Smelly legs: Arachnids with unusually glands on their limbs
"There is not a single paper describing either the morphology of the sexually dimorphic glands in these two species or how a sexually dimorphic gland is used in any species, in a suborder with over 4100 species.”
This quote, from a recent paper by Fernandes & Willemart, is referring to unusual glands on the legs of two species or neotropical harvestmen (Arachnida: Opiliones). This in itself is remarkable. It’s like taking ALL the mammal species in the world, looking at a couple of species of cetaceans, and wondering about the function and anatomy of baleens in those species. The Opiliones (or harvestmen, or daddy-long-legs, or grandfather greybeards) are a diverse yet poorly known Order of Arachnids. There’s much to gain by learning some fundamental natural history of the Opiliones (this was certainly one of the take-home message of the “Opliones Project”).
Fernandes & Willemart knew about sexually dimorphic glands on the legs of two species of laniatorids (a suborder of Opiliones) but knew nothing of their function. They looked closely at males of Gryne perlata and Gryne coccinelloides in part because "There is not a single paper describing either the morphology of the sexually dimorphic glands in these two species”. Fernandes & Willemart collected individuals, brought them to a laboratory for behavioural tests, and studied the external anatomy.
The authors found that although glands are present in both males and females (on some specific parts of only some of the legs, and not always on females), there is significant sexual dimorphism. For example, on the metatarsus IV, the males have extreme swelling of the areas with the glands, relative to the females (see figure, below). In behavioural tests, essential the harvestmen ‘rubbed’ their legs (in the specific regions with the glands) all over the place - on other parts of their bodies, sometimes on the bodies of the opposite sex, and sometimes on different substrates and they sometimes waved their legs in the air. From this, Fernandes & Willemart conclude that these behaviours were likely related to spreading pheromones (this behaviour is common across the animal kingdom). Rubbing different body parts may be allowing the chemical signature to be spread over the entire organism, and the authors suggest ‘leg waving’ may help release the volatiles into the environment. Given the sexual dimorphic nature of the glands, it’s likely these behaviours are related to mating or mate attraction, or perhaps to define a territory. However, although the world is now aware of the external morphology of these glands in two species, and some of the basic behaviours associated with these glands, the function remains largely unknown.
But the imagination does kick in: Hey baby, check out my leg glands… can you smell them?
You could argue that this work is just too preliminary, perhaps with limited sample size, or just too narrow to be generalizable, and despite the work, are we much further ahead? Despite such shortcomings, I think it’s still vital that this basic biology and natural history be investigated, even if the scope is limited. It leads to new questions, new avenues of study, and is a stepping stone for further study. It’s captured the curiosity, and further exposes the incredible lack of knowledge of one of the most common groups of Arachnids on the planet. We need this kind of research, and I applaud Fernandes & Willemart for tackling this question and starting the process of understanding weird-smelly-leg-glands on harvestmen. We should strive to learn about these funny leg-glands to the same degree that baleens have been studied and understood.
Let me finish with one other curious observation by Fernandes & Willemart: "At night they were seen leaving the termite nests. Nothing is known about the natural history of either of the species" Harvestmen and termites? What on earth could be the association between those two taxa? Look everyone, another research question!
Fernandes, N.S., R.H. Willemart, Neotropical harvestmen (Arachnida, Opiliones) use sexually dimorphic glands to spread chemicals in the environment, C. R. Biologies (2014), http://dx.doi.org/10.1016/j.crvi.2014.01.004