Parry, B. W., 1979, A revision of the British species of the genus Phthiracarus Perty 1841 (Cryptostigmata: Euptyctima), Bull. British Mus. nat. Hist., Zool. ser. 35, pp. 323-363: 350-352

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'Complete' and 'reduced' leg chaetotaxy - a possible basis for a subdivision of the genus Phthiracarus 

The earliest proposed subdivision of the genus Phthiracarus  appears to be that of Feider and Suciu (1957) who recognized two species groups based solely on the shape of the sensillus. Their ' anonymum  group' included species with spindle-shaped sensilli ( anonymum  , globosus  , lanatus  and piger  ) and their ' lentulus  group' species with thread-like sensilli ( baloghi  , italicus  , lentulus  , ligneus  , parabotrichus  and sellnicki  ). However, these do not appear to be satisfactory groupings since the present study has suggested that sensillar shape is not correlated with other features, such as the chaetotactic pattern of the legs or notogaster.

Eight years later and based on a study of three species, van der Hammen (1965) suggested that the number of notogastral fissures, the position of the vestigial seta f1, the number of adanal setae and the chaetotactic pattern of the legs were features which could be used in a future subdivision of the genus, and placed P. laevigatus  and P. nitens  (ip and ips present; f1 ventral to h1; ad1-2 absent; femur I with four setae; genu IV with a single seta) in a separate group from P. anonymum  (ip and ips absent; f1 dorsal to h1; ad1-2 present; femur I with three setae; genu IV without any setae). Although the number of adanal setae has been found to be uncorrelated with other morphological features, the present study supports van der Hammen's proposed division of the genus on the remaining characters. Of the eight British species having the combination 'femur 1-4, genu IV-1', only murphyi  and rectisetosus  have all the attributes of van der Hammen's 'laevigatus-nitens group', since the other species ( affinis  , clavatus  , globus  and juvenalis  ) all lack the fissures ip and ips. In the case of species having the combination 'femur I-3, genu IV-0', only P. anonymum  possesses all the characters of van der Hammen's second grouping.

Using numerical methods, Sheals (1969) examined the affinities of 19 Phthiracarus  species from Europe, Israel, Labrador and Morocco, recognizing three species groups based on the following combinations of characters: femur 1-4, genu IV-1, fissure ips present; femur 1-4, genu IV-1, fissure ips absent; femur 1-3, genu IV-0, fissure ips absent. It is noteworthy that P. anonymum  which in the present study was found to be rather atypical, formed part of a reasonably compact Cluster of five species in this grouping studied by Sheals ( P. anonymum  and four new species from France, Labrador, Sweden and Switzerland).

The evidence suggests therefore, that morphological differences, particularly in the leg chaetotaxy, could provide a useful basis for dividing this large genus into two species groups. In the present study all the British species (and all the type specimens examined) feil into either a 'complete' ( affinis  , clavatus  , globus  , juvenalis  , laevigatus  , murphyi  , nitens  and rectisetosus  ) or a 'reduced chaetotaxy group' ( anonymum  , flexisetosus  , membranifer  , serrulatus  and tardus  ). In addition to features of the leg chaetotaxy, a number of other characters were also used to distinguish between the two groupings. With the exception of P. affinis  (notogastral length 406 - 487 µm), the species in the 'complete chaetotaxy group' are all rather large (notogastral length 502- 1065 µm). Moreover, in this grouping, the vestigial seta f1 is generally ventral to seta h1 while on the aspis the rostral setae are always located rather close together. The fissures ip and ips are present in some members of the 'complete group' ( laevigatus  , murphyi  , nitens  and rectisetosus  ) while they are absent in affinis  , clavatus  , globus  and juvenalis  . By contrast, species in the 'reduced chaetotaxy group' are all relatively small (notogastral length 314- 507 µm), the fissures ip and ips are always absent, vestigial f1 is dorsal or slightly ventral to seta h1 and on the aspis the rostrals are often located relatively far apart.

Any formal subdivision of the genus will, however, have to be deferred until a new type species has been designated. Van der Hammen (1965) considered P. contractilis  (type) as being close to his 'laevigatus-nitens group' (and thus to the 'complete chaetotaxy group'), but the type material of P. contractilis  must be presumed to be lost and Perty's original description is such as to make the specific identity of his species impossible. Moreover, on the evidence available from the present study, P. anonymum  would not be a suitable typical specimen of the 'reduced chaetotaxy group'.