Spitfires revolt

This post was inspired by audience reactions to an image I saw on Instagram. The image showed a cluster of sawfly larvae sometimes known as ‘spitfires’.

Many people are afraid of insects because of their “creepy crawly” habits but few insects disgust entomophobes more than the sawfly larvae known as ‘spitfires’. This is due to their habit of rearing up en masse and vomiting thick yellow fluids (hence the name ‘spitfire’) when disturbed. This is a defence mechanism against animals which might try to eat them. What a predator (such as a bird or possum) sees is a revolting Medusa-like writhing vomiting mass. That’s enough to put any predator – or Instagram viewer – off their lunch!

The most commonly seen ‘spitfires’ in urban areas are the larvae of the steelblue sawfly Perga affinis, and the black sawfly Perga dorsalis. All told there are about 20 species of Perga, as well as several species of the less common Pergagrapta spp. and Pseudoperga spp. sawflies, whose larvae display similar defence mechanisms.

Sawflies are not true flies (i.e. members of the insect order Diptera) but are insects related to wasps and bees (Order Hymenoptera). The name ‘sawfly’ comes from the delicate serrated, saw-like ovipositor (egg-laying appendage) at the tip of the abdomens of female sawflies. They use this ‘saw’ to cut slits in leaves or stems of suitable plants into which they lay their eggs. The image below shows a female Pergagrapta polita laying her eggs.

Studies have shown that spitfires possess specialized mouthparts which separate toxic oils from food plant leaves and pass them into a special internal pouch known as a diverticulum.  The liquid is known to contain essential oils such as monoterpenes and, in some species, concentrations of sideroxylonal. Sideroxylonal in eucalypt foliage significantly influences the amount of leaves that some possums ingest – the higher the concentration of sideroxylonal the less likely the possum is to eat those leaves. It makes sense then that a possum may be deterred from a meal of juicy grubs if those grubs rear up and regurgitate a high concentration of a chemical the possum dislikes.

Now it gets interesting. Spitfires regurgitate these stored oils just before feeding or during feeding as well as when provoked by predators. The oils are toxic to the sawfly larvae themselves, so it makes sense that the diverticulum would have to be emptied when full. Is the purpose of the diverticulum a defence from toxins or defence from predators? Who knows?

Young Pseudoperga spp. sawfly larvae are defended by the mother sawfly (image below) for as long as it takes the larvae to build up defensive oil reserves. Studies have shown that it takes several days to fill the diverticulum with oil. A fair bit of oil it is too – studies carried out back in the 1970s showed that the oil accounted for nearly 20% of body weight in fully grown larvae.

Sawfly larvae also use stored oil when they spin their cocoons and pupate in the soil. By incorporating the oil (and faeces) into the silken wall of the cocoon they chemically shield their pupae.

Sawfly larvae are sometimes responsible for defoliating native trees, especially Eucalyptus spp. and related Myrtaceae. Depending on the species, larvae can grow to about 60 mm in length and are nocturnal feeders. Larvae are present over spring and summer and their feeding usually starts with the youngest leaves at branch tips. They can completely strip small branches of their leaves. Trees usually recover.

Larvae rest in groups during the day as a defence, and also climb down from trees in large groups to pupate in the soil, sometimes migrating along fences and across pathways in the process. Spitfires communicate with each other by using a kind of Morse code. When they spread out at night to feed they tap on the branches with their abdomens to let the rest of the group know where they are, so they can re-form their protective cluster before daylight.

I reckon they are pretty cool. What do you think?