Harvestmen  -  Order:   Opiliones
      As of January 2024, the Order: Opiliones, is said to contain 5 suborders (one of which is extinct),  70 families and 6,735 species of Harvestmen with a further 209 subspecies thrown in for good measure, according to   the 'World Catalogue of Opiliones' that is.  They are also known as Harvesters or Harvest Spiders although most people will know them as Daddy Longlegs.  Despite their resemblance to spiders they are not really that closely related.  If anything, they are likely closer to mites** than they are to any other kind of arachnid.  They are the third largest group of arachnids and it is estimated that only about 60% of all species are currently known leaving many yet to be discovered.
      Whereas fossils of Chelicerates are fairly common, the majority of these are Spiders.   Other groups such as the extinct Sea Scorpions, modern  Scorpions, and Acariform Mites are also well represented in the fossil record.  The Opilionids on the other hand make up only about one and a quarter per cent of Chelicerate fossils which is just one fifth of what one would expect based purely on the numbers.  The oldest fossils have been found in northern Scotland and date from the Early Devonian about 393 to 410 My ago.  Amongst other locations worldwide, the Koonawarra Fossil Bed of South Gippsland in Victoria has yielded a number of Opilionid fossils.
**   The prevailing opinion until about the 1980's and still held by many today.    There have since been proposals to group them with other arachnids but despite recent studies on the subject no definitive conclusions have been reached so far.
      All four of the Suborders of Opiliones are represented in Australia although one, the Cyphophthalmi or Mite Harvestmen,  counts only a couple of species from W.A. and northern Queensland and so is not discussed further on these pages.  
      Despite being amongst the oldest arachnids, over the past 400 My or so the Opilionids have not really changed much morphologically.  They are said to have been in 'stasis' for most of that time.   Palencia et al (2019) ascribes their lack of representation in the fossil record "as a combination of its low degree of 'potential fossilization' and the generally 'cryptic biology' of the extant members of the group."  (parentheses by editor) Potential fossilisation refers to the conditions for fossilisation, the most important one of which is burial by mud so as to exclude the presence of oxygen and thereby hindering or even preventing decomposition to take place.  Cryptic biology refers to individuals of different species being morphologically identical to individuals of other species.   No doubt the lack of vagility of Opilionids in general would have been a factor as well.
​      Most species of Opilionids are predators and despite their often fragile appearance are capable of taking down prey more formidable looking than themselves.  A Daddy Longlegs is actually quite capable of killing spiders such as Funnelwebs should the need arise.   Amongst Opilionids there are also scavengers, omnivores and even coprophages or dung eaters.  They are the only arachnids that can actually eat solids.  Opilionids have rather unusual defenses when threatened.  Self-amputation of limbs is a common but expensive means of trying to escape an enemy such as a predator. Expensive because the leg(s) can not be regenerated, not even in subsequent moults.  There are also species who can spray potential predator with a repugnant fluid produced in so-called repugnatory glands.  Yet others may release this repugnant fluid mixed with regurgitated fluids once in the predator's mouth to encourage it to spit out its' prey.
        There are a number of ways in which Opilionids can be distinguished from spiders, the most obvious being the fact that Opilionids appear to have only one body segment,  The cephalothorax is joined to the abdomen without a waist unlike spiders in which the two segments are clearly defined.   Other differences require much closer observation such as: Opilionids only having one pair of eyes which are often well set back. The odd shape of the 'head' in many species may even give the appearance that the eyes are raised on stalks.  Their chelicerae are often hidden and tend to resemble claws rather than fangs.   Opilionids do not produce silk and so, they have no spinnerets, nor silk glands.   Opilionids have no fangs and they produce no venom.  Thereby putting paid to the myth that "the Daddy Longlegs is the world's most poisonous 'spider' but its' 'fangs' are too weak to pierce human skin".   And finally, males have an elongated penis allowing direct copulation with their female counterpartss who themselves lay their eggs using an ovipositor.
References  and  links:
​
>      Dunlop, J.,  Penney, D.,  Tetlie, O.   and  Anderson, L.,   (2008).    'How many species of fossil arachnids are there?'   J Arachnol. Journal of Arachnology - J ARACHNOL. 36. 267-272.  10.1636/CH07-89.1.
>      Giribet, G.  and  Sharma, P.P.,  (2015).   'Evolutionary biology of harvestmen (Arachnida, Opiliones).'    Annual Review of Entomology, 60(1):157-175. https://doi.org/10.1146/annurev-ento-010814-021028
>      Kury, A.B.,  Mendes, A.C.,  Cardoso, L.,  Kury, M.S.,  Granado, A.d.A.,  Giribet, G.,  Cruz-López, J.A.,  Longhorn,  S.J.,  Medrano, M.A.,   and  de Oliveira, A.B.R.,  (2024).  World Catalogue of Opiliones (Version 2024-01-02). https://wcolite.com
>      Machado, G.,  and  Burns, M.,  (2022).    'Reproductive biology of harvestmen (Arachnida: Opiliones): a review of a rapidly evolving research field, Current Zoology, 2022;, zoac102,        https://doi.org/10.1093/cz/zoac102
>      Palencia, L.,  Peñalver, E.,  Prieto, C.E.,  and  Poyato-Ariza, F.J.,  (2019).    'First fossil harvestmen (Arachnida: Opiliones) from Spain and notes on the fossil record of Opiliones.'                  Palaeontologia Electronica 22.1.5A 1-18. https://doi.org/10.26879/855 palaeo-electronica.org/content/2019/2393-new-fossil-harvestmen
>      Pinto da Rocha, R.,  Machado, G.   and  Giribet, G.,  (2007).    ;Harvestmen: The Biology of Opiliones.'   Harvard University Press, London.
>      Ruppert, E.E.,  Fox, R.S.,  and  Barnes, R.D.,  (2004).    'Chelicerata.'  Pg. 554-604.  In Ruppert E.E., Fox, R.S., and Barnes, R.D. (ed). Invertebrate Zoology: A Functional Evolutionary          Approach  (7 th Ed.)  Thomson-Brooks/Cole. Belmont, C.A.
>      Sundevall, C.J.,   (1833).   'Conspectus Arachnidum.'   C. F. Berling, Londini Gothorum [= Lund], pp. 1-39.

      https://en.wikipedia.org/wiki/Coprophagia​
      https://en.wikipedia.org/wiki/Ovipositor​
      https://en.wikipedia.org/wiki/Punctuated_equilibrium
      https://en.wikipedia.org/wiki/Motility#Definitions​
      https://www.catalogueoflife.org/
      https://www.checklistbank.org/
      https://wcolite.com
      https://www.ento.csiro.au/education/allies/opiliones.html
      https://en.wiktionary.org/wiki/coprophage#:~:text=coprophage%20(plural%20coprophages),on%20feces%20or%20fecal%20matter.

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​Nick Lambert.
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