The Secret Life of Ants and Butterflies

Hey ya'll 

Not all relationships are brutal in the minuscule world of invertebrates, and sometimes, when it suits the needs of both parties, relationships can be mutually beneficial as well. Once such relationship is the textbook example of the caterpillars of the lycaenid butterfly and ants. As many a lepidopterist - or indeed anyone whose seen Sir David Attenborough's Life in the Undergrowth - might tell you, lycaenid butterflies are unique in that many species rely on the care of ants in order to mature as fully grown butterflies. Indeed, the species I am currently observing seem to be reliant on a particular species of weaver ant. 

From my observations I have documented that the caterpillars are herded by the ants during the day, much like cattle or other such life-stock, from branch to branch that they may feed off of the leaves. As you might imagine, as soon as one leaf seems to be damaged extensively by the voracious larvae, the ants would then move it to another leaf, and so forth in a similar fashion. 

weaver ant grazing lycaenid butterfly larvae

As the caterpillars seem to lack much means of movement, it is often up to the ants to carry them, when such a move is necessary. Indeed, the fat, green larvae often seem content to do nothing more than wave their heads left to right, quite content to chew whatever it is the ants direct their bodies towards. This seems to take much effort out of the poor ant and as the caterpillar grows into its final instars, it sometimes takes up to two ants to lift one such caterpillar (kind of  life a farmer attempting to graze a particularly lazy cow). But the ants don't seem to mind this, however, as the caterpillar rewards the ants by secreting a deliciously sweet fluid that is relished by the ant's own larvae. Because of this, the ants guard the caterpillars jealously. 

Usually, only one or two ants tend to each caterpillar at a time. One to stand directly above it (to protect it from things like wasps) and another to stand slightly in front. For a long time I was quite puzzled as to the purpose of the front most ant-sentry but I soon discovered what it was for. As I waved my fingers menacingly towards the caterpillar, the ant in front turned around, made a dash to the other ant and communicated with it in some way. It then pivoted back around to face my fingers and snap its jaws menacingly. If I moved my finger to the left, it would follow, and so would it, if I moved my finger to the right. For all intents and purposes the ant would not let me get past it without a fight. All the while I was doing this, the other ant was scampering furiously up the branch and back to the safety of its nest, with the caterpillar clutched firmly within its jaws. I also noticed belatedly that the other ants nearby were doing the same. 

A lycaenid butterfly pupa. I only managed to obtain it with 
much difficulty, and not without suffering from a few bites. 
Sorry ants! I swear I mean it no harm! 

When the time comes for the caterpillars to finally pupate, though, one might think that the mutual relationship between the two insects would end. The chrysalis, of course, cannot secrete anymore sweet fluids for the ants to feed on and would therefore be of no purpose to the overall welfare of the colony. But it is during this, the most vulnerable time of the butterfly's development, that the ants seem to redouble their efforts in living up to their end of this symbiotic bargain. More ants emerge from the nest to guard the chrysalis as the butterfly starts to develop within. Indeed in some instances I have observed as much as five to six ants guarding one particular chrysalis at a time, almost twice as many as the number usually found guarding the larvae. Because the chrysalids are tethered to the surfaces of leaves, the ants can no longer retreat with their precious quarry into the relative safety of their nests. As such, they will stand guard over them day and night. Occasionally, other ants will come to replace them but always there will be at least five or six of them per chrysalis. They even stand by the chrysalis and call for reinforcements when threatened as demonstrated when I waved my own finger menacingly at them once again to see what they might do.

An adult lycaenid butterfly

Eventually, the moment passes when the butterfly has to emerge. It is around this time that the ants begin to relax their vigil. As the chrysalis darkens, signalling the emergence of the adult butterfly, so to does the number of ants decrease. One by one they leave until eventually all that is left is a single bluish chrysalis on top of a leaf. A few minutes later and split will appear in the front portion of the chrysalis, a sign that the butterfly is about to emerge. I am still unsure if the coincidence between the timing of the ant's departure and the butterfly's eclosure is significant of anything but I might hypothesize that it has something to do with the difference in the sort of pheromones the butterfly would release, designed perhaps to protect it from the very ants that raised it, which might no longer recognize the butterfly as the same insect.  Conversely, this behavior might be something on the part of the ants themselves, evolved to protect the butterfly from the off chance that an ant attacks while it is spreading its wings. This ensures, of course, that the butterfly would survive thus leaving more eggs (and therefore more livestock) for the ants to tend to the following season.

(it should be of note that I am yet to find any eggs of the butterfly on the tree so it is really anyone's guess if the butterflies lay their eggs on the tree  which are then harvested by the ants, or if the ants bring the caterpillars to the higher parts of the tree, where the fresher, more tender leaves are, when they find them. Some lycaenids live part of their lives independently before finally moving in with the ants so this might be the case also. It would also be interesting to note that the caterpillars do not seem to thrive without the care of the ants. I once experimentally removed one caterpillar from a leaf in hopes that I might raise it myself to no avail. The caterpillar grew lethargic after 2 days and would not eat any amount of leaf I offered it. I suspect the ants play a crucial role in cleaning it which perhaps prevents infection or fungal attacks.  I promptly returned the caterpillar to its caretakers later where it eventually recovered)

Cheers,
Cyren