Folsomia similis is a rare species, and is closely similar to two other species (Folsomia diplopthalma and Folsomina palaeartica) whose occurrence in the UK is unclear. It is white spotted with black, has 1+1 ocelli (Fig. 1) and reaches a maximum length of 1.0 mm. The body setae are very short; the ratio of the length of the longest setae at the tip of the abdomen/length of mucro is about 2.0. The manubrium has 4+4 apical ventral (anterior) setae with 1+1 in a transverse row, 2+2 above these and 1+1 above these (Fig. 2; this is variable with 3+3 and 3+4 seen). The dens has 12-16 ventral (anterior) setae and 5 dorsal (posterior) setae. The post-antennal organ (PAO) is narrow and is longer than the width of ant1.
There are three specimens in the NHML, all collected by Bagnall. The holotype from Bridlington is a female of 1.0 mm in length. The other specimens are a male of 0.9 mm from Kew Gardens, and an individual (sex not clear) from a garden in Chiswick.
Folsomia similis is almost identical with Folsomia diplophthalma sensu Potapov & Dunger 2000. The only differences are that Folsomia diplophthalma has 4+4 latero-distal setae on the ventral tube and lateral teeth on the claws whereas Folsomia similis has only 3+3 latero-distal setae on the ventral tube (Fig. 3) and no lateral teeth on the claws. Previously Folsomia diplopthalma was stated to have 1+1 apical manubrial setae, but animals fitting this description are now classed as Folsomia palaearctica, a species not yet confirmed from the UK. Specimens of "Folsomia diplopthalma" in the NHM were found by Steve Hopkin to have 2+2 eyes (not 1+1) so were not even this species group, and Folsomia diplopthalma cannot be considered as being confirmed from the UK.
It is probably best to retain these three as separate species for the present, but it is possible that Folsomia similis may eventually be sunk as a junior synonym under a broader definition of Folsomia diplophthalma.