Pogonognathellus flavescens (formerly Tomocerus flavescens) seems to be a scarce species with few recent records. The only specimens definitely of this species in the NHML slide collection were taken from Punchbowl Pool in Hampshire in August 1954. However, literature records indicate that it is widespread and must be more common than records would indicate. Indeed, it was the commonest large springtail around Dundreggan "Trees for life" project north of Loch Ness in April 2013 (though a few Pogonognathellus longicornis were also present). The spines on the inner side of the dens are simple and there is a pair of large leaf-shaped scales present close to the point where the dentes join the manubrium (Figs. 1 and 2). The empodial appendage of the foot is about 0.7 times as long as the claw.
Steve Hopkin called this species Tomocerus flavescens, and I take some sympathy with his preference for this generic name, since it is certainly easier to spell! The distinction between Tomocerus and Pogonognathellus is however clear in several easily seen ways. In life Pogonognathellus can curl their antennae, while Tomocerus cannot. This is due to their antenna 3, which differ between the genera. Pogonognathellus antenna 3 tapers gradually, and under high power one can see minute scales on each annulation along this elongated segment; by contrast Tomocerus antenna 3 is a scale-free cylinder of constant diameter. Pogonognathellus flavescens can only get its antenna 3 into a bend, while Pogonognathellus longicornis alone can produce a true spiral. Also Pogonognathellus have a large leaf shaped scale each side of the inside of the dentes (lacking in Tomocerus), though these can be knocked off in handling.
Distinguishing between Pogonognathellus flavescens and Pogonognathellus longicornis is straightforward: P. flavescens has no filament on any empodium, and the antennae are shorter than the body. By contrast longicornis has a clear filament on each empodium and antennae longer than the body. When seen side by side flavescens tends to have a more yellow ground colour (as its name implies) but this is not dependable.