Sawflies are wasp-like insects that belong to the suborder Symphyta within the order Hymenoptera. Their common name comes from the saw-shaped ovipositor of the female, which she uses to cut into plant tissue when laying eggs. The largest and most diverse group of sawflies is the Tenthredinoidea, a superfamily that includes most of the approximately 8,000 described species in more than 800 genera worldwide.

Although Symphyta is traditionally ranked as a suborder, it is a paraphyletic grouping that represents several early lineages of Hymenoptera. The insects commonly called sawflies form a natural clade, however, this also includes the ancestors of the Apocrita—the ants, bees, and wasps. Adult sawflies are distinguished from the Apocrita by the absence of a narrow "wasp waist" or petiole between the thorax and abdomen. In sawflies, these segments are broadly joined, which gives the body a smooth profile.

Sawflies first appeared during the Triassic period, about 250 million years ago. The oldest known lineage, the Xyeloidea, is still represented by living species. Around 200 million years ago, some sawfly lineages evolved a parasitoid lifestyle, with larvae that prey on other insects’ eggs or young. Today, sawflies occur worldwide but they are most diverse in the temperate regions of the Northern Hemisphere.

Many sawflies are mimics of bees or wasps, and the female’s ovipositor is sometimes mistaken for a sting, though sawflies cannot sting. Adults range in size from about 2.5 to 20 millimetres (3⁄32 to 25⁄32 in), with the largest species reaching about 55 millimetres (2+1⁄4 in) in length. Most species are herbivorous, feeding on foliage or other plant parts, while members of the Orussoidea are parasitoids. Sawflies are preyed upon by birds, small mammals such as shrews, and predatory or parasitic insects including flies and other hymenopterans. The larvae of some species defend themselves by regurgitating irritating fluids or by clustering together for protection.

The larvae resemble caterpillars but can be distinguished by their greater number of prolegs and the absence of crochets on the feet. Sawflies undergo complete metamorphosis with four distinct stages, from egg, larva, pupa to adult. Adult sawflies live only about a week, but the larval stage may last from months to more than a year depending on species and environment. Many species reproduce through parthenogenesis, in which females produce fertile eggs without mating, though others reproduce sexually. Adults feed on pollen, nectar, sap, honeydew and sometimes on the hemolymph of other insects, using mouthparts adapted for these varied diets.

Females use their ovipositors to cut into plant tissue—or into the bodies of host insects in parasitoid species—and deposit eggs in clusters called rafts or pods. As the larvae mature, they seek protected sites such as soil or under bark to pupate.

Several species are significant pests. The pine sawfly can cause serious defoliation in forestry, while sawflies such as the iris sawfly damage ornamental plants. Outbreaks of larvae may lead to dieback or tree death. Control methods include the use of insecticides, natural predators and parasitoids, and manual removal of larvae.

Recent phylogenomic research has refined the evolutionary relationships among sawfly lineages, placing Xyeloidea as sister to all other Hymenoptera and revealing complex gene-tree discordance and introgression events. These studies highlight key evolutionary innovations such as the transition to parasitoidism and the development of the wasp waist, which underlie the diversification of the order.

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