Friday Fellow: Platine Shield Harvestman

by Piter Kehoma Boll

Beneath fallen logs and rocks or in the leaf litter of forests and gardens around the La Plata River in Argentina and Uruguay, you may find today’s fellow. Scientifically known as Discocyrtus prospicuus, it is a harvestman, a member of a group of arachnids that resemble spiders. As usual among small hidden invertebrates, it lacks a common name, so I coined the term Platine shield harvestman to refer to it.

Discocyrtus prospicuus in Buenos Aires, Argentina. Photo by Nicolas Olejnik.*

The Platine shield harvestman belongs to the family Gonyleptidae, which includes harvestmen with astonishing armored bodies and a prosoma (or cephalothorax) with a triangular shape resembling some sort of shield. It has a dark reddish brown color and two powerful hindlegs armed with several spines.

The Platine shield harvestman is found in several localities of Argentina and Uruguay, but especially in forested areas around the La Plata River and its tributaries. As usually among gonyleptid harvestmen, the Platine shield harvestman is dependent on environments with a considerable degree of humidity.

Different from most arachnids, harvestmen are usually omnivorous scavengers, feeding on dead animal and plant material, and the Platine shield harvestman is not different. In predator-prey relationships, they are usually the prey of other animals, especially spiders, such as wolf spiders that share the same habitat. When facing a large spider that is about to hunt it, the Platine shield harvestman can use a series of defense mechanisms. One of the simplest ways to avoid being eaten is remaining motionless or playing dead, a behavior called thanatosis. When facing an apparently dead harvestman, a wolf spider usually ignores it completely, as if it wasn’t even there. When this is not enough to stop the attack, the harvestman can use additional strategies, such as “showing its butt” to the spider by lifting its abdomen toward the predator and sometimes kicking the spider with its hind legs. Another common defense mechanism in harvestmen is releasing chemicals with a strong and repulsive scent but the Platine shield harvestman does not seem to use it often, at least not against spiders.

Little is known about the natural history of the Platine shield harvestman or of any of its close relatives. As I said several times before, we need more people studying the small creatures that live all around us.

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References:

Costa LE, Guerrero EL (2011) Geographical distribution of Discocyrtus prospicuus (Arachnida: Opiliones: Gonyleptidae): Is there a pattern? Zootaxa 2043: 1–24.

Fernandes NS, Stanley E, Costa FG, Toscano-Gadea CA, Willemart RH (2017) Chemical sex recognition in the harvestman Discocyrtus prospicuus (Arachnida: Opiliones). Acta Ethologica 20(3): 215–221. doi: 10.1007/s10211-017-0264-5

Segalerba A, Toscano-Gadea CA (2016) Description of the Defensive Behaviour of Four Neotropical Harvestmen (Laniatores: Gonyleptidae) Against a Synchronic and Sympatric Wolf Spider (Araneae: Lycosidae). Arachnology 17(1): 52–58. doi:10.13156/arac.2006.17.1.52 

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*Creative Commons License This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

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