The Imperial Blue butterfly
|Jalmenus evagoras is an Australian lycaenid butterfly, which like many lycaenids has an apparently mutualistic relationship with ants. In the case of J. evagoras, these are ants in the genus Iridomyrmex. My doctorate work, in the laboratory of Prof. Naomi Pierce, investigated the costs and benefits to both butterflies and ants of the interaction. |
Eggs of Jalmenus evagoras are laid on various species of Acacia , and the caterpillars feed on the leaves of these bushes when they hatch. The caterpillars go through five larval instars.
The first instar larvae donot possess any of the ant-associated organs typical of the lycaenids (see below) except for the pore-cupolae organs, and are largely ignored by the associated ants. From the second instar onwards, the caterpillars have the full set of ant-associated organs, and are actively tended by ants.
The picture to the right shows a fifth instar caterpillar of Jalmenus evagoras in the field, covered by about 15 workers of Iridomyrmex anceps. These ants are constantly with the caterpillar, and are very active (as you can see from the fact that many of them are blurred in my photograph). They are a very effective deterrent to arthropod predators (particularly other ants and wasps) and parasitoids (wasps and flies that lay their eggs in or on the caterpillars). They are also pretty good at deterring biologists!
If the tending ants are removed in the field, then the caterpillars are typically wiped out by predators and parasitoids within a few weeks, so it is clear that the caterpillars gain a lot of protection from the ants.
The caterpillars also pupate on the Acacia plant, and the pupae continue to be tended by ants.
What do the ants gain from the caterpillars in return?
If you strip the ants away from a fifth instar larva of Jalmenus evagoras, it looks like this:
The caterpillar possesses at least three sets of secretory organs that are used in its interaction with ants.
The first (1 above) is the Dorsal Nectary Organ (also called the "Newcomer's organ" or "Honey gland"). This produces droplets of a sweet secretion upon which the ants feed. The secretion also contains quite high concentrations of amino acids, the chemical constituents of proteins, which are hard for ants to obtain elsewhere.
The second (2) are a pair of organs known as the Tentacular Organs. These are rather like inside-out rubber gloves, and are normally kept inside the caterpillar - however, under some circumstances they are everted and produce volatile substances which affect ant behaviour - causing the ants to become excited, and sometimes to start biting indiscriminately. How they do this are still not known for certain, but they may produce substances that smell like the alarm pheromones produced by the ants.
The third (3) are small button-like organs called Pore Cupolae that are distributed all over the upper surface of the caterpillar, particularly around the Dorsal Nectary Organ and on the dorsal spines. In Jalmenus evagoras these are known to produce a secretion containing amino acids that is collected and eaten by the ants.
Hence it seems that what the ants get from the caterpillars in return for their protection is food - both sugars and amino acids - so that the relationship between the ants and the butterfly is a mutualism - one in which both partners benefit. This contrasts with the Alcon blue butterfly (Phengaris alcon), which is a parasite of ant colonies.