Species of Mesaphorura are very small (typically 0.7 mm in length) soil-dwelling Collembola. Until relatively recently, most Mesaphorura have been recorded under the name Mesaphorura krausbaueri but following work by Rusek and others, it is clear that there are several species 'hiding' under this name. Indeed, most of the slides in the NHML collection labelled 'Mesaphorura krausbaueri' are in fact Mesaphorura macrochaeta. Mesaphorura macrochaeta appears to be the most common and widespread species of Mesaphorura in the UK and Eire (although there are, as yet, no records from Wales). The post-antennal organ has about 40 simple vesicles (Fig. 1), the sensory organ on the third antennal segment has two inwardly pointing blunt sensory sensilla (Fig. 2), the mandibles have a stout molar plate (Fig. 3) and the pseudocelli (PSO) are rosette-shaped (Fig. 4).
     The PSO formula of Mesaphorura macrochaeta is 11/011/10011. Other diagnostic characters include five setae in tibiotarsal 'B' ring (Figs. 5 and 6), seta a2 present on th3 (Fig. 7), distance between p1 setae on abd4 shorter than the distance between the p2 setae (Fig. 8), seta m0 on abd4 absent (Fig. 8), 3+3 short setae on abd5 between the long a4 setae (Fig. 8), and anal setae l2' present (Fig. 9; this latter character is the only discernible difference from Mesaphorura krausbaueri in which anal setae l2' are absent - the legitimacy of using this character might be questioned but it is reasonably easy to see in most specimens if they have been cleared and orientated appropriately).

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Fig. 1 (above): Post-antennal organ (PAO) of Mesaphorura macrochaeta collected from Epping Forest in May 1951 by J.T. Salmon.

Fig. 2 (above): Antenna of the same specimen of Mesaphorura macrochaeta shown in Fig. 1. The club-shaped sensilla of the sensory organ (so) on the third antennal segment (ant3) point towards each other.

Fig. 3 (above): Mouthparts of the left side of the same specimen of Mesaphorura macrochaeta shown in Fig. 1.

Fig. 4 (above): Rosette-shaped pseudocellus (PSO) of the same specimen of Mesaphorura macrochaeta shown in Fig. 1.

Fig. 5 and 6 (above left and right): Foot of the second leg of the same specimen of Mesaphorura macrochaeta shown in Fig. 1 at two levels of focus. There are five setae in the tibiotarsal 'B' ring (1-5).

Fig. 7 (above): Dorsal side of the third thoracic segment (th3) of the same specimen of Mesaphorura macrochaeta shown in Fig. 1. Seta a2 is present.

Fig. 9 (above): Anus of the same specimen of Mesaphorura macrochaeta shown in Fig. 1. The setae are numbered after Fig. 156 in Arne Fjellberg's (1980) key to Norwegian Collembola. Note that the pair of setae l2' are present.

Fig. 8 (above): Dorsal side of the fourth (abd4), fifth (abd5) and sixth (abd6) abdominal segments of the same specimen of Mesaphorura macrochaeta shown in Fig. 1. The distance between the p1 setae on abd4 is shorter than the distance between the p2 setae, and there are 3+3 short setae (a1, a2, a3) on abd5 between the long a4 setae. PSO, pseudocellus.