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The occasional appearance of forms intermediate between the alate and the apterous castes of the Formicidm offers some ontological evidence of early stages in the origin of castes and has an important bearing on the question of development of organic forms through continuous or saltatory variation. The differentiation into castes is a condition which has existed for a long period of time since i is clearly shown in the many beautifully preserved ants of the Baltic amber from the Lower Oligocene Tertiary. In several genera of ants it is possible to trace all the transitional stages in the thoracic structure from that of the winged female to the wingless worker, except that the wings show no transitions, being perfectly developed in the typical female and entirely lacking in all the other forms of the series. In two genera gradational changes in the wing structure have been figured, one in the genus Monomorium (Wheeler, 17). and the other in the genus Pogonomyrmex (Tulloch, 30). The series of P. californicus which was given to the writer by Professor Harlow Shapley of the Harvard College Observatory exhibited various transitional stages of the wing from the completely venated type to a veinless wing sac. However, the appearance of these forms was considered to be pathological (since they only occurred in one colony) and, therefore, are perhaps not as instructive as the case noted by Dr. Wheeler in the genus Monomorium. Here it was discovered that at least two species exhibited a normal and unsuspected condition of subaptery in the female.

Thoracic Modifications Accompanying the Development of Subaptery and Aptery in the Genus Monomorium