History of Onychophorology, 1826-2020

Velvet worms, or onychophorans, are animals of extraordinary importance in the study of evolution. This is the first history of their study. They were described by Lansdown Guilding (1797-1831). This paper identifies the landmarks of their study, in a worldwide level, for almost 200 years. The beginning, 1826-1879, was based on describing their anatomy with light miscroscopy, mostly by famous French naturalists such as Milne-Edwards and Blanchard. In 1880-1929 peiord, work concentrated in anatomy, physiology, behavior, biogeography and ecology, but the most important work was Bouvier`s mammoth monograph. The next period, 1930-1979, was important for the discovery of Cambrian species; Vachons explanation of how ancient distribution defined the existence of two families; Pioneer DNA and electron microscopy from Brazil; and primitive attempts at systematics using embryology or isolated anatomical characteristics. Finally, the 1980-2020 period, with research centered in Australia, Brazil, Costa Rica and Germany, is marked by an evolutionary approach to everything, from body and behavior to distribution; for the solution of the old problem of how they form their adhesive net and how the glue works; the reconstruction of Cambrian onychophoran communities, the first experimental taphonomy; the first countrywide map of conservation status (from Costa Rica); the first model of why they survive in cities; the discovery of new phenomena like food hiding, parental feeding investment and ontogenetic diet shift; and for the birth of a new research branh, Onychophoran Etnobiology, founded in 2015,


INTRODUCTION
Velvet worms, or onychophorans, include placental species and, as a phylum, have survived all mass extinctions since the Cambrian. They capture prey with an extraordinary adhesive net that appears in an instant. There is not a single general history of this branch of science called onychophorology; the word does not even appear in dictionaries at the time I write this, but it has been used for decades by the Centre International de Myriapodologie in Paris. A definition is in order, so here is mine: Onychophorology is a field of biology that studies all members of the phylum Onychophora and all subjects related to them in all fields of research. These animals are extraordinary from the point of view of the evolution of life on Earth, but only interest a minuscule fraction of the scientific community. If all the onychophorologists active in the year 2020 were inside a bus, there would be many empty sits.
Other people probably saw onychophorans before Guilding (Costa Rican farmers refer to them as "slugs with legs"), but he was the first to describe them in a scientific paper (Guilding, 1826). It is hard to imagine the world in which he lived, where slavery was legal, Beethoven was still alive and no one knew that bacteria existed and caused disease. The Reverend Lansdown Guilding (1797Guilding ( -1831 was a British naturalist from the Caribbean island of Saint Vincent. He was mainly a botanist, a brilliant young man who corresponded with Darwin and Hooker; Hooker described him as "an arrogant, demanding, ambitious, and often conceited individual, all too ready to ask for unusual favors" (Howard & Howard, 1985). We know little else about Guilding, other than his first wife died "in childbed" leaving five children behind and that he died of unknown causes in 1831 while on vacation in another island (Howard & Howard, 1985).
Of the first and only onychophoran he ever saw, he wrote "it inhabits primary forests in Saint Vincent, often walks backward. If pressed, it releases viscous liquid from the mouth. Among the plants that I collected at the foot of mount "Bon Homme", I, astonished, discovered by chance the only specimen" (Monge-Nájera, 2019a).
Seven years after the publication, and two after Guilding's death, two French zoologists, Jean Victor Victoire Audouin (1797Audouin ( -1841 and Henri Milne-Edwards (1800-1885, a student of Georges Cuvier) moved onychophorans froms mollusks to annelids (Audouin & Milne-Edwards, 1833). Soon afterwards, the scientific world received the extraordinary news that the animal was also found half a world away, in South Africa (Gervais, 1836). Some pioneers in the field appear in Figure 1. Alejandro Solórzano (worm) and, for the portraits, https://commons.wikimedia.org/ During the 1840s the animals were studied by classical French luminaries Jean de Quatrefages and Émile Blanchard (Quatrefages, 1848;Blanchard, 1847). Blanchard also wrote the onychophoran chapter for the monumental book series Historia Física y Política de Chile edited by Claude Gay (1800-1873) a French naturalist who pioneered natural history in Chile (Blanchard, 1849).
The raising of onychophorans to their own phylum was done in 1853 by Adolf Eduard Grube (1812Grube ( -1880, then a lecturer in zoology in Dorpat, Germany. Grube, a student of the famous von Baer, father of embryology, was himself a recognized authority on invertebrates and specialized in Mediterranean polychaete worms (Grube, 1853).
The 1860s were poor in production, characterized by short notes, but for the first time in Europe (because it had been done earlier in Chile) we see the inclusion of the new animals in a general zoology, in this case, the Leipzig Handbuch der Zoologie (Carus, 1863).
