Isotomodes productus is a common and widespread soil species that reaches a maximum length of 0.9 mm. It iswhite in colour, has an oval post-antennal organ (PAO) and is eyeless (Fig. 1). There are two prominent setae (Fig. 2) on the dorsal side of the fifth abdominal segment which should be checked, because some other Isotomodes (Isotomodes bisetosus and Isotomodes templetoni) lack these. The furca is about the same length as the head and the mucro has two teeth (Fig. 3). The ventral side of each dens has a single seta .
There are 2 other Isotomodes species on the UK list (though one is unconfirmed); Isotomodes bisetosus and Isotomodes trisetosus. Additionally, Isotomodes templetoni is apparently confined to Ireland;. Isotomodes trisetosus is represented by three specimens in the NHML slide collection found by Harold Gough in Tarlton, Gloucestershire in February 1970. Unfortunately, they are very badly degraded and are impossible to identify, leaving this species in need of confirmation on the UK list. There is a single specimen of Isotomodes bisetosus in the NHML slide material, collected by Harold Gough in June 1970, from its only known locality among beach line grass on Iona. These are distinguished by the presence/absence of prominent setae on abd. 5 and by the number of setae on the ventral side of the dens, as follows:
Isotomodes bisetosus lacks the prominent setae on abd 5 and has 2 ventral setae on the dens.
Isotomodes trisetosus possesses the prominent setae on abd 5 and has 6 ventral setae on the dens.
Isotomodes templetoni lacks the prominent setae on abd 5 and has 4 ventral setae on the dens.