1. Structural changes simultaneously involving the
X-chromosome and an autosome have either not occurred at all in the Tettigonidae or at any rate have occurred far more rarely
than those involving the autosomes alone. The
X is consequently in a state of “evolutionary isolation”. The differences between telocentric and mediocentric
X’s when these occur in closely related genera are due solely to internal rearrangements in the
X and not to the mediocentric ones having acquired an extra arm from the autosomes (as has happened in
Drosophila).
2. These conclusions apply also to the Acrididae and the Mantoidea with the exception of the few genera whereXY orX
1
X
2
Y species have arisen fromXO ones as a result of “whole arm transfers” to theX.
3. In those subfamilies of Tettigonidae and Mantoidea where some of the species haveX’s of very different absolute length we must assume that duplications and deficiencies have arisen in theX in the course of its evolution. Such duplications and deficiencies may possibly have been beneficial to the species if they
readjusted the genic balance, upon which sex determination depends, to a slightly more advantageous equilibrium position.
This work was carried out at the John Innes Horticultural Institution, Merton. I am indebted to the Director, Dr C. D. Darlington,
for the facilities of the Institution after the evacuation of University College owing to the war.