All members of the genus Arrhopalites have only one ocellus on each side of the head. Arrhopalites principalis is a scarce species found inside and outside caves. The empodium of the foot is slightly longer than the claw (Fig. 1) and the fourth antennal segment (ant4) is divided into six clearly defined subsegments (Fig. 2). Both the inner and outer edges of the mucro are serrated (Fig. 3).
     Arrhopalites principalis is very similar in appearance to Arrhopalites cochlearifer. However, the female sub-anal appendage is 'feather shaped' in Arrhopalites principalis (Fig. 4) whereas it is 'spoon shaped' at the tip in Arrhopalites cochlearifer. In addition, there are six thickened setae on each side of the dorsal region of the head of Arrhopalites principalis (Fig. 5) whereas there are only four thickened setae of the same size on each side of the dorsal region of the head of Arrhopalites cochlearifer.

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Fig. 1 (above): Foot of the third leg of Arrhopalites principalis collected from Salen, Mull, Scotland in June 1970 by Peter Lawrence.

Fig. 2 (above): Antenna of the same specimen of Arrhopalites principalis shown in Fig. 1. The fourth antennal segment (ant4) is clearly subdivided into six subsegments (1 to 6).

Fig. 3 (above): Distal furca of the same specimen of  Arrhopalites principalis shown in Fig. 1. Both the inner and outer edges of the mucro are serrated.

Fig. 4 (above): Female sub-anal appendages (SAA) of the same specimen of Arrhopalites principalis shown in Fig. 1. The tip is 'feather' shaped.

Fig. 5 (above): Dorsal region of the head of the same specimen of Arrhopalites principalis shown in Fig. 1. There are six thick setae (1 to 6) on each side of the head (only one side visible).