A new genus of Neelidae (Collembola) from Mexican caves

Abstract The new genus Spinaethorax, whose proposal is based on specimens of Megalothorax spinotricosus Palacios-Vargas & Sánchez, 1999, is given a new name combination and a redescription. The type species comes from two caves in Campeche State, México. A new combination is also suggested for Megalothorax tonoius Palacios-Vargas & Sánchez, 1999. The new genus is similar to Megalothorax Willem, 1900 and Neelus Folsom, 1896, but it clearly differs from all genera within family Neelidae by a peculiar combination of characters and the presence of some new features, e.g. globular sensillum on Ant. III, sword-like macrosetae on oral fold. A comparative table and an identification key for all Neelidae genera as well as some summary tables of antennae chaetotaxy and legs setation for type species are provided.


Introduction
The family Neelidae comprises five genera and 41 species in the world. However, only 9 species in 3 genera are known from Mexico (Palacios-Vargas 1997;Palacios-Vargas and Sánchez 1999). This family is mainly a euedaphic group of Collembola, whose members are usually very small (0.3-1.0 mm), they have nor eyes or scales, however, their antennae are shorter than their heads. The main differences among the genera of this family are in the structure of forehead setation, antennae, sensory fields and furcula.
Members of this family are cosmopolitan. Therefore, they have been found in various localities mainly associated with soil and litter at different altitudes from sea shore up to 3,000 m a.s.l. (García-Gómez et al. 2009). They have frequently been recorded from caves, mainly in places with rich organic material. This family has two endemic genera: Acanthoneelidus, with only one species from Europe, and Zelandothorax from New Zeland plus three cosmopolitan genera. Megalothorax is the most diversified with 28 species, Neelus with 6 species Neelides with 5, both widely spread, too. The new genus described herein has two species distributed in Mexican caves.
The most remarkable contribution on this family from Mexico is the work by Bonet (1947), who revised the whole family of Neelidae. The catalog by Palacios-Vargas (1997) records 7 species in this family but some of them have to be revised in the light of new characters. The most recent contribution to the taxonomy of this family from Mexico was that by Palacios-Vargas and Sánchez (1999) who described the two new species that are revised herein.

Materials and methods
The present redescription of Megalothorax spinotricosus Palacios-Vargas & Sánchez, 1999, is based on original slides deposited in Facultad de Ciencias, UNAM. Specimens were obtained from samples of bat guano and soil that were processed by Berlese-Tullgren funnels and preserved in 75% ethylalcohol. Slides were mounted using Hoyer´s solution.
Body length was measured on slides excluding antennae and furcula. Lengths of unguis and unguiculus were measured between the most basal (proximal) point and the tip on their inner margins. Besides the common measurements, the ratio "unguis I, II, III (inner margin): Ti. I, II, III width (middle part)" that can be used as additional character, was also included.
We followed nomenclature used in last Neelus revision (Kováč and Papáč 2010) for labral setae and dental spines. Nomenclature used in the most recent revision of Megalothorax after Schneider and D´Haese (2013) was applied for arrangement of sensory fields, wax rod crypt (wrc1-8) on head, Th. and Abd. sensilla s1, s2, s3, subsegments of dens and chaetotaxy of antennae. Forehead chaetotaxy (presence of a0 seta) is applied according to Deharveng (1978) and posterior chaetotaxy of head is used according to Palacios-Vargas and Sánchez (1999).
Description. For lengths of different character see Table 1. Body length 0.5-0.7 mm. Habitus globular, as other members of Neelidae. No pigmentation, cuticle finely granulated, linea ventralis without crossing with integumentary channels on ventral head back. Mid and hind Abd. with numerous spine-like microsetae, stouter spines around all sensory fields on body and apex of head.
Variation. The young have only one proximal seta on dens.

Spinaethorax tonoius (Palacios-Vargas & Sánchez, 1999), comb. n.
Note. This species, described from a cave in State of Guerrero, shares with new type species of the genus similar generic characters like presence of sword-like macrosetae on oral fold, globular sensillum on Ant. III., stouter spines on Th., fusion of Ant. III and IV, 3+3 setae around Abd. s.f., 2+2 setae on proximal part of dens and absence of E3 spine on distal part of dens. The main differences consist in setation of hind Abd.
(numerous thickened macrosetae in S. tonoius comb. n. vs. numerous spine-like microsetae in S. spinotricosus comb. n.), in chaetotaxy of the apex of head (only one spine IL 1 in S. tonoius comb. n. vs. three spines IL 1 -IL 3 in S. spinotricosus comb. n.), in the structure of tenent hairs on Ti. (more developed in S. tonoius comb. n. vs. shorter in S. spinotricosus comb. n.) and in setation around Abd s.f. (absence of axial spine in S. tonoius comb. n. vs. presence of spine in S. spinotricosus comb. n.). Species description is based only on one adult specimen (holotype) and does not allow us to describe it in an appropriate way, along with drawings and measurements. Discussion. The family Neelidae was for a long time an overlooked group of Collembola, mainly due to its small size and lack of diagnostic characters. In spite of the foregoing facts recent years molecular phylogenetic analysis have revealed unexpected diversity within this family (Schneider et al. 2011). Recently Neelus has been revised (Kováč and Papáč 2010) as well as Megalothorax with a redescription of the nominal species Megalothorax minimus (Schneider & D´Haese, 2013). Soon after additional new taxonomical characters in genus Megalothorax were defined (Papáč and Kováč 2013). S. spinotrichosus comb. n. and S. tonoius comb. n. share many specific characters. These species were included in the genus Megalothorax (Palacios-Vargas & Sánchez, 1999), because some features (fusion of Ant. segments III and IV, absence of E3 spine on distal dens) indicated that those specimens belonged to this genus. On the other hand, some characters link those specimens to Neelus (dp with 2+2 setae -except N. fimbriatus, 1+1 neosminthuroid setae at the base of Abd. IV sternite). Schneider and D´Haese (2013) stated that chaetotaxy of M. spinotricosus clearly differs from the other Megalothorax by the presence of great number of microsetae on Abd. and pointed out that M. spinotricosus deserved its own genus. On the basis of recent diagnostic features, material of these two species was re-examined arriving to the same conclusion as Schneider and D´Haese (2013) that they represent a new genus in the Neelidae. Spinaethorax gen. n. differs from other genera by striking morphological features and combinations, which clearly separate them, e.g. sword-like macrosetae on oral fold, six setae on basomedian field of labium, Ant. III and IV fused, presence of small globular sensillum Sg on Ant. III, three marginal setae around abdominal sensory field, absence of spine E3 on dd and dp with two seate. For comparison with other genera see Table 4.

Identifiaction key to the World genera of Neelidae
The identification key is based on that of Bretfeld (1999 Bretfeld & Griegel, 1999 (Poland)