The strange relationship between Gossamer Butterflies and Ants

On this week's episode we are diving into the topic of "Myrmecopholi" in Butterflies. Specifically talking about the Lycaenid butterflies and how they manage to have ants tend to their every need. 

 

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Transcript: 

 

Welcome back to episode 35 of insects for fun! I’m your host Mitchel Logan and today we will be looking at a very interesting relationship between two unlikely species. 

 

In the insect world, there is a family of butterflies known as Lycaenidae which has around 6,000 species. It’s actually the second largest family next to Nymphalidae but the butterflies themselves are not so large. Commonly referred to as Gossamer wing butterflies, Lycaenids are very small with a wingspan often under 5cm. I guess gossamer in this case would mean delicate or dainty because of how small the butterflies are. 

 

These butterflies can be found across the globe and are divided into varying subfamilies that depend on where you are. In the United States for example we often see blues, coppers, and hairstreak lycaenids, but in more tropical places there is a greater variety. These common names refer to the appearance of their wings and I’m sure I don’t have to explain the difference between coppers and blues. Hairstreaks though get the name from the thin lines that are streaked across the underside of their wings. But I didn’t write up this episode to talk about all the small details regarding subfamilies because there is something far more interesting happening with around 70% of all the butterflies in this family, regardless of what their wings look like, and that would be their relationship with ants. 

 

Ants are usually predators when it comes to caterpillars and as we’ve learned in previous episodes of this podcast they can be quite formidable, especially against other insects, but the caterpillars of Lycaenid butterflies have evolved many intricate biological adaptations to bend ants wills in their favor. For example, many caterpillar for this family of butterflies have special pores on their skin called pore cupolas which are used to attract ants and tell them that the caterpillar is not prey but actually an ally. Others have nectar glands which ants can milk for honeydew. This is literally honey trapping for the gain of protective services by ants. Ants are extremely protective of their territories and resources, so much so that there are many species of ants in the rainforest that literally make a forest clearing by killing and removing all plants that could jeopardize the growth of special trees they rely on. Having these kinds of bodyguards is extremely handy when you’re a small vulnerable caterpillar. 

 

This ant-tending relationship between caterpillars and the ants is called myrmecophilous and translates to ant loving, and there are a few different ways in which these kinds of relations can occur. 

 

The first of these is known as facultative mutualism and this simply means the caterpillars don’t need the ants to survive, but they do make use of them while they’re around and form a beneficial alliance. 

 

The second is obligate mutualism which means the caterpillars require the help of ants in order to survive and make it to the butterfly stage. These kinds of interactions are not always mutualistic though. In fact there are quite a few species of lycaenid butterflies whose caterpillars act as a parasite and live off the ants' resources with no benefit to the ants at all and this is done in a number of ways. The one common factor between all these is that the caterpillar always tricks the ants into thinking it's one of them or even brain washing them with laced honeydew made from their bodies.  

 

Let’s first look at one of Great Britain's most rare butterfly species known as a Large Blue. Large blue butterflies actually did go extinct on the island and had to be reintroduced from mainland Europe. Thankfully this reintroduction has been successful and the populations are once again stable! Anyway, this butterfly’s caterpillars use both scents and sounds to trick ants into thinking that they are one of them. They don’t do this right away though! Typically speaking caterpillars within the large blue genus start out by feeding on their natural host plants until they reach their 4th instar or molt. At this point the caterpillar drops to the ground and emits a pheromone that tells ants of a specific species that it is one of them. The ants then carry this caterpillar into the nest where 1 of 2 things happens. Either the caterpillar gets fed by the ants as if it were a baby queen or the caterpillar takes a hard 180 and begins feeding on ant larvae. Survival rate is much higher as you’d expect in the species which feign being a larval queen and get fed by the ants as opposed to the caterpillars which eat the nest. 

 

The caterpillars which feign being a larval Queen are referred to as cuckoo caterpillars and they produce the sounds a genuine queen ant larvae would make which causes the ants to put the caterpillar in high priority and abandon genuine ants within the colony. However this tactic really is only useful for specific species. Let’s say for example one of these caterpillars gets taken in by a slightly different species. The chances of being predated by those ants highly increases simply because the mimicry isn’t perfect. Predatory caterpillars on the other hand are more versatile but have lower odds of survival when compared with a cuckoo species that made it into the correct nest. This is simply because predatory behavior is a lot more invasive in negative ways than pretending to be a Queen. What I find even more incredible about both of these caterpillars though is that even the pupae have glands which continue to tell the ant that it is one of them. Only once the butterfly ecloses does it need to hurry out of the nest because at that point the butterfly is out of the bag in this case. 

 

Now let's go to Australia and look at one of their rarest butterflies known as the Bathurst Copper or Purple Copper butterfly. This one, unlike the large blue, doesn't use the ants as a food source but actually has the ants protect them in exchange for special honeydew which the caterpillar excretes from a gland on its back. The caterpillar feeds on a host plant known as blackthorn and can only be found in a limited range at altitudes greater than 850m. The caterpillars are also rarely seen because the ants actually carry them into their nests during the day to protect them from predators and then at night the ants carry them out and on to their host plants where the caterpillar will resume feeding. Once the caterpillar is ready to pupate it once again returns to the ants nest where it stays dormant until it's ready to eclose months later.