Hand Lens - October 23, 2020

Issue №9: Low Harvests

by W.L.

Leaves russet and fall
A harvest beneath the fog
Rustled by long legs

Sitting on the other side of the arachnid family tree from spiders, opiliones--commonly known as “harvestmen” or “daddy long-legs”--are the gasoline car to the spider’s electric. The two orders bend the malleable arachnid bodyplan into the eight-long-legged, compact-bodied form factor we are accustomed to seeing scuttle along the floor, with four smaller limbs at the mouth (the chelicerae and the pedipalps) moving with disconcerting grace on whatever has proven unfortunate enough to fall within their range. The diagnostic distinction between the two orders--that what in harvestmen is simply another segment of the body is, in spiders, a narrow waist-like “pedicel” dividing the body into cephalothorax and abdomen--seems functionally almost trivial. Under the hood, however, the two organisms are vastly different.

Most of the meaningful distinctions can only be made clear with a microscope--the vast differences between their respiratory, muscular, and reproductive systems--or a carefully-planned experiment--the two eyes of a harvestman appear to roughly correspond to the pair of eyes most spiders have that communicates only regions of light and dark, while their other eyes resolve more detailed images. Even differences that to us would appear so obvious as to be definitional were difficult to establish--scientific papers as late as 1938 (T.H. Savory’s “Notes on the Biology of Harvestmen&rsdquo;) erroneously claimed to have found silk produced by harvestmen. Yet anyone who has been lucky enough to see a harvestman with their prey will immediately grasp how different their digestive systems are: The venomless harvestmen have no patience for the spider‘s drawn-out process of external digestion, and instead simply rip apart and chew down their food, be it plant matter, live small insects, or in this case, an already-dead (and lightly-smushed) wolf spider.

Unfortunately, the images that I took do not offer enough detail to resolve the tips of the chelicerae--the larger pair of limbs by the opilione‘s mouth. In spiders, these limbs terminate in fangs, but harvestmen instead have miniscule pincers that allow them to break apart their food before their pedipalps--the smaller pair--maneuver it into their mouths.

Accordingly, harvestmen are at substantially more risk of, effectively, contracting food poisoning--this literature review by Cokendolpher provides the long list of parasites and microorganisms researchers have collected from harvestman intestines.

“The frequent grooming of the legs by the harvestmen may also lead to the ingestion of oocysts and spores.”

It is impossible to observe harvestmen for any prolonged period of time without noticing the peculiar relationship they have with their own legs. The easy majority of adult harvestmen lack at least one of their long limbs, and seem largely capable of living their lives unimpaired on only five--or occasionally four--of their original eight. The joint connecting each leg to the body is notable weak to tension stress, able to detach if the limb has been caught by one of the many, many species too large to fear its chelicerae. Indeed, if the limb has been injured but has not fully detached, a harvestman may simply pull off the leg itself. Yet, despite this willingness to part with them, opiliones frequently clean their legs by drawing them through their mouths, a movement not unlike that of a bow being drawn across a violin.

In addition to clearing off the parasites that accumulate on their legs, this cleaning ensures that the second pair of harvestman-legs--substantially longer even than the other pairs--is free of contaminants as it senses out the world around the harvestman. A harvestman in motion will often have its second pair of legs moving independently from the other six; gently, quickly tapping the ground with its flexible limb-ends, the two legs orthogonal to each other, they sweep back and forth--a motion which often persists, albeit at an almost imperceptible pace, when the harvestman is otherwise still.

Another species of New England harvestman, with notably shorter legs--in more tropical climates, harvestmen display even more variation. In fact, the largest sub-order of opiliones, Laniatores, live primarily in the tropics and are characterized by short limbs, additional pincers, and the generous application of spines.

While precisely what counts as the first spider is a contested issue, the currently-known fossil record accounts opiliones as tens of millions of years older than anything we would recognize as a spider--and the oldest known arachnid of any type is only ten million years older than the oldest known harvestmen. Found in Scotland’s Rhynie Chert, a Devonian strata approximately 410 million years old, the “Rhynie Chert Harvestmen” were buried and silica-preserved by geysers and hotsprings, along with the contemporary plants and arthropods which they would have fed upon--including springtails--which were among the first sequence of complex lifeforms to evolve exclusively-terrestrial lifestyles.

Rhynie Chert Harvestmen from Dunlop et. al., “A harvestman from the Early Devonian Rhynie cherts, Aberdeenshire, Scotland.”

Some of the Rhynie Chert Harvestmen exhibit another first--the first organ in a terrestrial lifeform that could creditably be called a “penis.” To this day, reproducing with a penis separates opiliones from the other arachnids, which instead transfer sperm in packets. It is, to me, a curious coincidence that this is the arachnid that our language is sure, in both of its common names, must be a man. Curiouser still, it is one of the rare arachnids--and indeed, rare arthropods--to exhibit forms of paternal-care behavior. While the etymology of the names is somewhat unclear--one account of the origin of the term “harvestman” holds it to be related to an old shepherd’s trick of watching you flock from atop stilts, but I personally have my doubts--and would appear that the implied masculinity of this little arachnid is no more than an artifact of how the English language genders occupations (and, perhaps, that the word “daddy” is fun to say, daddy-o). No--deliberate implications of masculinity, you need only read articles in the popular press, and even from some scientists. As so often happens when some aspect of an animal’s biology corresponds, however roughly, with that of human beings, even articles in National Geographic cannot help but engage in facile anthropomorphism, imagining male opiliones to have the same anxieties and ecstasies that we culturally attribute to the experience of being a man. Never mind that, were we to observe a similar organ in a fellow mammal, we would scarcely be able to discuss it outside of how physiognomically alien it is to its human analog, only trivially similar in form! One could have a field day in any sociology of gender class with these articles--but beyond that, it seems a true failure of empathy. Empathy begins with recognizing what is shared, but must account for what is not; the vast differences between human and opiliones. What do we understand of the harvestman’s experience, of a life spent wandering among the dead leaves? What does it mean to an opiliones to clean the eggs of his offspring? Is it something like contentment that drives that behavior, something like fear, like joy, like unfeeling instinct? Does it compare at all to the sensation of cleaning your own leg with your toothless mouth, your two long, sensitive probing legs? More observation is needed.