that have reverted to the ancestral 

 pattern. No male form of bidecoratus has yet been found, 

 the male ruhrocaudatus only existing with these so far as 

 is known, and this male is in perfect harmony with the 

 Odyneri of the woods. On tlie Mtillerian theory I should 

 say that the more easily changed male of ruhrocaudatus 

 arrived at a very perfect and stable state of mimetic 

 resemblance to the Odyneri of the woods, but that the 

 more conservative female had never reached so perfect a 

 condition — as shown also by its hyaline wings — and that, 

 owing to its conservativeness, it had not reached the stable 

 condition of the male abdominal colouring, when the 

 causes leading to the mimicry (viz. bird attacks) were 

 removed or much abated. I should look on it as a species 

 of which the ancestrally coloured hidecoratus form might 

 easily in future times become dominant again. 



I have made a crude sketch of a distinctus female, from 

 which you can judge how different it appears from an all- 

 black-bodied ?iotostictus var., and the brightest female 

 Nesocrabro ruhrocaudatus hidecoratus has almost a yellow 

 abdomen, the black is so reduced. 



[The accompanying drawing of the ^ X. distinctus showed 

 that the following structures and markings are yellow : 

 the pronotal collar, a transverse spot on the scutellum and 

 another on the post scutellum, a curiously shaped spot on 

 the 1st abdominal segment, a band on the 2nd, 4th and 

 5th, a minute lateral spot on the 3rd, not really visible in 

 a strictly dorsal view. The var. notostictus possesses the 

 above-described thoracic markings, but is without the 

 abdominal, although intermediates occur. Another draw- 

 ing, of the basal abdominal segment of Nesocrahro ruhro- 

 caudatus var. bidecoratus, showed the similar character of 

 the variable yellow spot to that of X. distinctus. 



Dr. Perkins added : — ] 



The typical ruhrocaudatus is entirely black, but in some 

 examples the thorax may have the yellow markings of the 

 var. bidecoratus, without any abdominal markings. If ab- 

 dominal markings are present, thoracic ones are invariably 

 developed. 



[Concerning the tendency of the females to lose the 

 white or yellow bands on the abdomen, Dr. Perkins wrote, 

 Nov. 15, 1911 :— ] 



In Odj/nerus, the species of the structural group of 

 0. sociabilis and the group of 0. nautarum have always 



TRANS. ENT. SOC. LOND. 1912. — PART IV. (FEB.) 8 A 