The 1870s had a marked increase in productivity, mostly about anatomy, but also with a pioneering study about the embryology of Peripatopsis capensis from South Africa (Gegenbaur, 1874) and an early attempt on the evolutionary history of the group and its possible relationship with the origin of insects (Wood-Mason, 1879).

They are all over the world! Bouvier enters the scene: 1880 to 1929
The already notable growth in publications from the previous decade was followed by an even more spectacular increase in the 1880s, with papers on anatomy, embryology, ecology, geographic distribution and behavior. It was also the time of the oldest thesis on these animals that I could find: a study on the anatomy and histology of "peripatus" from the University of Breslau, in what is now Wrocław, Poland (Gaffron, 1883) The embryology papers were about species from South America (Sclater, 1888), South Africa (Balfour, 1883;Sedgwick, 1884) and New Zealand (Sheldon, 1887). Others dealt with compared anatomy of the brain (Saint-Remy, 1889) and the farynx (Nicolas, 1889), the origin of metamerism (Sedgwick, 1884), and the first study focused on how the number of legs varies, this one from South Africa (Peters, 1880).
Ecological papers described the habitat of species from New South Wales (Bell, 1887) and New Zealand (Kirk, 1883), and reverend Adam Sedgwick, Darwin's geology teacher, published the first The 1900s started well, with the first paper on spermatogenesis (Montgomery, 1900) and also the first one on population density (Duerden, 1901). But undoubtedly what marks this period is the appearance of a large mass of literature by Bouvier, including evolution (Bouvier, 1902), and his monumental monograph (Bouvier, 1905(Bouvier, , 1907, where he summarized all that was known about the animals, with emphasis on formal species descriptions. His species descriptions may be insufficient according to the standards of the 21st century, but are extraordinary if one considers the equipment and resources available to him at the time, and the fact that Eugène Louis Bouvier (1856Bouvier ( -1944 was a busy man working mostly on crabs at the time. The son of a watchmaker, Bouvier moved up the social ladder through hard work, first as a teacher in a primary school, then by teaching about mosses and lichens in a Pharmacy School (Anonymous, 2012). It was only in 1895 that he got the chair of entomology at the National Museum of Natural History in Paris; the chair was previously occupied by Émile Blanchard (who had written the onychophoran section for the Chilean monograph mentioned earlier; Anonymous, 2012).
At the museum, Bouvier established what today would be called a "citizen science" program to enlarge the collections, and he also wrote a textbook of natural history for colleges that came out at the same time as his onychophoran monograph. He was then with the Prince of Monaco expedition, studying deep water crustaceans from the Sargasso Sea (Anonymous, 2012).
Later in his life, Bouvier wrote popular books about insect behavior (perhaps under pressure, because he had written little in the field of insects, despite keeping the entomology chair), but in science he is best remembered for his studies of crabs and onychophorans, which appeared to have been his real love (Blanckaert & Hurel, 2017).
This is also the decade in which researchers studied how onychophoran bodies process waste (Bruntz, 1903) and did more comparative studies, considering the parapodia of onychophorans and millipedes (Lankester, 1904); at the same time Sedgwick made the first attempt to associate the systematic relationships of onychophorans with their distribution around the world (Sedgwick, 1908a, b). Perhaps the most extraordinary products from this time were the lessons by C.E. Porter, in which he taught the natural history of onychophorans to students of the Naval Officers School of Chile, lessons that were later collected in the Chilean Journal of Natural History (Porter, 1905) and that probably are not part of the curriculum in naval schools today.
Their known distribution in Asia and Oceania was expanded in the decade of 1910 (Horst, 1910;Annandale, 1912); a curious observation on the discharge of mitochondria from the spermatozoon was reported (Montgomery, 1912) and Clark (1915) analyzed their world distribution, while the natural history and bibliography of the Chilean species were reviewed by two authors (Johow, 1911;Porter, 1917). An interesting finding from the time was that they could also be found in the forest canopy, according to Costa Rican microbiologist Clodomiro Picado (Picado, 1911).
The 1930s had the first papers on Cambrian onychophorans (Hutchinson, 1930;Walcott, 1931), as well as a study of the local variation of a species, a problem that troubled Bouvier and still troubles us (Brues, 1935). A rare report on the parasites of onychophorans (Vincent, 1936) has not, unfortunately, been followed by much work afterwards, leaving this as an almost virgin territory for exploration.
This period is marked by the first papers by authors that would become important in next decade, such as Marcus in Brazil and Manton in England (Marcus, 1937;Manton, 1937), as well as by Snodgrass' classic work on the compared evolution of onychophoran and arthropod bodies (Snodgrass, 1938).
