1964] 
Wilson and Taylor — Fossil Ant Colony 
IOI 
merits. Also, the leakeyi pupae are not enclosed in cocoons, a negative 
character shared with the modern species of the genus. The absence 
of cocoons is a rare and probably derived character within the sub- 
family Formicinae (Wheeler, 1915). Cocoons are omitted by many 
of the diverse species of Polyrhachis that are arboreal and use silk 
produced by their larvae for nest construction. They are also omitted 
by certain twig-dwelling species belonging to such genera as Campono- 
tus ( Colobopsis) and Gesomyrmex ; the latter genus is included on the 
basis of a single naked pupa recorded by Wheeler (1929). Cocoons 
are also lacking in two related terricolous genera, Prenolepis and 
Paratrechina. The great majority of formicine genera, however, are 
both terricolous and cocoon-makers. The absence of cocoons is, there- 
fore, correlated, but not perfectly, with the arboricolous habit. Addi- 
tional evidence favoring the proposition that the leakeyi colony was 
arboricolous is the very fact that the colony was preserved intact. It 
is very difficult to imagine how a subterranean or log-nesting colony 
could have been preserved as a unit; but it is much easier to imagine 
how an arboreal nest, especially the kind constructed by modern Oeco- 
phylla , could have broken off, dropped into the water, and been pre- 
served with little further disturbance. The small ratio of workers to 
immatures suggests that the former were able to escape in part while 
the latter remained trapped inside the drowned nest. 
Although the foregoing considerations are admittedly tenuous, some- 
thing more definitive can be said about the habitat in which the leakeyi 
colony lived. The species was part of a rich arthropod fauna. The 
Leakeys (in litt.) found it in association with many hundreds of other 
soft-bodied insects of diverse orders, as well as arachnids. Very little 
of this interesting fauna has been studied. There is also abundant 
associated plant material. In a preliminary study of the Mfwangano 
and Rusinga Islands plant fossils, Chesters (1957) discerned a mini- 
mum of 17 families and 21 genera of which five are fern genera. The 
majority of the fossils represent living African tropical genera. “Much 
of the material awaiting detailed examination will probably prove to 
be unidentifiable owing to its mode of preservation as crystalline casts. 
But the report here published does give a representative picture of a 
Miocene flora closely akin to that of tropical Africa at the present day. 
. . . The large number of lianas suggests a gallery-type forest in which 
trees festooned with climbers overhung the water-course.” 
Explanation of Plate 12 
Fossil pupae of 0. leakeyi of various sizes. The actual total length 
of the smallest specimen is about 4.0 mm. 
