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Wonderful Creatures: life is a gamble (inside a caterpillar) for the trigonalid wasp

Among the huge diversity of insects there are some bewilderingly complex life cycles, but few can compete with the trigonalid wasps for the seemingly haphazard way they ensure their genes are passed to the next generation.

In most cases, a female parasitoid wasp deposits her eggs on or in the host, but this is far too pedestrian and safe for the trigonalids (image 1). These mavericks of the wasp world, which are also parasitoids, like to make things more difficult for themselves. So, the female deposits her eggs on plant leaves. If this were the end of the story these wasps would not have lasted very long. No, the egg-in-the-leaf-trick is merely a ruse. The female trigonalid uses her short ovipositor and the unique structure of her abdomen to punch out a small piece of tissue from near the margin of a leaf, replacing it with a flattened egg. She deposits a few eggs in one leaf before moving onto another leaf perhaps on a nearby plant. This goes on until she gets eaten by a predator or exhausts her eggs, which in some species of trigonalid can be as many as 10,000—a huge number for a parasitoid, which underlines just how haphazard this strategy is.

Trigonalys species (female). A species from the Central African Republic. These wasps have quite a distinctive appearance. Note the unusual shape of the abdomen that enables the female to press out a small piece of leaf tissue. Photo courtesy of Simon van Noort/

The whole purpose of going through the laborious act of laying thousands of eggs in leaves is so her offspring can get swallowed by a caterpillar or the larva of a sawfly. If the very tough egg is lucky enough to get swallowed it hatches. This is thought to be triggered by the physical action of being chewed and/or salivary secretions.

For most parasitoids, getting into a plump caterpillar would be mission completed and cause for celebration, but no such luck for the tiny trigonalid larva (Image 2). The tiny larva winds up in the caterpillar’s gut, but wastes no time in breaking out of there to gain entry to the caterpillar’s haemocoel, the insect body cavity that is bathed in haemolymph, analogous to our -blood. The trigonalid makes its way through this liquid searching for its real quarry—another parasitoid that is already living in the caterpillar. Lots of parasitoids, such as other wasps and flies, are dependent on the larvae of Lepidoptera and sawflies and it is these the trigonalid larva is after. If it’s lucky it will find its prey, attack them and eat them, but in many cases the trigonalid that has so far defied the odds to get swallowed by a caterpillar will find nothing suitable to predate once inside. Either that or the only parasitoid within the caterpillar will be too large for the trigonalid to tackle. In both cases the poor little trigonalid is doomed. However, some species of trigonalid are able to sit tight inside the caterpillar until it gets parasitized by an ichneumon or a tachinid fly.

But this isn’t the only bizarre life-cycle of the trigonalids. Another strategy used by some species also hinges heavily on coincidence, but the supporting cast is slightly different. These species still depend on a caterpillar or sawfly larva, but this time the caterpillar must be captured by a vespid or eumenid wasp, butchered and fed to one of the wasp’s grubs back at the nest. In the flesh of the dead caterpillar are the eggs of the trigonalid and once swallowed they hatch to feed on the unfortunate host larva.

In the world of wasps, trigonalids are something of an enigma. Only around 100 species are known, the adults don’t live very long and their precarious way of life means they are rather rare, so we don’t know a great deal about them. Their geographic distribution, appearance and lifestyle suggest they are very ancient and some entomologists have theorized they may be something of a missing link between the sawflies and bees, ants and wasps. Indeed, the oldest trigonalids are known from 100 million year old lumps of Cretaceous amber, which shows this way of life, precarious as it may be, has been going on for some time.

Like all insect larvae, immature trigonalids are unsightly to say the least. On the left we have a third instar larva of Bareogonalos jezoensis from Japan. This instar has huge mandibles for making short work of the prey. On the right is the final instar of the same species. The big mandibles have gone and instead we have a chubby beast with a tiny head. This species is a parasitoid of vespid wasps. Illustrations by: Seike Yamane, 1973.

Dr. Ross Piper is a zoologist and author and has recently presented on the BBC/Smithsonian TV production, Wild Burma: Nature’s Lost Kingdom, soon to be shown in the USA. You can read an interview with Ross Piper here: Animal Earth: exploring the hidden biodiversity of our planet.

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