170 
Psyche 
[December 
them. The rest of the Pterygota comprise the division 
Neopterygota, characterized by their ability to lay the 
wings back along the body in repose. They consequently 
develop a basal fold in the wing, and have more than two 
or three axillary sclerites. Their wings were originally 
heteronomous, with an anal fan in the hind wing (though 
this is lost in many of their descendents) , and a neala is 
developed in the postero-basal region of the wings. This 
division of the Pterygota is a much more fundamental one 
than the usual division into Exopterygota and Endoptery- 
gota (on the basis of the external or internal development 
of the wings) which makes an unnatural separation of 
closely related forms, and lumps together others which are 
not at all closely related. 
The common ancestors of all of these forms are repre- 
sented in the diagram by the hypothetical “Protopalseo- 
dictyoptera”. The Palseodictyoptera are the nearest known 
representatives of these common ancestors, and represent 
their direct descendents. The Ephemerida were probably 
derived from the common ancestors of the group by way of 
the Protephemerida, while the Odonata were derived from 
them by way of some unknown, extinct forms, to which the 
Protodonata are very closely related ; and the line of descent 
of the Megasecoptera apparently branched off from the 
common Palseodictyopteriod stock near the origin of the 
ancestors of the Odonata. The Odonata are the most 
“Orthopteroid” of the above-mentioned insects, while the 
Ephemerida are the most primitive living representatives 
of the group. The Ephemerida and Protephemerida might 
be grouped into a superorder called the Panephemeroptera 
(or Ephemeropteria) , characterized by the occurrence of 
three caudal filaments in many members of the superorder, 
while the Odonata and Protodonata might be grouped in a 
second superorder, the Pantyloptera (or Tylopteria) char- 
acterized by the skewness of the thorax etc., but too little is 
known of the morphological details of the fossil forms to 
enable us to group them correctly at this time. 
The Neopterygota may be grouped into three divisions 
called the Orthopteroid insects (Paurometabola or Orthop- 
teradelphia) , the Hemipteroid insects (Parametabola or 
Hemipteradelphia) , and the Neuropteroid insects (Holo- 