The decade of 1940 is important because it has the first studies about a region that would later become a center of onychophoran research, Central America; the articles mostly resulting from strong research activity in the Panama canal area during, and after, World War II (Brues, 1941;Dunn, 1943;Clark & Zetek, 1946;Hilton, 1946;Arnett, 1947).
Significant studies from the period considered how onychophorans produce leukocytes (Arvy, 1954), "muscle pharmacology" and possible uses of these animals in pharmacology (Ewer & van der Berg, 1954;Trindade, 1958); crural gland microanatomy (Gabe, 1956); brain secretions (Sanchez, 1958;Mendes & Sawaya, 1958); oxygen consumption in relation to size, temperature and oxygen tension (Mendes & Sawaya, 1958) and the formation of sperm cells (Tuzet & Manier, 1958;Gatenby, 1959). Perhaps the most innovative study from this decade was the first biogeographic analysis that explained the division into two families, as the results of historical separation at the time of Gondwana and Laurasia (Vachon, 1954).
Attempts were also made to disentangle systematics by comparing isolated aspects, such as locomotion (Manton, 1972), cuticle (Hackman & Goldberg, 1975), compared anatomy (de la Fuente, 1975) haemolymph (Gowri & Sundara Rajulu, 1976), and the Golgi complex (Locke & Huie, 1977). These attempts had little chance of succeeding because the resulting phylogenetic trees produced by any particular character were incompatible with the trees produced by other characters. This is also the time of the first general study about the habitat (Lavallard, Campiglia, Parisi Alvares, & Valle, 1975). Lawrence (1977) summarized research in South Africa, while Peck (1975) reviewed the species of the American continent and Delle Cave and Simonetta (1975)   Much study was done in the 1980s on the anatomy of onychophorans: musculature and innervation (Hoyle and Williams, 1980); morphometry of the tracheal system (Bicudo and Campiglia, 1985); the application of scanning electron microscopy to systematics (Read, 1988); and the curious cephalic pits and palps of some Australian species (Ruhberg, Tait, Briscoe, & Storch, 1988).
This period is also marked by the first strongly evolutionary focus on their reproduction (Morera, Monge-Nájera, & Saenz, 1988;Havel, Wilson, & Hebert, 1989), and seems to also be the first time that onychophorans appear in the IUCN Invertebrate Red Data Book (Wells, Pyle, & Collins, 1983). New possible onychophoran fossils were reported from Illinois (Thompson & Jones, 1980) and France (Rolfe, Schram, Pacaud, Sotty, & Secretan, 1982), but perhaps the most important publication from this decade is the second monograph on the family Peripatopsidae (Ruhberg, 1985). The 1990s was characterized by research in genetics and systematics. RNA and DNA sequences were combined with morphological data to assess systematics, using antennal circulatory organs (Pass, 1991;de Haro, 1998), and a paper recommended rejection of the "Uniramia" hypothesis, in which Sidney Manton joined Hexapoda, Myriapoda and Onychophora as a single monophyletic group (Wägele, 1993).
The second area of this decade, by productivity, was ecology and biogeography. Findings from Singapore showed that recent introductions could complicate the reconstruction of onychophoran historical biogeography (van der Lande, 1991), and pioneering computerized techniques were used to understand how climate and paleo-vegetation define their current world distribution (Monge-Nájera, 1994a), as well as how climate affects body characteristics (Monge-Nájera, 1994b) This decade was also marked by the first detailed study of the geographic variation of habitats (Monge-Nájera & Alfaro, 1995), and a model of onychophoran biogeographic history from the Jurassic through the Pliocene (Monge-Nájera, 1996). Other salient studies were a comparative analysis of evolutionary trends in onychophorans and scorpions (Monge-Nájera & Lourenco, 1995), and the first in-depth studies of the onychophoran adhesive secretion, with electrophoresis (Mora, Herrera, & León, 1996); chemical characterization (Benkendorff, Beardmore, Gooley, Packer, & Tait, 1999) and a comparison with arthropod silk secretions (Craig, 1997).
In the field of conservation, the finding of a single large aggregation in New Zealand (Harris, 1991) and a unconfirmed estimate that the population of a single species reached millions, led to the unjustified generalization that these animals "are not rare" (Mesibov, 1998), an error that can lead to dangerous implications for conservation (Monge Nájera, 2019d).
