In Collembola, PAO is a paired organ located dorsally on the head, behind the antennae. It is probably a sensory organ of smell, humidity or temperature (Altner and Thies 1976). In some group, e.g. Onychiuridae and Neanuridae, PAO is morphologically diversified and highly important for taxonomy (Hopkins 1997). In contrast, most Entomobryomorpha have this organ poorly developed (e.g. Isotomidae) or absent (e.g. most Entomobryidae, Tomoceridae, Paronellidae and Cyphoderidae).

PAO in Tomocerinae was firstly recorded by Mills (1948) in a new genus and species, Tomolonus reductus Mills, 1948. Goto (1956) showed that PAO is present in the first instar of Pogonognathellus longicornis (Müller 1776) but disappears in subsequent instars. Christiansen (1964) claimed this organ did not always appear in all specimens of Tomolonus reductus and accordingly rejected the generic state of Tomolonus, but later studies (Yosii 1967, Rusek 1977) supported Mills’ original position.

Until now, the only known species of Tomocerinae with PAO in the mature stage was Tomolonus reductus, from North America. The purpose of this paper is to describe a second species of this subfamily with well developed PAO, based on material from Vietnam.

All type specimens were collected in caves with aspirators. After being photographed with a Jenoptik ProgRes C10+ camera mounted on a Leica MZ 16 stereomicroscope, specimens were cleared in lactic acid and mounted in Marc André II solution. The head, furca and legs were cut off from the trunk and mounted separately for detailed observation. The slide-mounted specimens were studied using a Leica DMLB microscope.

The pattern of dorsal cephalic chaetotaxy used here is modified from that Wang et al. (2013): the two posterior macrochaetae placed in the postocular area by these authors (Figure 2F in Wang et al. 2013) are here considered to belong instead to the posterior area. We follow Fjellberg (2007) for maxilla lamellae numbering and Christiansen (1964) for macrochaetotaxy. The dental spine formula follows that of Folsom (1913), in which the dental spines are arranged from basal to distal, with a slash indicating the separation between basal and medial subsegments and the Roman numerals referring to spines that are noticeably larger.

Most tomocerids have four prelabral chaetae, which is also common number in other groups of Entomobryomorpha. The taxonomic significance of the prelabral chaetae in Tomocerinae was firstly discovered by Yosii (1966, 1967), who described several Plutomurus species with more than four such chaetae. As far as we know, tomocerids other than Tomocerus postantennalis sp. n. with six or eight prelabral chaetae belong to two genera: Plutomurus (Plutomurus grahami Christiansen, 1980, Plutomurus ehimensis Yosii, 1956, Plutomurus kawasawai Yosii, 1956, Plutomurus gul Yosii, 1966, Plutomurus iwatensis Yoshii, 1991, Plutomurus ortobalaganensis Jordana and Baquero, 2012, Plutomurus kelasuricus Martynova, 1969 and Plutomurus marmorarius Yosii, 1967) and Lethemurus Yosii, 1970 (Lethemurus finitimus Yosii, 1970). They all inhabit caves and most of them are eyeless, with the sole exception of Plutomurus kelasuricus, which has small ocelli. Although the function of prelabral chaetae is still unknown, the increased number of these chaetae appears to be a troglomorphic adaptation. The protruding prelabral chaetae probably provide a tactile sense in front of the mouthparts, and more prelabral chaetae may increase the sensitivity, which is important for living in darkness.

So far, only two species of Tomocerinae have the PAO developed in adults: Tomolonus reductus (Mills) and Tomocerus postantennalis sp. n. The former has the most reduced eyes of non-cave species (3+3) and lives in soil, while the latter has the maximum number of ocelli for the subfamily (6+6) and lives in a cave, where its eyes are useless because of darkness. PAO development in adult may therefore be assumed to compensate for deficient vision performance in both species. Under this assumption, we would expect PAO among other cave species of Tomocerinae, a presence difficult to detect given the inconspicuousness of the organ when present.

The PAO of Tomocerus postantennalis sp. n. is similar in shape to those of juvenile Pogonognathellus longicornis (Goto 1956) and some adult isotomids (Potapov 2001), but it differs from the compound PAO of Tomolonus, which is constituted of a small central vesicle and three larger tubercles (Mills 1948, Yosii 1967, Rusek 1977). Besides this structural difference, the PAO of Tomocerus postantennalis sp. n. is relatively larger than those of Tomolonus reductus and juvenile Pogonognathellus longicornis. The morphological differences in PAO between Tomocerus postantennalis sp. n. and Tomolonus reductus may reflect a functional difference or represent two evolutionary alternatives—enlargement of a simple organ or complication of a small organ, to enhance a similar function. In any case, the different PAOs in the two species are likely to be adaptive convergences rather than synapomorphies, since they are distantly related based on strong morphological differences.

Despite the presence of PAO, Tomolonus is more closely related to Plutomurus, since both have two posterior macrochaetae on the thoracic segments (Christiansen 1964, 1980, Jordana et al. 2012), a well developed trochanteral-femoral organ, strong outer basal chaetae on the dens and simple dental spines. The new species fits Tomocerus better than other genera because it has three posterior macrochaetae on the thoracic segments, poorly developed trochanteral-femoral organ, no strong dental chaetae and a toothlet on the outer basal tooth of the mucro.

The genus Tomocerus Nicolet, 1841 is so far poorly defined by a single character: the presence of a toothlet on the outer basal mucronal tooth, thus all species conforming to this criterion and lacking the diagnostic character of other genera have been assigned to this genus, resulting in a wider range of intrageneric diversity than in other genera. For instance, all types of dental spines from simple to strongly furcated can be found in Tomocerus, whereas in Pogonognathellus, Plutomurus and Monodontocerus the shape of dental spines are constant within genus. On the other hand, the single generic character is not exclusive for Tomocerus since Aphaenomurus interpositus denticulatus Yosii, 1956 and Plutomurus vigintiferispina Lee, 1974 also have a toothlet on the mucronal outer basal tooth. The situation of Tomocerus will remain problematic until a comprehensive and detail investigate is carried out.