(257FObis) Folsomia bisetosa (Gisin, 1953)

Folsomia 'bisetosa' is a scarce species with only two confirmed - and very different - UK localities (Skokholm Island and Wolverhampton). It is white and blind (0+0 ocelli) and reaches a maximum length of 1.1 mm (Skokholm specimen). The body setae are very short and the ratio of the length of the longest setae at the tip of the abdomen/length of mucro is only about 2.0. The manubrium has 2+2 apical ventral (anterior) setae arranged longitudinally (Fig. 1). The dens has 12-19 ventral (anterior) setae and 5 dorsal (posterior) setae with three at the base of the dens and two in the mid dorsal region (Fig. 2; hence bisetosa). The PAO is quite broad and is shorter than the width of ant1 (Fig. 3).
Potapov (2001) says that Folsomia bisetosa is a circumpolar species and that records from 'more southern areas' ….. 'probably refer to other, undescribed species'. Thus although the British and Irish specimens of Folsomia 'bisetosa' correspond closely with descriptions of Folsomia bisetosa in the literature, it seems likely that they represent another (possibly undescribed) species.

 

 

 

Fig. 1 (above): Manubrium (ma) and dens (d) of Folsomia 'bisetosa' collected in September 1969 by Harold Gough from Skokholm Island. The manubrium has 2+2 apical ventral (anterior) setae arranged longitudinally (arrows). The specimen is squashed under the cover slip.

Fig. 2: Furca of Folsomia 'bisetosa' collected in September 1969 by Harold Gough from Skokholm Island. The dens has 5 dorsal (posterior) setae with three at the base of the dens and (note especially:) two dorsal setae in the mid region (arrows). The two setae in the mid region of the other dens are out of the plane of focus.

Fig. 3: PAO of the same specimen of Folsomia 'bisetosa' shown in Fig. 1

Fig. 4: Dorsal side of the first abdominal segment of Folsomia 'bisetosa' collected in September 1969 by Harold Gough from Skokholm Island. Folsomia 'bisetosa' is one of the species of Folsomia in which the sensory setae (arrows) are IN the p row of setae on the dorsal side of the abdominal segments.

 
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