In fossils, the most important findings were the reinterpretation of Hallucigenia, now thought to be an onychophoran that was originally interpreted upside-down by Morris (1977) and corrected by Hou and Bergström (1991); and the opposite case, a Palaeozoic "onychophoran" reinterpreted as a stalked echnoderm (Rhebergen & Donovan, 1994). But perhaps the most unfortunate error of the time was G. Poinar's creation of new "onychophoran families" with specimens that lacked the body parts needed to define families (Poinar, 1996(Poinar, , 2000. In paleoecology, a quantitative study of an exquisitely preserved Cambrian community from China found extraordinary similarities with an extant community in Costa Rica (Monge-Nájera & Hou, 2000), and experimental taphonomy identified which fossil onychophoran "structures" are real and which can be just artifacts (Monge-Nájera & Xianguang, 2002).
Other outstanding reports included the use the head to insert the spermatophore (Tait & Norman, 2001) and that female-dominated hierarchies (Reinhard & Rowell, 2005).
This decade was also marked by the first study of the inflammatory process in onychophorans (Silva, Coelho, & Nogueira, 2000) and the identification of immune inducible genes (Altincicek & Vilcinskas, 2008), but perhaps the most innovative work was the first study of the physics of the adhesive secretion (Jerez-Jaimes & Bernal-Pérez, 2009).
The most recent decade, the 2010s, has been marked by the discovery of the largest species ever and by the first study of the mechanism by which onychophorans produce their adhesive "hunting nets" (Morera-Brenes & Monge-Nájera, 2010). The adhesive skin exudate of some frogs was found to be similar to that of onychophorans (Graham, Glattauer, Li, Tyler, & Ramshaw, 2013), and the adhesive net mechanism was finally cracked and found to be produced by passive hydrodynamic instability (Concha et al., 2015).
Other studies from this time dealt with the nanostructures of the solidified adhesive secretion (Corrales et al., 2017), the assembling of fibers by electrostatic interactions in phosphoproteins (Baer, Hänsch, Mayer, Harrington, & Schmidt, 2018), and a mimic of onychophoran skin, which is slippery to the adhesive, produced with microporous porphyrin networks (Ryu et al., 2018).
In this decade, onychophorans were found in unexpected places, like Vietnam (Bái & Anh, 2012), lava tubes (Espinasa et al., 2015) and urban vegetation (Barrett, 2013), and the first model to explain and predict their survival in highly disturbed habitats, based on their size and habits, was published (Monge-Nájera, 2018). The only tropical African species was collected again after more than a century (Costa & Giribet, 2016), and the period was also marked by advances in the study of onychophoran genetics and conservation in Brazil (Lacorte, De Sena Oliveira, and Da Fonseca, 2011;Costa, 2016;Cunha et al., 2017;Costa et al., 2018).
Costa Rica became the first country to fully map the distribution of its onychophorans and to indicate where they were preserved and the strength of conservation measures for each species (Morera, Monge-Nájera, & Mora, 2018). The first field monitoring study covering several years found that the relationship between onychophoran hunting activity, humidity and light, was not as expected (Barquero-González, ). Food hiding, parental feeding investment and ontogenetic diet shift were also reported for the first time for the whole phylum (Barquero-González, Vega-Hidalgo, & Monge-Nájera, 2019).
A new branch of onychophorology was also born in this decade, the Ethnobiology of Onychophorans, with the first study of onychophoran representations in folklore and art (Monge-Nájera & . The period ends with a series that proposes evolutionary explanations for several previously unexplained anatomical, physiological, behavioral and ecological characteristics, such as why do onychophoran spermatozoa swim for years, why do some Australian onychophorans have bizarre heads, why are there no onychophorans in Cuba and why ovoviviparity may be the ancestral form of reproduction in velvet worms (Monge-Nájera, 2019b, 2019c, 2019d.
In a period of climatic change and fear of mass extinctions, perhaps the most significant paper of the decade may be one that presents a way to rapidly and cheaply distinguish and name onychophoran species for their protection and long term conservation (Sosa-Bartuano, Monge-Nájera, & .

CONCLUSION
In these two centuries, Onychophorology has been built by contributions from many men and women. A few stayed long enough in the field to write numerous contributions (e.g. Barquero-González, Bouvier, Campiglia, Daniels, Mayer, Monge-Nájera, Morera-Brenes, Lavallard, Oliveira, Ruhberg, Sedgwick, Storch, Sunnucks, and Tait), but we ought the bulk of our knowledge to the mass of men and women who entered the field only briefly. There is a lesson here: for Uniciencia. Vol. 36(1), Jan-Jun, 2025: 1-XX onychophorology to prosper, we need to attract as many researchers from other fields as possible, even if each produces only one paper, for it is in their brains that the future of onychophoroly will be born.