Ant-worshiping spider cult?
Or, just a case of predator and pray?
Araneae: Myrmecicultoridae
Myrmecicultor chihauhuensis
An ant-worshiping cult of spiders in the desert? Sounds like a reason to stay home, lock the door, and turn out the lights; or, maybe the plot of an apocalyptic science fiction movie. Embellished with a measure of dramatic literary license, my title is instead a nod to an unusual new spider species found living among ants at localities in Texas and Mexico — and its scientific name. Guests in ant nests are nothing new, nor are associations of spiders with ants. There are many species of insects and other arthropods found living among ants, within their colonies. Some are mere interlopers, although casually entering a nest is risky business. Species that are adapted to live in the nests include many obligate symbionts that benefit the ants in some way, such as cleaning up the garbage, as well as benefitting from life in the colony. Among such uninvited guests are species of mites, bristletails, springtails, more than a dozen families of beetles, flies and cockroaches, among others. In all, it has been estimated that there are perhaps 10,000 such mymecophilic species of arthropods.
Figure: Several Myrmecicultor chihuahuensis with their ant host Pogonomyrmex rugosus, close to the entrance to its nest near Cuatro Cienegas, Mexico. Photo: courtesy of Dr. David Lightfoot.
What inquilines eat varies. Some feed on dead ants or detritus in the nest. Some feed on the ants’ provisions, such as fungus gardens. Others feast on ant eggs, larvae or pupae, or, less often, adults. Many inquilines remain to be discovered. The rate of discovery of inquilines is not as fast as it could be. But, digging up ant nests, that can go deep into the ground, while being swarmed by thousands of angry ants, is an acquired taste, and among the small number who engage in the sport, many are more focused on mapping the architecture of the nest than specifically collecting inquilines.
Species that become obligate ant nest inhabitants generally live under reduced selection pressure for certain traits, like pigmentation or visual acuity — it is, after all, dark in the depths of a subterranean ant nest — and increased selection pressure for ways to avoid attack from ants aggressively guarding their colony. Resulting adaptations may involve secretions that chemically signal to ants that they belong in the nest and tactile behaviors that reassure the same message.
The combination of losing some morphological features, and evolving other peculiar ones, often make obligate inquilines look very different from their free-living relatives. One example is the South American spider genus Attacobius that can be observed clinging to, and riding upon, host ants on the march. Their morphology was so confusing that species were assigned to no fewer than five different genera and several families.
Both ants and spiders are common in many habitats, so it is not surprising that they interact with one another. Associations of spiders with ants generally fall into three categories. The first is mymecophily, literally “ant-loving,” such as the new inquiline species we are focused on. The others are myrmecophagy, that is, eating ants. And finally, myrmecomorphy, that is, looking like ants. The more elaborate cases of such mimicry are surprisingly convincing with elongated narrow and constricted bodies visually screaming ant, not spider. We will eventually meet one of these weirdos.
The new species was described from the Chihauhaun desert, in the Big Bend region of Texas and near Cuatro Cienegas in Mexico. They were first found in pitfall traps, cups dug into the soil leaving their opening flush with the surrounding ground so that arthropods running along fall in and can’t climb back out. It is now known that the spider is associated with three species of harvester ants: Pogonomyrmex rugosus, Novomessor albisetosis, and Novomessor cockerelli.
When the little spiders, significantly smaller than their ant hosts, were first noticed about fifteen years before their description, they stumped those attempting to identify them. Turns out, for good cause. Not only are these spiders a new species, they represent a new genus and a new family as well, bringing the total number of spider families to one hundred and twenty.
The new species was named Myrmecicultor chihuahuensis. The genus name is derived from the Greek myrmex for “ant” and the Latin cultor for “worshiper.” The specific epithet, of course, refers to the desert in which they live. These spiders share an unusual mixture of characters which, in combination, don’t clearly align with any other spider family. An analysis of both morphological and DNA data still leaves the nearest relative of the new genus uncertain, only adding to the mystery of this little spider. One interesting character is the tapeta of the posterior, median eyes. This membrane of reflective cells is oriented at ninety degrees, a peculiar character shared by gnaphosid spiders, and their relatives. This peculiar attribute has been shown to function as a sky compass in gnaphosids, and is suggestive of one possible position of the new family.
It is evident that this new spider is tolerated by its ant hosts. Given the spider’s small size, they appear not to feed on ants. Feeding on larvae is possible, but considered unlikely. It is speculated that they may feed on other ant nest symbionts, such as collembola, that are more clearly in an appropriate prey size range. Data suggests that the spiders come to the surface following rainfall, and most of the specimens so far collected have been found after and close to a rain event. It isn’t known whether the spiders make their way to surface on their own, or are carried by worker ants. Winged, reproductive ants appear at the surface during the same times as the spiders.
Acknowledgment
Thanks to Dr. David Lightfoot, University of New Mexico, for permission to use his photograph of the new species.
Further Reading
Cushing, Paula E. (2012) Spider-ant associations: An updated review of mymecomorphy, myrmecophily, and myrmecophagy in spiders. Psyche, 22 April. https://doi.org/10.1155/2012/151989
Parker, Joseph (2016) Myrmecophily in beetles (Coleoptera): evolutionary patterns and biological mechanisms. Myrmecological News 22: 65-108.
Ramirez, Martin J., Grismado, Cristian J., Ubick, Darrell, Ovtsharenko, Vladimir, Cushing, Paula E., Platnick, Norman I., Wheeler, Ward C., Prendini, Lorenzo, Crowley, Louise M., and Norman V. Horner (2019). Myrmecicultoridae, a new family of myrmecophilic spiders from the Chihuahuan desert (Araneae: Entelegynae). American Museum Novitates, no. 3930, 24 pp.