CORRELATION BETWEEN SEGMENT LENGTH AND SPINE COUNTS IN TWO SPIDER SPECIES OF ARztNEUS ( ARANEAE : ARANEIDAE ) *

Observations made on several hundred adult male spiders o.f two species o.f Araneus indicate a highly significant correlation (po.ooI) between rhe length of a segment (tibi.a o,f the second leg) and the number of macrosetae ("spines") present on the segment. This result is further supported by observations on the first tibiae of about twenty male A. trifolium, one of the two species, and by a few observatio,ns on immatures of the two species. A short summary of the methods used in taking the measurements and making the calculations is folio,wed by discussion of the implications. this correlation with reference to species determination and geographic variation.

In addition, lengths and spine counts were taken for the first tibiae of 23 of the spid.ers, yielding 4 observations. Similarly, 2o specimens of A. marmoreus yielded 2o second tibiae; the length was measured, and two spine counts were made.: the total number on the tibia, and the number of modified, "dentiform ''1 spines (see Figure 3). The samples o,f both species were museum collections, and represented almost the entire known range o.f each in North America, extending from coast to coast and roughly from the 35th to the 55th parallels.
The spines of the second tibia, like all the spines of these spiders, are actua.lly setae in the entomological sense; they are set in a socket *Manuscript received by the editor January 10, 1973 1Term used by Locket & Millidge (1953), pp. 120, 121.  In the two species examined, the spines are uniformly larger than the hairs which are also present, the o, rmer having a diameter at the base o roughly 0.03 to 0.06 ram, while the latter are five to ten times smaller. In A. marmoreus, there is a second type o spine, described as dentiform, which is roughly 0.07 to o.Io mm at the base. Since this constituted a distinct group, it was counted separately.
Finally, the spines in both species are arranged in a.irly constant patterns, particular to the species (see . This makes it possible to recognize each spine, which further eliminates any uncertainty as to the number o spines. These ttiree actors, the clear difference between spines and hairs, the presence of a socket whether or not the spine itself has been lost, and the possibility o recognizing each spine, make the spine counts unambiguous and as accurate as possible within the limits of observor error. The length o the tibia was measured along the dorsal midline o the segment, between points "a" and "b" as shown in Figures and 3. A grid in the microscope eyepiece, each cell o which measured 0.325 mm on a side (as determined with a stage micrometer), permitted accuracy to -+-0.02 mm, or about -+-1%.
The treatment o.f the data ollowed standard statistics texts; the actual calculations were performed by the Harvard SDS 940 digital computer.  (967) or Adriana: the spine count is quite constant within each species, though few specimens are actually identical. Furthermore, in the two species of Araneus, as in Ariadna, almost no individuals are completely symmetrical in pattern, and most are asymmetric in actual spine counts. Beatty attributed such differences within and between individuals to developmental "accidents"; it is clear, ho.wever, that in the case of A raneus some of the variation between individuals is specifically related to difference in size. But for any single specimen the difference in s.pine count (between left and right legs) seems not to be correlated with the difference in segment length. (This co.rrelation is calculated as r2 in Table in II; the values, though positive, are not significant at the 5% level.) Thus Beatty's assertion is correct for individual spiders; differences between left and right legs do seem to be due to developmental accidents, and quite independent of each other. This point will be important in the folio.wing discussion.

DISCUSSION
It is important first to note that the above correlation does no.t in itseli: imply cause and effect; this is clear rorn the fact that o,r any individual, segment length and spine count are no.t correlated. It is likely that both l:he length of the segment and the number of spines are dependent on some other factor, such as general body size, etc.
One obvious possibility is that both measurements are related to the degree o,f development, that is, to. the number of molts the spider has undergone. In most spiders raised there is some variation in the number of preadult instars within a species. Furthermore, it is nt known with certainty at what stage these spiders mature or hw many molts occur after maturation, so this possibility cannot be examined with the data available. All the calculations here are based on sexually mature specimens, but their ages cannot be determined more precisely. Consequently, part of the spine count variation may be dependent on this unmeasured v.ariable; of course., size. is somewhat dependent on this variable too.
On the basis of the data presented here, the best statement is simply that spine count is very significantly correlated with segment length, in these two species of ztraneus.
Then there is the question of geographic variation. The samples studied represent a pool of many local populations in North America, and it is possible that the relationship between segment length and spine count is different in different regions. (Preliminary examination of the data with regard to this question indicate that this is in  tion is open to further study; its significance will be mentioned below. CONCLUSION The correlation between the number o.f spines on a. segment and the length of the segment is important to at least two aspects of Araneology: taxono,my and the study of geographic variation. Macrosetal counts have often been used to distinguish between different genera of spiders2, as well as between species of one genus such as d raneus. If the situation described in this paper is a general one, then clearly any character based on setal counts should be used for taxonomic purposes only after careful study. In general, it would seem from observations on these two species of draneus that the number of spines alone is not highly reliable, but the pattern is quite constant within a species (or at least recognizable, though spines may be missing, or present in "unusual" locations). This is supported by observations made on species of the genus Neoscona (Berman and Levi 1971 ), p. 467).
Secondly, in studying geographic variation, it is necessary at least in this case to consider the mean dimensions of local po,pulatio.ns as well as the spine counts. A marked variation in spine count between :For examples, see Kaston (1948)