This is how Solifugid Sun Spiders mate. The male puts the female into a trance by massaging her with his pedipalps, then flips over her abdomen to insert a spermatophore (or droplet of sperm). The female gonopore is located just below the Prosoma on the upper part of the Abdomen (underside) See: http://www.solpugid.com/Courtship%20and%... and http://lanwebs.lander.edu/faculty/rsfox/... for Solifugid anatomy. The male is on the left with his head under her body and not visible. The female is on the right, her head is visible and her abdomen is full of eggs, which can be seen through the integument. They are both about 2 cm in length, although the female is more robust. Family Ammotrechidae. The last 3 pictures are of the same species from the same place taken in October 2016 to show the entire body. The underside is visible in the last picture. Notice the prominent Racket Organs or Malleoli on the femora of the last pair of legs, used for sensory detection. The female gonopore is under 2 genital opercula just below the 4th pair of legs.
I found them hanging upside-down from the outside overhanging eve of the tool house. I have seen them both frequently in this area and it is probably their preferred territory. Semi-rural residential area, San Cristobal de Las Casas, Chiapas, Mexico, 2,200 meters.
This species is described from San Cristobal de Las Casas, with the type specimens in the California Academy of Sciences in San Francisco. The pale white ring at the base of the last segment of the pedipalp (visible on the female) is characteristic of this species. See http://www.solpugid.com/Ammotrecha%20chi.... This is supposed to be a rare species known only from Museum specimens, but I see them frequently and definitely more are on the way! See also: https://www.desertusa.com/insects/solpug.... http://www.projectnoah.org/spottings/107... http://www.projectnoah.org/spottings/185...