Part IV: Chemical Defenses of Harvestmen
Longlegs fact #4: Harvestmen have repugnatorial glands that produce compounds used in chemical defense.
These repugnatorial glands are also known as defensive glands, scent glands, stink glands or odoriferous glands. The repugnatorial glands are a major synapomorphic character of Opiliones. This means that the glands are a derived character, shared among all Opiliones (and their most recent common ancestor), but not among other arachnid groups (though other arachnids may have different chemical defense mechanisms). The glands produce chemical compounds that are meant to deter predators. The chemical compounds produced are very diverse but many are forms of quinones and phenols. The openings of the glands are on the body near the second pair of legs. The harvestmen usually release this secretion when threatened or disturbed.
The chemical compounds produced by some species can actually be detected by our own senses. When I collect harvestmen by hand I will sometimes smell them just out of curiosity (despite the crazy looks I get). The chemical compounds produced by some species are surprisingly potent! If I were a natural predator of harvestmen I would think twice about consuming them after getting a whiff of this. And no, I have not tried tasting them! Yet.
A species of Cosmetidae using its chemical defenses. The yellow droplet seen on the legs was first produced from the glands on the body and then transferred to the legs. A clear droplet can be seen on the body between legs I and II (on the right side).
Eupoecilaema magnum (Cosmetidae), one of the largest cosmetids, from La Selva Biological Station, Costa Rica.
Another species of the family Cosmetidae. There are over 700 species just in this one family!
Although there will be many more interesting stories and facts that I will share about harvestmen in future posts, this post will conclude my series on the introduction to the biology of harvestmen.
Part III: How many species of daddy longlegs?
Longlegs fact #3:
With more than 6,500 species, Opiliones represent the third largest order of Arachnida. The largest arachnid order is mites/ticks (order Acari) with more than 50,000 described species, followed by spiders (order Araneae) with more than 40,000 species. For a better frame of reference consider this: there are approximately 5,700 species of mammals in the world while there are more than 350,000 species of beetles in the world.
Almost every time I travel to a new place in Central America to collect harvestmen, new species are collected. So there’s plenty more work to be done in understanding and describing the diversity of this group.
Below are more photos of harvestmen from Costa Rica.
Meterginus inermipes (Cosmetidae) from Costa Rica
Cynortellana oculata (Cosmetidae) from Costa Rica
Unknown species, likely a new species, of Cosmetidae
Prionostemma sp. (Sclerosomatidae) from Costa Rica
The truth about daddy longlegs
The arachnid order Opiliones has several common names in English including daddy longlegs, grand daddy longlegs and harvestmen. I have many wonderful things to share with you regarding the natural history of Opiliones so I’m going to post it in a series of about five posts. Here is the first fun fact to whet your appetite. Keep an eye out for the others very soon!
Longlegs fact #1: Daddy longlegs are NOT spiders.
They are also not insects. So what are they? Well, I’m glad you asked. Daddy longlegs are arachnids belonging to the order Opiliones. The subclass Arachnida includes spiders, scorpions, mites/ticks, daddy longlegs, and several smaller groups, all of which belong to different orders. So daddy longlegs are distantly related to spiders, but are actually more closely related to scorpions!
These incredible arachnids are easily overlooked, as they are most active at night and many species are very secretive in their behavior. Here’s a few examples of species from Costa Rica:
Prionostemma sp. feasting upon what remains of an insect
Cynorta marginalis eating an earthworm as a second individual sneaks up.
Poecilaemula signata (male) with enlarged chelicerae
The orange harvestman, an undescribed species
For more arachnid photos see my Arachnid gallery.
danielproud 