The Cytology of Tasmanian Short-Horned Grasshoppers ( Orthoptera: Acridoidea)

Mainly because of their large size, and general suitability for cytological study the chromosomes of the short-horned grasshoppers (superfamily Acridoidea) have been the subject of wide research. In the largest and most widely studied family, the Acrididae, early workers (McClung, 1905; Davis, 1908) reported the male number as being uniformly twentythree rod-shaped chromosomes, but Granata (1910) showed that Pamphagus possessed nineteen rod-shaped chromosomes. With few exceptions an XO sex chromosome sy~tem is found. Later work has shown that one group of subfamilies of the Acrididae is characterised by the male diploid number of· nineteen rod-shaped chromosomes, whilst another and larger group is characterised by the male diploid number of twenty-three. These are usually called the ten and twelve chromosome groups, and correspond to the Chasmosacci and Cryptosacci groups of subfamilies (Roberts, 1941). Cytologically the Chasmosacci is a very uniform group as has been shown by Rao (1937) and Powers (1942). The twelve chromosome group, however, has some cytological variability. In more than forty genera the characteristic male diploid chromosome number of twenty-three is found (White, 1945) ; but" centric fusions" (White, 1945) have been responsible for lowering the chromosome number of some species, although the characteristic twenty-three arms are still found. The most notable of these is the Stenobothrus, Chorthippus, Stauroderus, Gomphocerus, Chloealtis, Chrusochmon group where the number has been reduced to seventeen by three centric fusions (McClung, 1914; Robertson, 1916; Carlson, 1936; Darlington and Dark, 1932; Klingstedt, 1939). Recognition of this type of change was first due to the work of Robertson (1916). Reduction in chromosome number due to centric fusions reaches its maximum in Philocleon amomalus where the haploid number is six the X being fused to an autosome so that an XY sex mechanism is attained (Helwig, 1941). In some few species genuine reduction in chromosome arm