Perga dorsalis Leach, 1817
Steel Blue Saw Fly

Don Herbison-Evans
Stella Crossley

(updated 7 November 2008)

(Photo: courtesy of Merlin Crossley)

These are not true Caterpillars, but are the larvae of a wasp. When young, these larvae are so gregarious they are often found in a knotted ball of many individuals. When disturbed, they all wriggle their tails and exude a nasty yellowish fluid from their mouths.

(Photo: courtesy of Sharon Adnum, Bowral)

This latter habit has given them the common name of “Spitfires”, although they do not actually spit the fluid, just dribble it. They use the tapping of their tails to communicate with each other.

They can be either all blue-black, or pied as in the pictures above. They are often found feeding on:

  • Gum Trees ( Eucalyptus, MYRTACEAE ).

Other Saw Fly species feed on other plants. Different species vary in size, some growing to a length of about 5 cms.

They pupate in a dark brown cocoon in the leaf litter, and the pupal duration can be two years.


An adult wasp (it is misnamed as a ‘fly’) of the genus Perga is big and dark brown or black, with a white spot on the thorax, and a wing span of about 4 cms. The female lays her eggs in a slit she cuts in a leaf of a food plant.

The larvae and adults of this family are quite harmless to people. They do not sting, as their cousins the communal wasps do. Various species of Saw Flies are found all over the world, for example :

Further reading :

Greg Pyers,
Insects, Reed International Books Australia, Port Melbourne, 1999, pp. 5,10-11.