CHROMOSOME NUMBERS IN METAZOA 65 
Sex chromosomes in insects—Continued 
Z2 TO 
5 3 GROUP FAMILY EXCEPTIONS shar EXCEPTIONS 
XY | Coleoptera Buprestidae Ist 
Cerambycidae 
| Cincindellidae 
Coccinellidae 
Lucanidae 
Melandryidae 
Meloidae 
Scarabaeidae 
Staphylinidae 
Tenebrionidae 
Some Carabi- 
dae 
Some Chryso- 
melidae 
One Silphidae 
V. HETEROCHROMOSOMES 
The most conspicuous heterochromosomes are the sex chromo- 
somes, an unpaired X or an unequal XY pair, which occur most 
characteristically in certain groups of insects. As may be seen 
from the accompanying table, an unpaired X occurs in practically 
all the Orthoptera and Hemiptera homoptera and in some families 
of the Hemiptera heteroptera and Coleoptera; an XY occurs in 
practically all the Diptera and in some families of the Hemiptera 
heteroptera and Coleoptera. In some families of the Coleoptera, 
an X is found in some genera and an XY in others. The sex 
chromosomes undergo their differential division in the first 
maturation in the Orthoptera, Hemiptera homoptera (except 
Enchenopa?), Diptera and Coleoptera (except Photinus), and in 
the second maturation division in the Hemiptera heteroptera (ex- 
cept Archimerus and Tingis). Sex chromosomes have not been 
described in any Hymenoptera, and are not of general occurrence 
in the Lepidoptera, though here an equal XY in the @ has been 
described in several species, an XY in the @ in Phragmatobia and 
an X in the 2° in Abraxas. Among the other Arthropods, an X 
