6 4 
Psyche 
[March-June 
of the cuticle, and are movable. Thus even when the spine itself 
becomes detached and lost, as is common with preserved specimens, 
its presence or absence can be determined unequivocally by the pres- 
ence or absence of the setal socket. 
In the two species examined, the spines are uniformly larger than 
the hairs which are also present, the former having a diameter at 
the base of roughly 0.03 to 0.06 mm, while the latter are five to 
ten times smaller. In A . marmoreus, there is a second type of spine, 
described as dentiform, which is roughly 0.07 to 0.10 mm at the 
base. Since this constituted a distinct group, it was counted sep- 
arately. 
Finally, the spines in both species are arranged in fairly constant 
patterns, particular to the species (see Figs. 1-4). This makes it 
possible to recognize each spine, which further eliminates any un- 
certainty as to the number of spines. These three factors, the clear 
difference between spines and hairs, the presence of a socket whether 
or not the spine itself has been lost, and the possibility of recogniz- 
ing each spine, make the spine counts unambiguous and as accurate 
as possible within the limits of observor error. 
The length of the tibia was measured along the dorsal midline 
of the segment, between points “a” and “b” as shown in Figures 1 
and 3. A grid in the microscope eyepiece, each cell of which meas- 
ured 0.325 mm on a side (as determined with a stage micrometer), 
permitted accuracy to ± 0.02 mm, or about =+= 1%. 
The treatment of the data followed standard statistics texts; the 
actual calculations were performed by the Harvard SDS 940 digital 
computer. 
Results 
Table I presents the correlation coefficient for the number of 
spines versus the segment length in the four different samples, as 
well as the coefficient of regression (b) and the results of the t-test 
for b = o. These values indicate that in all four cases the correla- 
tion is highly significant (i.e. significant at the 0.1% level), b, the 
coefficient of regression, quantifies the relationship established here; 
it is the slope of the estimated regression line drawn in Graphs I 
and II. The line is: 
(number of spines) = a + b X (length of tibia) 
Table II gives the mean and variance for the two variables in 
both species, and the mean absolute difference between right and 
left legs of individual specimens. In general the results are very 
