Lawrence (1973) sunk Folsomia thalassophila as a junior synonym of Folsomia sexoculata. However, I am certain that they are both distinct taxa. The most important difference between the species is in the ratios of the length of the longest setae at the tip of the abdomen/length of mucro. In Folsomia sexoculata this is never more than 2.5 whereas in Folsomia thalassophila the ratio is between 3.0 and 3.8. I have seen mature males and females of both species.
     Folsomia thalassophila is a scarce species which has been found exclusively in the littoral zone in Scotland and Ireland. It is predominantly white with small black flecks, has 3+3 ocelli (two close together near the PAO and one clearly separate on each side of the head; Fig. 1) and reaches a maximum length of 1.4 mm. The body setae are relatively long (see above). In all the specimens I have seen, the manubrium has 2+2 apical ventral (anterior) setae in a transverse row (Fig. 2; much less variable than in Folsomia sexoculata). The dens has 12 ventral (anterior) setae and 6 dorsal (posterior) setae (4 at the base of the dens and 2 in the mid region). The PAO is narrow and is about as long as the width of ant1 (Fig. 1).
     Folsomia thalassophila is a senior synonym of Folsomia norvegica Altner, 1963 (confirmed by Arne Fjellberg).

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Fig. 1 (above): Folsomia thalassophila collected from Maghee Island, County Down, Ireland in November 1943 by Bagnall (this specimen is 1.3 mm in length and is a male). There are two anterior ocelli set close together near the post-antennal organ (PAO) and a single posterior ocellus.

Fig. 2 (above): Folsomia thalassophila collected from South Queensferry, Scotland in December 1934 by Bagnall (this specimen is 1.4 mm in length and is a male). The manubrium (ma) has 2+2 apical ventral (anterior) setae (*). d, dens.