Folsomia quadrioculata is very common and widespread. There has been some controversy over the legitimacy of separating Folsomia manolachei from Folsomia quadrioculata but it is now generally accepted that Folsomia manolachei is a good species. Folsomia quadrioculata is pale grey (sometimes with black spots) with 2+2 ocelli (Fig.1); it reaches a maximum length of 1.9 mm in British and Irish specimens (up to 2.5 mm in mainland Europe). The ratio of the length of the longest setae at the tip of the abdomen/length of mucro is usually between 4.0 and 5.5 (rarely as short as 3.5). The manubrium has 1+1 apical ventral (anterior) setae (Fig. 2). The dens has 8 ventral (anterior) setae (Fig. 3) and 3 dorsal (posterior) setae (Fig. 4; two at the base of the dens and one in the middle). The PAO is narrow, constricted in the middle and is about the same length or slightly longer than the width of ant1. An important and very reliable character is the position of the sensillum in the posterior corner of the dorsal side of the second thoracic segment (Fig. 5). In Folsomia quadrioculata, this sensillum is NOT in the posterior row of setae and is displaced anteriorly. In Folsomia manolachei, this sensillum is IN the posterior row of setae.
While working through the Folsomia material in the NHML collection, Steve Hopkin discovered several misidentifications of Folsomia quadrioculata for Folsomia manolachei and vice versa. At least some of the literature records on the adjacent map must be for Folsomia manolachei which is clearly an under-recorded species. Both species are sometimes found together in the same site.
In well-cleared specimens, the posterior ocelli may be very difficult to see (Fig. 1).