Pseudoscorpions  -  Order:  Pseudoscorpiones
"He who would study the false scorpions, either biologically or morphologically, will find his reward in the fascination of the bizarre and the little known, for indeed they constitute one of the most peculiar and one of the lesser known groups of animals."     
Joseph Conrad Chamberlin,  (1898 - 1962),   American arachnologist,   in:   'The Arachnid Order, Chelonethida' (1931).
      Also, and more commonly so, known as False Scorpions and Book Scorpions, these small to medium-sized arachnids, at first glance, resemble ticks with scorpion-like claws.   Averaging little more than about 3 mm in bodylength, the size range is between 1.5 - 8 mm with a few notable exceptions such as the species Garypus titanius from Boatswain Bird Island*, which can grow to about 11 -12 mm long.   In general Pseudoscorpions are short-legged with pear-shaped bodies.  Their defining feature is a set of long pedipalps that terminate in a pair of pincers that are very similar to those of Scorpions. 
     These small creatures are found on all continents except Antarctica.  They are however absent or, sparse at best, in desert areas.  Their wide dispersal means they can survive in a number of different habitats, including our homes, and no doubt are more numerous than most people might realise.   Worldwide there are 4,253 ** species in 490 genera in 25 families as of January, 2024 and more species are discovered regularly.  They are an ancient group of arachnids with a moderately extensive fossil record going back to the Middle Devonian, 388 Mya however, there are no Australian fossils on record yet.
    
Picture
Image 1. Fossil Cheiridium hartmanni (Menge, 1954) preserved in amber. Cropped section of image taken from Harms & Dunlop (2017) reproduced here under Creative Commons 3.0
      Most Pseudoscorpions produce venom from a gland located in the 'moving part' of their claws.  The venom is used primarily when hunting for food and serves to immobilise the pseudoscorpions' prey which usually consists of small insects and larvae thereof.  
​They are also capable of producing silk but, unlike spiders who produce silk from the rear of the abdomen, the pseudoscorpions extrude the silk -which is produced in glands in the cephalothorax, - through a tube at the end of the moveable section of the chelicerae (aka the 'finger').  The silk is used to make small shelters for mating purposes but also to protect themselves from predation during moulting episodes and to wait out extended periods of cold weather.
And finally, they secrete a fluid from their mouth to help digest their prey externally but they do not need to wait for the process to finish.  With their chelicerae they can tear off bits and masticate them prior to 'swallowing'.
Continued below.
      The Order: Pseudoscorpiones contains three suborders as per Catalogue of Life' in January 2024.  'iNaturalist' counts only two with the two small families that, together, make up the third suborder in the 'Catalogue', listed as a superfamily in 'iNat'.  The arrangement on 'iNaturalist' is the one with the most recent updates and therefore is taken as the one to use here.  (See: Harvey, 1992)
      Whereas telling apart species often requires the use of a microscope to compare the chelicerae, claws and other features, the families and, often, the genera therein can sometimes be dermined by the number of segments of the legs and the relative length of the pedipalps.   Their colouring is not at all any guide as they are often quite similar in that aspect across a number of taxons.  Pale ochres to reddish and dark browns to nearly black are most common and also most appropriate considering they can be found in leaf litter and under rocks and bark where they find shelter and forage for their food.
Picture
Image 2. Pseudoscorpion chelicera from The Spider Book by Comstock, J.H., 1913.
      Mating among Pseudoscorpions is usually no-contact but in a number of species it follows an elaborate mating dance after which the the male delivers a sperm packet to the female.  Some species use the sperm straight away whereas others may store it in a special organ for this purpose, for up to six months.  A brood of offspring is produced at least once a year and sometimes more often.  The mother carries the eggs and the hatchlings with her sometimes up until they moult into their second instar.  During that time the mother feeds them with a liquid secreted from her ovary ***.
      Many pseudoscorpions ​have formed associations with other animals; some even to the point that they have become   'obligate commensals'   i.e. they have become totally dependent on the organism they live with.  Many species from the genera  Lasiochernes and Megachernes for instance, spend their entire lives in the nest or fur of certain mammals.   Other species seem a bit more opportunistic in their associations and the most common among Pseudoscorpions is the manner in which they disperse.  They will literally 'hitch a ride' with flying insects by attaching themselves to a leg or other part of the insects' body and just wait for their ride to take them somewhere.   This particular behaviour is called  'phoresis'  and can be either obligate (part of their lifestyle) or  facultative (in response to environmental circumstances).
      Joseph Conrad Chamberlin (see quote at the top of the page), was an American arachnologist who specialised in pseudoscorpions.   After earning his PhD at Stanford Universitity, California, in 1929 he commenced working for The U.S. Dept. of Agriculture where he remained for most of his career.  He was the nephew of Ralph Vary Chamberlin (1879 - 1967), one of the most prolific taxonomists with over 4,000 species describedand named in 400 publications,  of which 1,400 were arachnids.  On occassion, uncle and nephew, co-authored papers on arachnids.
     *  =  Boatswain Bird Island is a small 13 acre island just off the coast of Ascencion Island in the Atlantic Ocean.  It is a barren, steep walled rocky outcrop with a flattish top not unlike a mesa.   It is the last refuge of G. titanius which was driven to extiction on the main Island of Ascencion by the introduction of rats, mice and other exotic animals.  
     **  =  According to the World Pseudoscorpiones Catalogue.
​     ***  =  This is not an 'accidental' feature of pseudoscorpions but rather a thoroughly developed and well-structured one that allows the female pseudoscorpion to provide nutrients to her offspring shortly after the eggs have been deposited in the eggsac and well before they hatch.   She even develops a special 'pumping organ' that facilitates the delivery of the nutrional fluids.   (See:  Jędrzejowska, I.  and  Garbiec, A.,  (2020))
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References  and  links:

>      Image  1.   Fossil Cheiridium hartmanni (Menge, 1954) preserved in amber. Cropped section of image taken from Harms & Dunlop (2017) reproduced here under Creative Commons 3.0  
>      Image 2.   Pseudoscorrpion chelicera. An image from The Spider Book by John Henry Comstock, 1913.  

>      Chamberlin Ph.D., C.J.,  (1931).    'The Arachnid Order Chelonethida.'    Stanford University Publications, Biological Sciences, Vol. VII, No. I, Aug. 1931. The Canadian Entomologist.  1931;63(12):294-294.  doi:10.4039/Ent63294a-12
>      Comstock, J.H.,  (1913).   'The Spider Book.'  Doubleday, Page and Co.  Garden City,  New York.
>      Harms, D.  and  Dunlop, J.A.,  (2017).   'The fossil history of pseudoscorpions (Arachnida: Pseudoscorpiones).'   Foss. Rec., 20, 215–238.  https://doi.org/10.5194/fr-20-215-2017
>      Harvey, M.S., 1991.   'Catalogue of the Pseudoscorpionida.'   Manchester University Press, Manchester, U.K.
>      Harvey, M.S.,  (1992.)   'The phylogeny and classification of the Pseudoscorpionida (Chelicerata: Arachnida).'    Invertebrate Systematics, 6 (6), 1373 – 1435.
>      Jędrzejowska, I.  and  Garbiec, A.,  (2020).   'Adaptations for matrotrophy in the female reproductive system in the pseudoscorpion Chelifer cancroides (Chelicerata: Pseudoscorpiones, Cheliferidae).'     https://doi.org/10.1002/jmor.21238

      https://www.ascension.gov.ac/map-marker/boatswain-bird-island
      https://www.catalogueoflife.org/data/taxon/B6LL9
      https://en.wikipedia.org/wiki/Commensalism
      https://en.wikipedia.org/wiki/Devonian
      https://en.wikipedia.org/wiki/Mesa
      https://inaturalist.ala.org.au/taxa/172373-Pseudoscorpiones
      https://museum.wa.gov.au/catalogues-beta/pseudoscorpions
      https://www.mindat.org/taxon-1161.html
​​      https://wac.nmbe.ch/
​      https://wac.nmbe.ch/order/pseudoscorpiones/families/3​
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