PHYLOGENY 97 



recognise that the Odonata probably arose along two separate 

 lines of modification from common ancestors. These ancestors 

 were evidently related to the Protodonata, but the relationships 

 of the Protodonata with the Megasecoptera are obscure. 



Arising apparently as another offshoot of the Pala^odictyoptera 

 are the Protoperlaria, which exhibit a type of venation fore- 

 shadowing the recent Plecoptera. The more advanced types of 

 Protoperlaria, such as Artinska Sell., show features ancestral to 

 the more primitive of the living Perloids and indicate that the 

 latter were probably descendants of the former. The presence of 

 prothoracic lobes and 5 -segmented tarsi in the Protoperlaria affords 

 characters which are well known to be wanting in all recent 

 Plecoptera. The discovery of a true Plecopteron, viz., 

 Ste7ioperlidium (Tillyard, 1935a), which clearly belongs to the 

 living family Eustheniidse, is one of the more striking disclosures 

 in insect palaeontology. The fact that a representative of a living 

 family of insects should be preserved in Palaeozoic rocks (Upper 

 Permian) is only paralleled among the Blattida:. 



The Ephemeroptera reveal evident Pala^odictyopteroid 

 characters in their primitive venation and their long cerci. The 

 Upper Carboniferous genus Triphlosoba. which constitutes the 

 order Protoephemeroidea of Handlirsch, appears to be related 

 to the genus Homaloneura of the Palaeodictyoptera. These 

 forms lead to the true May-flies of the Lower Permian epoch 

 which belong to the genus Protereisjua. In the latter insects 

 the fore and hind wings were closely alike in size and venation, 

 and the archaic living family of the Siphlonuridae has not diverged 

 very markedly from these early fossils. 



Another ancient order — the Protohemiptera (Fig. 43)— had a 

 type of wing venation which suggests that it may have arisen 

 from a specialised family of the Palaeodictyoptera, but it combines 

 primitive venational characters with specialised Hemipterous 

 mouth-parts. Up to the present the fossil record has afforded 

 no indication as to how the latter organs were derived from 

 generalised biting jaws. 



The Protohemiptera may, or may not, have been on the direct 

 line of descent of modern Hemiptera, but they indicate the 



R.A. ENTOMOLOGY. 4 



