1930] An Unusual Nest of Pogonomyrmex 67 
bearing segment may have been evolved from a wingless 
segment. Snodgrass (1926) has suggested that the insect’s 
ancestor may have been a soft-bodied, segmented animal, 
resembling in its segmentation the soft-bodied larvae of 
some modern insects. Modern adult arthropods, unlike 
their hypothetical ancestors, have the thorax modified as 
the locomotor region of the body, and are in general hard- 
shelled forms, having developed an external skeleton formed 
of calcareous or chitinous matter, and the hardening of 
the body wall has had a profound influence on the struc- 
ture of the segments and on the general mechanism of the 
animal. The skeletal deposits have taken the form of 
segmental plates, of which the principal is a dorsal, or 
tergum and a ventral, or sternum. These two plates are 
separated on the sides of the segment by a membranous 
pleural area. The condition found in a wingless thoracic 
segment is fundamentally of this structure, except that a 
chitinous pleural plate is present and connected to the 
tergum and sternum by membranous regions. In the winged 
thoracic segment these plates have become modified to 
strengthen the thorax in order to provide solid attachment 
points for the muscles of the wings. This has come about 
by the formation of chitinous antecoxal and postcoxal 
bridges connecting the pleuron and sternum, as well as 
prealar and postalar bridges connecting the pleuron and 
tergum. The tergum has become secondarily divided to pro- 
vide muscle attachment points. Thus the transition from 
the wingless thoracic segment to the winged segment has 
been one of strengthening the thorax to accommodate the 
muscle stresses concomitant with the acquisition of wings. 
The embryonic history of insects substantiates the fact 
that the thorax was first differentiated as the locomotor 
region of the body by a specialization of three pairs of 
segmented appendages as the principal organs of progres- 
sion, this being accomplished by the modification of the 
gnathal appendages to feeding organs and by the suppres- 
sion of most of the abdominal appendages. Flight being a 
comparatively recent development as a further mode of 
progression, the development of wings and the perfection 
of the mechanism meant a further and much greater alter- 
nation in the structure of the wing-bearing segment than 
