Atractides nodipalpis, THOR, 1899

Gerecke, Reinhard, 2003, Water mites of the genus Atractides Koch, 1837 (Acari: Parasitengona: Hygrobatidae) in the western Palaearctic region: a revision, Zoological Journal of the Linnean Society 138, pp. 141-378 : 161-163

VARIATION: A. NODIPALPIS THOR

During an early stage of this revision the classification of Atractides from streams in southern Germany suggested the seasonal vicariance of several species, mainly of A. spinipes sensu K. Viets (1936) and A. nodipalpis Thor, 1899 . In order to obtain further insight into this problem, year-round collections were taken from the Goldersbach (Gerecke, 2002) and nodipalpis -like Atractides were studied using several statistical methods.

The results of the PCA indicate that in both sexes the first component is mostly influenced by the measurements of I-L-5 (dL, HA–HC) and P-2. In males it also involves L I-L-6, and in females L P-4. However, at around 60%, the explanatory power of the first three components is low. An ordination based on the character scores of the first two components when labelled with the month of collection does not produce seasonal groups: the males are spread over all four quadrants, and the females tend to group with strong superposition. Thus, variation in the measurements of the palp and I-L is largly independent of season. Slight differences between spring and autumn females are best explained by the presence of two consecutive generations (Gerecke, 2002). In agreement with this observation, scattergrams of all 23 variables used in this test, plotted individually against month of capturing, did not show any significant seasonal change.

In order to investigate whether an a priori grouping gives different results, the dataset was split into spring–summer and autumn–winter groups. The chisquare transformation of Wilks’ lambda resulted in only one variable in the male group (S-1 L); no variable in the female group showed a significance level below 0.02. There is little or no difference between the means of the two seasonal groups: in the males, 32% are misclassified in spring–summer, and 39% in autumn–winter. In females, the respective values are 27% and 40%.

This observation demonstrates that there is a very slight morphometric differentiation between the spring–summer and autumn–winter specimens. The combined results indicate that the A. nodipalpis population from Goldersbach is genetically homogeneous. Therefore, the palp and I-L measurement range observed there can be used in this revision to: (1) interpret the diagnostic significance of these characters in closely related (sub)species; (2) provide an example of the possible variability of these characters for taxonomically distant species.

Table 1 provides a survey of the mean values and range of variability of the studied I-L and palp measurements in the Goldersbach population.