The Harvestman with Horns
The ubiquitous species of harvestman Phalangium opilio was named by Carl Linnaeus in 1758. P. opilio is now distributed all over North America, where it has been introduced from Europe (thankfully it is not a pest!). The species range has also apparently spread into parts of Asia and Africa. This species thrives in a number of different habitats, including natural habitats, such as forests, as well as anthropogenic habitats/structures, such as under bridges, in gardens or green spaces.
Males of P. opilio have large horns on their chelicerae (pointing outward, away from the anterior end). These structures are used as weapons in male-male contests which most often results in the loser fleeing and the winner having a chance to mate with a nearby female.
Below are a few more photos from a field trip to Toft Point, WI during the 2012 meeting of the American Arachnological Society at UWGB.
Notes on image below: The cheliceral horns are the long pointed cones sticking off of the first pair of appendages (the chelicerae). The chelicerae bend downward, each terminating in a movable, pincer-like claw that is not visible here. Note that the male pedipalps are very long, unarmed, and appear as if they are an additional pair of legs (here the pedipalps are more gold in color, while the legs are black). This male is also missing the second walking leg on the near side of the body.
Image below: Same male, dors0lateral view.
Image below: Female from a dorsal view. The pedipalps are shorter and are held close to the body partially covering the smaller chelicerae of the female.
Harvestmen: The Biology of Opiliones. 2007. R. Pinto-da-Rocha, G. Machado & G. Giribet (Eds.). Harvard University Press.
Willemart RH, Farine J-P, Peretti AV, Gnaspini P. 2006. Behavioral roles of the sexually dimorphic structures in the male harvestman, Phalangium opilio (Opiliones, Phalangiidae). Can. J. Zool. 84: 1763-1774.