1968] Roth Cohen — Chromosomes of Pycnoscelus 55 
100% alcohol for i minute each, and finally in xylene for about 5 
minutes. Slides were made permanent by using a mounting medium 
such as Permount or Permaslip. 
The photographs of the chromosomes (mitotic metaphase) were 
taken with Kodak Contrast Process Pan 4X5 sheet film. Develop- 
ment time was 3.5 min at 70°C in Kodak D-11. Enlargements 
were made up to 2600X on Kodak Polycontrast paper. 
For karyotyping, the photographs of the chromosomes were cut 
out and roughly arranged by type and size. Centromeric indices 
were then calculated and the values were used to place the chromo- 
somes in their respective classes (Levan et al ., 1965). No attempt 
was made to pair the chromosomes and they are simply arranged by 
size, in each class. 
RESULTS AND DISCUSSION 
Pycnoscelus indicus. — The numbers of chromosomes found in P. 
indicus are summarized in Tables 1 and 2. Most of the specimens 
examined from our laboratory cultures have one of two modal diploid 
numbers of chromosomes: 2n ( c? ) — 35 or 37 (Table 1; Figs. 1, 
2) and 2n ($) = 36 or 38 (Table 2; Figs. 3, 4). Of the 21 
female nymphs examined, only 1 had 37 chromosomes (Table 2, 
nymph no. 18). It is possible that this female resulted from a cross 
between a male with 2n = 35 and a female with 2n = 38; a 
successful cross between these two forms should give females with 
37 chromosomes. 
Whether these 2 chromosomal forms exist in nature is unknown. 
The original P. indicus culture was started from several females origi- 
nating in Hawaii in 1954, and additional specimens from the same 
island were added in 1958 (Roth, 1967). It is possible that both 
forms were originally established in the laboratory; but it is also 
conceivable that one of the chromosomal forms arose during the 14 
years that this culture has been maintained (approximately 25 gen- 
erations — thousands of individuals were produced during this 
period). The scarcity of individuals with intermediate numbers of 
chromosomes, i.e., males with 36 (none were found among the 14 
nymphs examined) and females with 37 chromosomes (only one 
found), suggests that the 2 chromosomal forms are not interbreeding 
successfully. There is apparently no way of distinguishing the 2 
forms other than by chromosome count. Reddish or black nymphs 
may have either numbers of chromosomes (Tables 1, 2, footnotes 
e and f). We are attempting to establish separate cultures of the 
