Tuesday, 27 June 2006

Ant-ics

Many species of blue butterflies (Lycaenidae) have something going with ants. The adults are free-flying but caterpillars live in ant nests. Mostly the relationship is mutualistic—ants shield caterpillars from predators and parasites, caterpillars provide ants with a sugar secretion. (Protection honey?) And sometimes it all goes horribly wrong for one partner. The ungrateful moth butterfly (Liphyra brassolis) dines on its benefactors.

Ant-friendly caterpillars release a pheromone from tiny glands called pore cupola organs. The scent encourages ants to tolerate the caterpillar. (It has been described as an appeasement or adoption signal.) Instead of feeding on the intruder, their attention is rewarded with food. Each caterpillar has a dorsal nectary organ that exudes a liquid rich in sugars and amino acids when prompted. It’s a fair exchange.

Caterpillars of the ciliate blue butterfly (Anthene emolus globosus) live with green tree ants (Oecophylla smaragdina). These ants occur throughout SE Asia and tropical Australia, where they build nests from leaves sewn together with silk produced by their own larvae. They are aggressive, active and willing to bite. If sinking the jaws in isn’t enough to deter an enemy, they spray formic acid as well.

Anthene caterpillars are flat and slug-like. They produces ant-nosh at a high rate200 to 300 droplets an hour. They also drum to attract ants, just in case the food and pheromone aren’t enough.

Ants keep the big predators away—only the brave or foolhardy will take them on—but they aren’t always so effective at deterring the smaller ones. Braconid wasps are parasitoids. (Parasitoids have life cycles similar to those of parasites but there’s always an unhappy ending for the host.) Adults lay eggs on or in caterpillars and other insects. The wasp larva hatch and develops inside the caterpillar, eating its way through the internal organs as it grows. Eventually, the larva pupates and metamorphoses into an adult wasp. Depending on the species of parasitoid, the caterpillar may be dead before the wasp pupates. Or not. (The life cycle of Dan O’Bannon’s Alien was based loosely on that of parasitoid wasps. Real parasitoids are much worse on their own miniature scale.)

When the braconid Apanteles emerges from Anthene, it spins a cocoon beneath the caterpillar. The host remains partly functional for a long while after. The DNOs secrete sugary fluid for up to three days but the caterpillar corpse remains attractive to ants for another one or two days. Drumming continues unabated by a lack of insides. (It eventually stops after three to five days.)

The braconids dine on their hosts. At the same time the ants protect them from hyperparasitoids while they are pupating. (Hyperparasitoids specialise in giving parasitoids a taste of their own medicine.) The caterpillars finally cease the ant-attracting activity a day or two before the wasps emerge from their cocoons. Apanteles is then free to go about its business—looking for hosts for the next generation.

It’s a jungle out there.


Read more
Fiedler, K., Seufert, P., Pierce, N.E., Pearson, J.G. & Baumgarten, H-T. (1992). Exploitation of lycaenid-ant mutualisms by braconid parasitoids. Journal of Research on the Lepidoptera 31(3-4): 153–168.

Saarinen, E.V. (2006). Differences in worker caste behaviour of Oecophylla smaragdina (Hymenoptera: Formicidae) in response to larvae of Anthene emolus (Lepidoptera: Lycaenidae). Biological Journal of the Linnean Society 88: 391–395.

No comments: