The Long Kiss Goodnight: Following an Invasive Ant-mimicking Spider from Texas to California

By: Zach Phillips, Ph.D., Terrestrial Invertebrate Conservation Ecologist

In the past few decades, the spider Falconina gracilis has become established in California (Valle et al. 2013). Although Falconina is a widespread invasive predator, we don’t know much about its biology or ecological impact. What little we do know includes the following: it comes from South America, mimics and eats ants, and seems to have adapted well to human environments (Fowler 1981; Valle et al. 2013). In other words, the spider is a bit of a hanger-on in two different kinds of societies: those of ants and people (Photo 1).

1: A Falconina gracilis spider literally hangs on to an ant between its jaws.

Photo: Alex Wild

Falconina and I share this in common – I’m also a bit of a parasite on ants (and on people, if you want to be mean about it). During graduate school in Austin, Texas, I studied Texas leaf-cutter ants (Atta texana), collecting them for research, shamelessly observing their private lives, and being a general pest. This led to my first encounter with Falconina on a leafcutter nest mound (Photo 2), where the spider was pretending to be just one of the ants.

2: The author annoys a Texas leaf-cutter ant (Atta texana) colony at Brackenridge Field Laboratory in Austin, Texas.

Photo: Larry Gilbert, Ph.D.

In stillness, Falconina doesn’t look much like an ant. In motion, however, it is a consummate artiste. The spider moves its front legs like antennae (ants have antennae, spiders don’t) and lifts and “bobbles” its abdomen like many ants do, and it can adopt the general saunter of an ant. I was bewitched, and over the next few years I discovered a few things about Falconina’s basic biology (Phillips 2021), including details of how it eats (Kiss of Death) and what it eats (Queen Killer), findings that can inform predictions of Falconina’s impact here in California (Falconina in California).

Kiss of Death

During the first pandemic lockdown, I brought my research home with me (i.e., I covered the apartment with spiders). I had time — too much time — to study Falconina’s predation in detail, and I discovered that Falconina eats ants in an exceptional way. Typically, the spider follows an ant from behind, bites the ant’s posterior, and then retreats until its venom takes effect. So far, nothing special. What happens next is the strange part. Falconina maneuvers the incapacitated ant into a mouth-to-mouth position and feeds on the contents of the ant’s head.

3, 4, 5: The spider’s kiss of death: Falconina gracilis maneuvers a Texas leaf-cutter ant (Atta texana) forager into a mouth-to-mouth position and proceeds to eat the contents of its head

Photo: Alex Wild

Photo: Alex Wild

Photo: Alex Wild

Ant heads are full of high-value nutrients but well protected by a thick cuticle. As a consequence, they can be difficult for predators to breach. The mouth, however, is a weak spot. Falconina may have evolved its “kiss of death” as a way to gain easy entry to ant heads and the riches within.

Queen Killer

When a predator eats a few ant foragers, it doesn't do much harm to the colony that they belong to; the effect on the colony is like the effect a mosquito bite has on you or me. On the other hand, if a predator kills the colony’s queen, it destroys the colony.

My fieldwork in Texas suggests Falconina is a queen killer. Not a killer of just any queens, but of new queens starting new colonies with no nestmates to protect them. At my research sites, I found Falconina occupying the new nests of carpenter ant (Camponotus sansabeanus) queens. Occasionally, I would find a queen’s carcass, still intact but dried out, just outside her nest entrance. Other times I found the decapitated heads of queens, along with other insect body parts, attached to nearby Falconina egg sacs (this gory egg sac ornamentation may be a form of Falconina maternal care, deterring potential egg sac predators and parasitoids) (Photo 6).

6: Insect body parts, including the head of a carpenter ant (top left), covering a Falconina egg sac.

Photo: Zach Phillips, Ph.D.

Furthermore, in feeding trials conducted in the lab (i.e., my apartment), Falconina quickly incapacitated and ate carpenter ant queens (Photo 7). Collectively, these findings indicate Falconina can acquire big meals and valuable shelters from new queens; however, it’s unclear if the spiders specifically target new queens or attack them opportunistically.

7: Falconina gracilis captures and eats a carpenter ant (Camponotus sansabeanus) queen during feeding trials.

Photo: Zach Phillips, Ph.D.

Falconina in California

Falconina remains relatively unknown and understudied in California. For instance, we don’t really know what Falconina eats here, essential information for predicting an invasive predator’s ecological impact.

If Falconina kills ant queens in California, it may indirectly give invasive Argentine ants (Linepithema humile), arguably our most damaging invasive insect, an even greater competitive advantage over native ants. Unlike new queens of many native ant species, new queens of Argentine ants are always protected by nestmates — workers accompany them when they leave their parental nest in a process called “colony budding” (analogous to vegetative reproduction in plants). Thus, new Argentine ant queens are likely far less vulnerable to Falconina predation than new queens that must fend for themselves. If this is the case, Falconina might disproportionately kill California’s native ant queens, such as those of Harvester ants (Pogonomyrmex spp.), further enabling the spread and dominance of Argentine ants.

Another important bit of information we don’t know is the extent of Falconina’s invasive range. Observations on iNaturalist.org suggest Santa Barbara County or Ventura County marks Falconina’s northern range limit on the west coast (https://www.inaturalist. org/observations?taxon_id=70360). The spider is nocturnal and cryptic, so these few observations could represent just the tip of the Falconina iceberg (apologies for the image, arachnophobes and icebergophobes). If community naturalists like yourselves continue to post Falconina observations on iNaturalist.org, it can help resolve where the spider lives and breeds and how common it is in our area. Eventually, this data could be used to track Falconina’s possible northward range expansion.

‘‘Falconina gracilis, there’s no need to kill us’’

Ants are brimming with poetry. They can’t stop reciting the stuff. It’s unnatural.

Late one night in Austin, I overheard a colony – one especially prone to lyrical outbursts – begging Falconina to leave it alone. Begging in rhyme, of course. I doubt the spiders heeded the message:

In morbid jest

We call you “Success”

Because you go straight to our heads

In all seriousness

You’re really a pest

Have you tried eating crickets instead?

People say they’re good,

A sustainable food!

A growing part of humanity’s diet

And if you could

We know that you would

Ape the bipeds – don’t you deny it

But you’re stuck

Mimicking us,

An homage we could never disdain

We make a fuss

Only because

You won’t stop eating our brains

Sincerely, Ants

Works Cited: Fowler, H. G. 1981. Behavior of two myrmecophiles of paraguayan leaf-cutting ants. Rev. Chilena Ent. 11:69–72. Phillips, Z.P. 2021. Dispersal of Attaphila fungicola, a symbiotic cockroach of leaf-cutter ants. University of Texas at Austin. Valle, S. J., C. B. Keiser, L. S. Vincent, and R. S. Vetter. 2013. A South American spider, Falconina gracilis (Keyserling 1891) (Araneae: Corinnidae), newly established in Southern California. The Pan-Pacific Entomologist 89:259–263. Contact If you think you’ve seen Falconina in the area, please contact the author. If you are an ant brimming with poetry, please do not contact the author. Acknowledgements Thank you Alex Wild for permission to include your photographs.