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VIII. The Opisthomeres and the Gonapophyses in me 

 Dermaptera. By Malcolm Burr, D.Sc, F.E.S. 



[Read November 4th, 1914.] 



The Opisthomeres, 



If the forceps of an earwig be removed, it will be found 

 that there is a strongly chitinised plate that extends from 

 the posterior margin of the last tergite, bent downwards ; 

 this is the pygidium, the peculiar processes of which have 

 long been familiar to Dermapterists as affording useful 

 specific characters, especially in the Eudermaptera. 



But if the pygidium be separated from the last tergite, 

 it will be found that there is a second, and occasionally 

 a third, smaller segment attached to it, which is tucked 

 away above the penultimate sternite, and so only dis- 

 cernible upon dissection. This second plate in the meta- 

 pygidium, and the third, when present, is the telson. 



Verhoeff was the first systematist to realise the impor- 

 tance of these segments. He points out that Brunner 

 refers to the telson under the name of sub-anal plate, but 

 it was more probably the metapygidium that he saw, as 

 the telson is but rarely developed. At the same time, 

 Verhoeff called attention to these characters, which are 

 better developed in the more primitive Protodermaptera, 

 and gradually degenerate in the Eudermaptera. 



The pygidium is the rudiment of the eleventh tergite, 

 the metapygidium of the twelfth, and the telson of the 

 thirteenth. These three segments are called by Verhoeff 

 the opisthomeres. The first two are always present, but 

 the telson is rarely chitinised : it is usually absent, or repre- 

 sented by a hyaline, delicate membrane. 



The telson is an independent chitinised plate in the 

 Pygidicranidae and Pyragrinae, and probably in the Di- 

 platyinae. The figures of the Allostethine opisthomeres, 

 and those of Adiathetus tenebrator, show an ill-defined 

 membrane which I take to be the degenerate telson. But 

 in the metapygidium and pygidium there are often visible 

 transverse sutures, which seem to suggest a fusion of two 

 plates. 



TRANS. ENT. SOC. LOND. 1915 — PARTS III, IV. (DEC.) S 



