The amazing Ichneumon wasps
January 18th 2008 05:16
Did you know that most wasps are parasites? More accurately, they’re parasitoids, or organisms whose larvae are parasites. Ichneumon wasps (belonging to the superfamily Ichneumonoidea) comprise well over 80,000 species, far outnumbering social wasps. Many use their ovipositor – a sting-like projection extending from the abdomen – to inject their larvae into other organisms. Others paralyse their victim and lay an egg on its abdomen, from which the larva hatches and then consumes the still living but helpless organism. Depending on the species of wasp, hosts can be many different types of invertebrates, including caterpillars, aphids, cockroaches, mature tarantulas, and butterfly larvae.
One of the most utterly amazing feats of animal design I have ever heard about is manifested through one of these wasps, called Ichneumon eumerus. The story begins with another insect, the Alcon Blue butterfly (Maculinea alcon), which lays its eggs on Gentian plants. When the larvae hatch, they drop down into the soil, where passing ants are “fooled” through means of an allomone released by the larvae (an allomone is a chemical produced by an organism to affect the behaviour of an individual of another species), which mimics those released by ant larvae. The M.alcon larvae even make sounds like those of the ants'. Taking the butterfly as one of their own, they carry it back to their nest, where it is fed and groomed. That’s interesting by itself, but here comes the wonderfully sinister side to the story. An I.eumerus wasp in the vicinity can detect the nests, among many others, that harbour the butterflies. Naturally, the ants are not want to accept an intruder into their domain, but the wasp has another ace up its sleave: it releases a chemical that causes the ants to turn on each other, giving it enough time to go off and look for the precious items. And yes, inject them with its future. The wasp then leaves, and things soon return to normal for the warring ants. They continue to feed and groom the M.alcon larvae, oblivious to the intrigue that is about to unfold. During chrysalis, the ants care for the butterfly as one of their own pupae (the next stage in the ant life cycle), scurrying around the casing, still intoxicated by the allomone. But, of course, it is not always an adult butterfly that emerges. In those unfortunate grubs that were injected by I.eumerus, a wasp emerges. And it goes on, of course, to do as its parent did. This wasp is a parasite of a parasite, for it exploits and consumes an organism that is itself an exploiter.
This story highlights two things. The first is that parasitism can be an excellent means of making a living for an animal; to be sure, it seems highly unglamorous to us, but that doesn’t enter into evolution’s accounting. These sinister strategies can become very successful, largely because nature is such a target rich environment for exploitation. It saves the expense of having to look after one’s young, for one thing, and in many cases, parasites are morphologically simplified, having lost many of their ancestral features, retaining only the bare necessities of reproduction and propagation (of course, this isn’t so much the case with parasitoids, as they need to fend for themselves once they are done with their hosts. You will have noticed that a wasp is a very complex piece of biological machinery). In fact, about a quarter of all animal species are reckoned to be parasites; clearly a good strategy for many.
Another lesson is that ant colonies – highly organised societies serving as excellent sources of food and shelter – are vulnerable to exploitation by a raft of organisms. Ants have evolved ways of countering this harassment and loafing, but they don’t always succeed. This has given rise to evolutionary arms races, in which both lineages evolve features to stay one step ahead of each other.
One of the most utterly amazing feats of animal design I have ever heard about is manifested through one of these wasps, called Ichneumon eumerus. The story begins with another insect, the Alcon Blue butterfly (Maculinea alcon), which lays its eggs on Gentian plants. When the larvae hatch, they drop down into the soil, where passing ants are “fooled” through means of an allomone released by the larvae (an allomone is a chemical produced by an organism to affect the behaviour of an individual of another species), which mimics those released by ant larvae. The M.alcon larvae even make sounds like those of the ants'. Taking the butterfly as one of their own, they carry it back to their nest, where it is fed and groomed. That’s interesting by itself, but here comes the wonderfully sinister side to the story. An I.eumerus wasp in the vicinity can detect the nests, among many others, that harbour the butterflies. Naturally, the ants are not want to accept an intruder into their domain, but the wasp has another ace up its sleave: it releases a chemical that causes the ants to turn on each other, giving it enough time to go off and look for the precious items. And yes, inject them with its future. The wasp then leaves, and things soon return to normal for the warring ants. They continue to feed and groom the M.alcon larvae, oblivious to the intrigue that is about to unfold. During chrysalis, the ants care for the butterfly as one of their own pupae (the next stage in the ant life cycle), scurrying around the casing, still intoxicated by the allomone. But, of course, it is not always an adult butterfly that emerges. In those unfortunate grubs that were injected by I.eumerus, a wasp emerges. And it goes on, of course, to do as its parent did. This wasp is a parasite of a parasite, for it exploits and consumes an organism that is itself an exploiter.
This story highlights two things. The first is that parasitism can be an excellent means of making a living for an animal; to be sure, it seems highly unglamorous to us, but that doesn’t enter into evolution’s accounting. These sinister strategies can become very successful, largely because nature is such a target rich environment for exploitation. It saves the expense of having to look after one’s young, for one thing, and in many cases, parasites are morphologically simplified, having lost many of their ancestral features, retaining only the bare necessities of reproduction and propagation (of course, this isn’t so much the case with parasitoids, as they need to fend for themselves once they are done with their hosts. You will have noticed that a wasp is a very complex piece of biological machinery). In fact, about a quarter of all animal species are reckoned to be parasites; clearly a good strategy for many.
Another lesson is that ant colonies – highly organised societies serving as excellent sources of food and shelter – are vulnerable to exploitation by a raft of organisms. Ants have evolved ways of countering this harassment and loafing, but they don’t always succeed. This has given rise to evolutionary arms races, in which both lineages evolve features to stay one step ahead of each other.
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