1970] 
Kukalova — Palaeodictyoptera 
3 
forms on both the thorax and the abdomen, as they are still partially 
present in some Recent Thysanura (Lepismatidae) . He considered 
the lateral abdominal expansions of Stenodictya to represent a very 
primitive stage of this development at a time when they were not 
yet completely reduced. 
My examination of the Commentry dictyoneurids throws an 
entirely different light on the problem. The abdominal tergites of 
Stenodictya are heavily sclerotized with pointed and even prolonged 
posterolateral angles directed obliquely backwards. They are pro- 
vided with oblique ridges, which, of course, have nothing to do 
with the “oblique furrow” of trilobite pleura, but are secondary 
structures, undoubtedly serving to strengthen the lateral parts of 
tergites. The longitudinal suture, separating the lateral parts of 
tergites in some other families of Palaeodictyoptera, is completely 
missing. Since the abdomen of Stenodictya was relatively broad, 
decomposition processes had a marked effect on the relationship of 
abdominal segments, as preserved. They were widely separated 
from each other so that much of the intersegmental membrane 
between the tergites is visible in most specimens. The prolonged 
posterolateral angles of tergites consequently overlap the following 
segments much less and they protrude much more towards the 
sides. Simply by cutting out single segments of an exact figure and 
arranging them back to normal position, the abdomen becomes 
shorter, with the tergites slightly protruding. The abdomen, so 
reconstructed, is very suggestive of that of many Neoptera (e.g., 
roaches) and undoubtedly represents a derived, specialized state of 
development. Other body parts of Stenodictya were heavily sclero- 
tized as well, with deep, dense pits, exceptional for the Palaeodic- 
tyoptera. This strengthened cuticle probably provided protection 
against injury. Much more primitive, in my opinion, is the abdomen 
of some Palaeodictyoptera that have lateral lamellae present, i.e., 
with lateral parts of tergites separated by a longitudinal suture ex- 
tending from the notum. These structures are probably homologous 
with lateral lamellae of some living mayfly nymphs but their function 
is not yet explained. 
Martynov considered the venation of the Dictyoneuridae as the 
most primitive in the Palaeodictyoptera because of the presence of 
only a few branches (Comstock and Needham, 1898-99). The 
geological record, on the other hand, supports Redtenbacher’s (1886) 
concept of the richer wing venation being present in the more ancient 
groups. This idea was followed and developed further by Lameere 
