CHROMOSOME NUMBERS IN METAZOA 63 
somes of different species and to determine whether fusion of 
chromosomes has actually taken place. 
The Orthoptera have been carefully studied by McClung and 
his students, and they have found a great constancy, especially 
among the Acrididae. ‘The type group for the Orthoptera is 12 
including X. In Stenobothrus (Chorthippus), which has 9, 
Robertson has shown that three of these are really compound. In 
Chortophaga, there is a union of chromosomes in the diploid 
groups (McClung, 714). In two species of Hesperotettix (Mc- 
Clung, ’17), X is fused with another chromosome, while in an- 
other species, H. viridis, it may be fused or free, and fusion may 
occur among other pairs, correspondingly decreasing the number 
of chromosomes. Also in Mermeria bivittata (McClung, *17) X 
is fused with another chromosome, while in other species it is 
free. Among the Locustidae, Jamaicana (Woolsey, ’15) shows 
steps in change of number. Some individuals of J. unicolor have 
31 rod-shaped chromosomes and 2 Vs. in the diploid groups; 
some individuals of J. subguttata have 33 rods and 1 V; other 
individuals of these two species and all of J. flava have all rods 
and a diploid number of 35. Robertson (’16) has suggested that 
the 2 V-shaped chromosomes of Steiroxys are represented by 4 
rods in Decticus, giving a total of two more in the diploid group 
of the latter. An unpublished account of Mohr agrees with this 
in showing the two Vs in Steiroxys and he also shows them in 
Locusta viridissima (diploid number 29) whereas no Vs are pres- 
ent in several other genera whose diploid number is 31. Robert- 
son also points out that several Vs are present in Gryllus do- 
mesticus, which, if counted as twice as many rods, would give 
the number of chromosomes in G. assimilis; as no figures are 
given of the latter, this cannot be verified. 
There is therefore considerable evidence from the Orthoptera, 
Diptera and Hemiptera that chromosome numbers change by 
the splitting and fusion of chromosomes. The splitting of the 
sex chromosomes, which occurs in many other groups than the 
insects (see p. 66) indicates the probability that a similar proc- 
ess may take place among the other chromosomes. 
JOURNAL OF MORPHOLOGY, VOL. 34, No. 1 
