1978] 
Burnham — Social Insects in Fossil Record 
93 
tion under bark” seems immensely feasible; not only is isolation 
(and, hence, inbreeding) possible, but selective pressures leading to 
dependence on a cellulose diet would also be high. It seems an 
excellent explanation for the early separation of the termites and 
cockroaches from a common protorthopteran (protoblattoid) an- 
cestor as long ago as the Late Paleozoic. More definite conclusions 
on the origin of the Isoptera must wait until termites or termite-like 
insects have been found in pre-Cretaceous strata. 
HYMENOPTERA 
The Hymenoptera belong to the major subdivision of the Insecta 
known as the Endopterygota. There are no clues elucidating the 
nature or precise age of the earliest endopterygote insects, but the 
fossil record does provide insight into the history of the group as a 
whole. Representatives of two endopterygote orders, Neuroptera 
and Mecoptera, are found as far back as the Early Permian, some 
280 million years ago. This occurrence suggests an origin of the 
Endopterygota approximately 100 million years after the origin of 
the true insects. 3 
The earliest known Hymenoptera have been found in Triassic 
beds of Central Asia (Rasnitsyn, 1964) and Australia (Riek, 1955). 
These fossils establish a minimum age for the order of about 220 
million years. All the specimens known from this period belong to 
the suborder Symphyta, and surprisingly enough belong to the 
existing family Xyelidae. 
A major advance in the evolution of the Hymenoptera occurred 
with the development of a constriction between the first and second 
abdominal segments; this presumably had the selective advantage of 
increasing the flexibility of the abdomen, important for both 
oviposition and defense. Hymenoptera which possess this adapta- 
tion, a diagnostic character of the suborder Apocrita, are first 
known from Upper Jurassic deposits of Central Asia (Rasnitsyn, 
1975, 1977). These specimens have been assigned to the more 
primitive division of the Apocrita known as the Terebrantia or 
3 The oldest known insects, found in Upper Carboniferous deposits, comprise 11 
orders and include the Apterygota (Thysanura), Paleoptera and Exopterygota. It 
should be noted that here the use of the term insect does not include the Collembola, 
Protura or Diplura. 
