Bugs within bugs, part 1

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by Patrick
Publish date
28 April 2014
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To support our collection of almost 100 invertebrate species on display at Melbourne Museum, Live Exhibits staff must occasionally collect bugs from the wild, and some of these can hold unwanted surprises inside.

Chalcid w A Chalcid wasp, newly emerged from the cocoon of its host caterpillar.
Image: Patrick Honan
Source: Museum Victorai
 

At least one in every ten insect species is a parasite on other insects. In Australia this adds up to several tens of thousands of parasitic insect species, all living their lives on the outside or, more commonly the inside, of other insects. Most of these parasites are wasps and flies, and most of their hosts are butterflies, moths and beetles.

Tachinid fly eggs on beetle Tachinid fly eggs on the outside of a doomed Leaf Beetle larva (Paropsis species).
Image: Patrick Honan
Source: Museum Victoria
 

Parasitic insects usually lay their eggs directly onto or into the body of their host, but may lay eggs on a food plant in the hope that they will later be ingested by a potential host. Once inside, the parasitic larva consumes the host’s internal tissues while the host continues to go about its business. The parasite usually avoids the vital organs until the last minute – then polishes these off and finally kills its host.

Aphid ‘mummies’ Aphid ‘mummies’ – aphids that have been parasitised and glued to the leaf by Braconid wasps. When fully developed, the wasps will cut a perfectly circular hole in the back and emerge as adults.
Image: Patrick Honan
Source: Museum Victoria
 

Parasitic wasps are able to find their hosts no matter where they might hide. In order to lay eggs on its host – Sirex Wood Wasp (Sirex noctilio) – Megarhyssa (Megarhyssa nortoni) can drill through 9cm of solid wood with its ovipositor (egg-laying organ). Megarhyssa females find their hosts using infrared detectors on their antennae; the outer bark of a tree is 0.5oC warmer where a larva lies underneath.

female Megarhyssa A female Megarhyssa (Megarhyssa nortoni) drills through a pine tree to deposit an egg on the Sirex Wood Wasp within.
Image: Patrick Honan
Source: Museum Victoria
 

A parasitic wasp will sometimes inject venom that paralyses the host but keeps it alive, allowing the wasp to hide it away for its offspring as a sort of living larder. A large wasp dragging a larger, paralysed, huntsman across open ground is a familiar sight during summer.

Pompilid wasp drags a huntsman A Pompilid wasp drags a huntsman (Isopeda species) from its hiding place under bark, before paralysing it and inserting an egg.
Image: Patrick Honan
Source: Museum Victoria
 

A caterpillar can sometimes dislodge a parasite’s eggs with its mandibles, but wasps usually lay their eggs just behind the caterpillar’s head where they can’t be reached. Many hosts are able to ‘encapsulate’ a parasite already inside their bodies, localising its damage and starving it of oxygen. When more than one parasite is laid inside a host, the larvae fight to the death within the host’s body for the rights to its organs.

wasp cocoons inside the empty shell of a Cabbage White Butterfly caterpillar Thousands of parasitic wasp cocoons inside the empty shell of a Cabbage White Butterfly caterpillar (Pieris rapae). Each cocoon will soon produce a tiny wasp that flies off to find other caterpillars to parasitise.
Image: Patrick Honan
Source: Museum Victoria
 

Parasites are themselves subject to attack from hyperparasites, insects that lay eggs inside an already-occupied host. The emerging larva seeks out the parasite in residence and burrows into its body. These in turn may be parasitised by superhyperparasites, and so on.

“Great fleas have little fleas upon their backs to bite 'em,

And little fleas have lesser fleas, and so ad infinitum…”

Augustus de Morgan (after Jonathan Swift)

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Siobhan 28 April, 2014 16:44
Wow. Thanks Patrick, that last image of the Cabbage White caterpillar is going to stick with me for a while. Fascinating though!
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