Caterpillars , the larvae of butterflies and moths, are often attacked by flies. The grubs (larvae) of some fly species in the family TACHINIDAE specialise in feeding inside caterpillars.

close-up showing white egg-shells
(Photo: courtesy of Steve Pearson, Airlie Beach, Queensland)

Some flies lay their eggs on the caterpillar, and when the egg hatches: the fly grub burrows its way into the caterpillar.

caterpillar of Hippotion celerio with discolorations where fly larvae are feeding under the skin
(Photo: courtesy of Steve Pearson, Airlie Beach, Queensland)

The fly grubs are careful to avoid feeding on vital organs, so that the caterpillar is not killed immediately, but can continue feeding and growing.

close-up of discoloration on the back where a fly grub is feeding under the skin
of a caterpillar of Hippotion celerio (Photo: courtesy of Steve Pearson, Airlie Beach, Queensland)

In due course, the fly grubs do kill their host caterpillar, when the caterpillar is mature or even after it pupates.

a fly grub that has just emerged from the pupa of Hippotion celerio
(Photo: courtesy of Peter Street, Niagara Park, New South Wales)

The fly grubs are parasitoids. Note that 'parasitoids' are parasites that actually kill their host, whereas 'parasites' just feed off it but leave it alive. In due course the fly grubs bore their way out of the caterpillar or its pupa, and pupate themselves.

pupa of dead Hippotion celerio with pupae and adults of fly grubs that have emnerged from it
(Photo: courtesy of Steve Pearson, Airlie Beach, Queensland)

The exact relationship of the various species of Lepidoptera caterpillars with the various parasitic species of flies is often very specialised, and is of great importance in pest control. If you should get some flies and/or wasps emerge from your caterpillar or pupa, you might consider donating them to your local natural history museum or university entomology department, who may have study programs on these parasites.

Clania ignobilis larval case opened to show pupa of parasitic fly (Tritaxys species)
(Photo: copyright of Brett and Marie Smith at Ellura Sanctuary, South Australia)

Flies from the genus Tritaxys in the family TACHINIDAE lay eggs on the caterpillar foodplant that are swallowed but not digested by the caterpillar. In due course: the eggs hatch inside the caterpillar, and eat it from inside out.

Tachinid fly recently emerged from its chrysalis
(Photo: courtesy of Steve Pearson, Airlie Beach, Queensland)

When a caterpillar is infected with a parasite, its behaviour changes, just as ours does when we get a cold or a fever. So though normal caterpillars will keep hidden, caterpillars when infected sometimes start wandering aimlessly about. So if you find a caterpillar wandering about, it is probably infected already.

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Australian Lepidoptera

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(written 4 October 2017)