Ptenothrix atra is very rare. Although there are a few scattered literature records for this species, it is quite easy to confuse with Dicyrtoma fusca. There is only one definite specimen from Britain in the NHML slide collection found at Boxhill in Surrey by Bagnall in October 1943. Unfortunately, the antennae of this specimen are poorly preserved.
     Members of the Dicyrtomidae are characterised by having a very short fourth antennal segment. In Ptenothrix atra, the third antennal segment (ant3) is clearly subdivided (ant3 is not subdivided in Dicyrtomina fusca). The claw on the foot is not wrapped in an outer membrane and the empodium bears a long filament that reached beyond the tip of the claw (Fig. 1). The inner and outer edges of the mucro are serrated (Figs. 2, 3) and the setae on the outer edge of the dens are smooth (Fig. 4) (these setae are feathered in Dicyrtoma fusca). The spines on the abdomen are relatively long (Fig. 5) and the dorsal side of abd6 (Fig. 6) bears long erect setae (not present in Dicyrtoma fusca).

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Fig. 1 (above): Foot of the third leg of a specimen of Ptenothrix atra collected from the Netherlands in July 1904.

Fig. 2 (above): Tip of the furca of Ptenothrix atra collected from Boxhill, Surrey in October 1943 by Bagnall.

Fig. 3 (above): Mucro of the same specimen of Ptenothrix atra shown in Fig. 2.

Fig. 4 (above): Smooth (non-feathered) setae on the outer edge of the dens of the same specimen of Ptenothrix atra shown in Fig. 2.

Fig. 5 (above): Long abdominal spines of the same specimen of Ptenothrix atra shown in Fig. 2.

Fig. 6 (above): Posterior abdomen of the same specimen of Ptenothrix atra shown in Fig. 2. Note the long erect setae on the dorsal side of the sixth abdominal segment (abd6).