Gala Solari Keller sent me this picture of two harvestmen head to head, asking if this was predatory or mating. A "female harvestman" is a bit of an oxymoron, as would be calling her "daddy longlegs" or "granddaddy longlegs." For that reason I will refer to them by their order name, Opiliones. Not only are there female "daddies," some species' females can reproduce by parthenogenesis without male fertilization. Take that, you sexist naturalists!
|Opilione - note combined cephalothorax and abdomen into a single body segment.- Wikimedia|
|Brown recluse body-|
Separate cephalothorax and abdomen
There is a common misconception, perpetuated in some web sites, that Opiliones can bite humans or that their venom is more toxic than that of a spider. They actually don't have venom glands or even suck their food as liquids like a spider does. They actually take in small pieces of insects, plant material, fungus, or even dung. These are stalkers, not trappers- they don't make webs and don't even have silk glands.
As slow moving creatures they are vulnerable but have several interesting defenses. They have scent glands which disuade predators such as ants and even birds. They may bob up and down when disturbed. They also will sacrifice a leg (after all they have 8), a practice called autotomy. Because there is a separate pacemaker in the femur, the leg can twitch for a prolonged period, much like the tail of a skink, distracting the predator while the victim sneaks away.
A predator is not the only cause of the loss of legs. This video shows the perils of sex in Opiliones. Males may fight over the female's charms and use their jaws to amputate a rivals legs. Notice at each attempted mating the pair are face to face.
|Mating - note the red mites, a common problem among Opiloines|
Now back to the original question - dinner or sex? It turns out that Opiliones mate face to face as seen in this video and lots of pictures including Gala's. The male has a penis and mates by direct copulation. This research paper has more details on the anatomy describing how this occurs and may be more than you want to know. The important thing is that they know it.
David Darling's Encyclopedia of Science.