Schizomida is one of the so-called “smaller” or “lesser known” arachnid orders (e.g., Harvey 2003), comprising two families, three subfamilies and about 360 described accepted species (Monjaraz-Ruedas et al. 2020) restricted to tropical and subtropical regions around the world. With an amber fossil record extending back to the Cretaceous (Selden & Ren 2017; Müller et al. 2020), in addition to the earlier known fossils from Miocene Dominican amber (Dunlop 2010; Krüger & Dunlop 2010), their phylogenetic relationships to other arachnid orders are well established. Schizomida is the sister group of Uropygi, from which they diverged around the Upper Carboniferous, and started diversifying during the Permian–Triassic (Clouse et al. 2017). Together with Amblypygi, they constitute the clade Pedipalpi (e.g., Latreille 1810; Shultz 1990, 1999; Giribet et al. 2002; Shultz 2007; Garwood et al. 2017; Huang et al. 2018), a clade supported (although sometimes without sampling Schizomida) in broad-scale phylogenomic analyses (Sharma et al. 2014; Fernández et al. 2018; Ballesteros et al. 2019; Ballesteros & Sharma 2019; Lozano-Fernandez et al. 2019, 2020). Pedipalpi is characterized, among other features, by having a brood sac attached to the ventral surface of the female's opisthosoma, the embryos feeding on the yolk of the eggs. This differs from brooding in Pseudoscorpiones, in which embryos depend on maternal secretions (Weygoldt 1969; Shultz 1990).

Despite being a synapomorphy of Pedipalpi, little is known about the brooding habits of schizomids when compared to those of Uropygi or Amblypygi (e.g., Weygoldt 1971, 2000). In Schizomida, the female glues her eggs to the abdomen using a secretion (Alberti & Palacios-Vargas 2015) and carries her first instar young (Gravely 1915; Rowland 1972), but details about where the secretion is produced and the timing of the entire process are not well understood. At least in some regions, schizomids reproduce throughout the year (Adis et al. 1999, 2001).

Gravely (1915: Pl. XXIV, fig. 27) illustrated the underground brood chamber of Schizomus crassicaudatus (O. Pickard-Cambridge, 1872) from Sri Lanka kept in captivity, the female carrying seven eggs arranged in a ring of five, one in the center, and another one on top of the central egg. His illustration shows a female with the opisthosoma bent 90° upwards carrying five eggs in the sternal side (see Fig. 1A here). The eggs did not hatch.

Figure 1.

A, Reproduction of the original illustration of the brooding chamber of the female Schizomus crassicaudatus by Gravely (1915); B, Reproduction of the original illustration by Rowland (1972) of the brooding chamber of a female Hubbardia pentapeltis, with eggs (left) and young (right); C, D, Hansenochrus tobago (MCZ IZ-144144), female with young instars, Gilpin Trail, Tobago, Trinidad & Tobago, May 23^rd, 2017 (photographs by GG).

The only study following the whole reproductive process, from fecundation to postembryonic development, is the widely cited paper on Hubbardia pentapeltis Cook, 1899 (as Trithyreus pentapeltis) by Rowland (1972). This study placed a male and a female from Riverside Co., California, in a glass chamber with 5 cm of loosely packed soil and 2 cm of oak leaf litter on top, with artificially made holes, where the individuals spent hours motionless. After 55 days, the female spent five days making a round chamber, the radius of her body, in one of the holes. On day 71, the female was noticed to have a distended opisthosoma, with the developing eggs inside. On day 72, the female was found on the bottom of the chamber with a hemispherical mass of 30 eggs attached to the venter of the opisthosoma (Rowland 1972: fig. 1). Until day 102, the female remained active within the chamber; by then the eggs had elongated, with the long axis parallel to the female's body. By day 108, the young had already hatched, the appendages were recognizable but not the telson, and they surrounded the mother's opisthosoma, aligned along the A–P axis of the female, parallel to her (Rowland 1972: fig. 2). The young kept developing slowly until the first two molted on day 146 (74 days after the eggs were first observed). On day 147, about half of the young dropped off the mother and were on the floor of the chamber, and by day 148, all had dropped off the mother, sluggishly moving about inside the chamber. By day 157, the mother finished the rearing by breaking a hole in the chamber wall, exiting to the surface, followed by all but four of the young. There are some indications that the male may have eaten some of the young; the female was found dead on day 167.

Rowland (1972) observed six instars, one embryonic and five postembryonic. He also examined two vials at the American Museum of Natural History, New York, with two collections containing young from caves in Mexico collected by R.W. Mitchell and J. Reddell, and it is unclear whether they used chambers. He also mentioned not having observed the animals to eat anything during the time he kept them in captivity, but Gravely (1915) did observe an adult schizomid eating a symphylan (Scutigerella sp.).

However, other than these two studies, observations on schizomid reproduction and brood care are rare and few have been reported. Under the Comments section of their redescription of Schizomus crassicaudatus, Reddell & Cokendolpher (1991: 7) mention a series of papers discussing the “habitat and behavior (including egg brooding and defensive secretion) of this species”. From the cited papers (Gravely 1911, 1915; Silvestri 1947; Rémy 1961), only Gravely (1915) and Silvestri (1947) have information on brooding, the other two focusing on descriptions of the habitat where specimens were found, but Rémy (1961) did report on collecting several young in Sri Lanka, during the wet season in 1959. We therefore provide a compilation of the existing but scattered literature as well as additional personal observations related to the reproductive behavior of Schizomida (see also Table 1).

Table 1.

Summary of observations reported with information on taxonomy, locality and date.

Here we have compiled published (mostly obscure) and novel observations on schizomid brooding including the citizens science portal iNaturalist, which is becoming a powerful tool for biodiversity research (e.g., Heberling & Isaac 2018). Both authors have spent many hours searching for edaphic fauna, indicating that these observations are rare. Most studies show the existence of three to multiple young aligned with the opisthosoma of the female, converging on the flagellum. One study, however, has showed an illustration with many small young surrounding the mother's opisthosoma in multiple layers (Rowland 1972: fig. 2 [see Fig. 1B here])—as if ornaments on a Christmas tree. This position seems at odds with all other observations (Gravely 1915; Brach 1976; Bellmann 2016), including the ones provided here (Fig. 1C–D). While this could be due to the large number of eggs/young reported for Hubbardia pentapeltis, the illustration is a drawing and should be confirmed with new observations. Another aspect worth studying seems to be their brooding chamber, reported for some of the species (Gravely 1915; Rowland 1972), as well as in some of the observations reported here. Similar chambers have also been reported for Rowlandius potiguar Santos, Ferreira & Buzatto, 2013 kept in captivity, although in this case they were supposedly built for shelter (de Oliveira & Ferreira 2014: fig. 6). Rowland (1972) also provides the possibility that cave species may brood without the use of chambers in caves, but this remains to be studied, as an observation from Hawaii reports the building of an earthen cell attached to the underside of a flat rock in a collapsed sinkhole (Howarth & Montgomery 1982).

Schizomids continue to be a fascinating order of arachnids, with only a few aspects of their biology well characterized (Beccaloni 2009). Few studies have focused on the transfer of spermatophores and mating (e.g., Sturm 1958, 1973), and a few more provide anecdotal data on brooding. More detailed experimental studies are certainly needed, both in the laboratory and in the field. We hope this report stimulates behavioral and developmental work on this poorly understood group of soil animals.