Sawflies are a group of Hymenoptera, one of the holometabolous insect orders. Holometabolous insects undergo complete metamorphosis and have four life stages: egg, larva, pupa, and adult. Female sawflies typically lay eggs on or inside plant tissue using a long, sclerotized ovipositor. The name “sawfly” comes from this distinct ovipositor, which looks and functions similarly to a saw blade. One family, the Orussidae, are parasitic on wood-boring beetles and woodwasps, and lay their eggs on or near their prey.
Examples of the four life stages of a diprionid sawfly:
Examples of an ovipositor “saw”, the egg-laying apparatus of the female sawfly:
The active immature stage in sawflies is the larval stage. Larvae are morphologically distinct from adults. In sawflies, the larvae are generally caterpillar-like with visible sclerotized heads, sclerotized legs attached to the thorax, and fleshy prolegs on their abdomen. The larvae of leaf-mining and wood- or stem-boring taxa, as well as the Pamphiliidae, frequently have reduced or absent legs and prolegs. Like all insects, transitions between life stages occur during a molt. This includes multiple larval stages, or instars, with between five and nine instars across the group.
The pupal stage occurs during metamorphosis from larva to adult. Pupae are largely immobile, and look somewhat like a mummified version of the adult. Legs, wings, and other adult body parts are visible, but are pressed closely to the body and mostly immoveable.
Adult sawflies are generally winged, and most species have two sexes. Body form varies widely, from stout, hairy, and bee-like, to long, slender, and wasp-like. Many species have aposematic coloring and look like their venomous cousins, wasps. While some sawflies can deliver a jarring bite, no species of sawfly can sting.
Sawflies do most of their feeding during their larval stage. Nearly all sawflies feed on plants, in almost every fashion imaginable. Most are external leaf feeders, but many are internal wood borers, stem borers, gall formers, leaf rollers, or leaf miners. Many taxa are characterized by a consistent feeding behavior within their group. For example, most Heterarthrinae in North America are leaf miners, while Siricidae are all wood borers. Wood-boring larvae (Siricidae, Xiphydriidae, Anayelidae) are associated with fungal symbionts, which are usually transmitted by ovipositing sawflies. The fungi attack the host plants, and facilitate wood digestion by sawfly larvae or become food themselves. Many sawfly genera are specialists or narrowly oligophagous, feeding only on a single or limited number of plant species. Several genera have diversified in association with a single group of plants, such as Neodiprion on Pinaceae. Other complex groups of closely related genera have diversified across certain plant genera. In North America this is exemplified by the diversity of Nematus, Pontania, and Eurra species associated with willows.
Sawflies in the subfamily Tenthredininae have been observed feeding on other insects as adults (see page for Rhogogaster and Tenthredo). The only other regularly predaceous sawflies are in the Orussidae, of which some species are parasitoids of wood-boring beetle larvae (see page for Orussus).
Though no sawflies are social in the sense of eusociality exhibited by many bees and wasps, sawfly larvae will often feed in aggregations, presumably for defensive purposes. Larval defensive behavior also includes coordinated movements, such as the jerking behavior exhibited by Neodiprion sertifer larvae and the toxic compound excretion exhibited by some Arge species.