A sawfly is an insect having the normal compliment of six legs and three body parts (head, thorax and abdomen) but it is also in the order Hymenoptera which means that is is closely related to wasps, bees and ants. However, it doesn't have a sting but the females have a structure at the end of their abdomens (at their tail) which they can extend and which has, along one side, teeth just like on a carpenter's saw. She uses her 'saw' to cut slots in plants, usually the stem, into which she lays her eggs. This is the only type of insect of which it can be said that it has "a saw on its bottom".
Sawflies are usually considered to be the most primitive of the insect order Hymenoptera, which includes the bees, wasps, ants and parasitic wasps. The sawflies form a complete suborder known as the Symphyta and are characterised by the absence of a 'wasp-waist' constriction between the thorax and the abdomen (in sawflies the thorax runs into the abdomen in almost a straight line). All sawflies have two-pairs of wings, as in most of the other Hymenoptera (apart from those that have no wings at all), but the venation of their wings is considered to be similar to the ancestral hymenoptera and is extremely variable with an individual sawfly often having differences in venation between the left and the right wings.
All adult sawflies, apart from those in the Cephidae family, have a pair of "cenchri" behind the scutellum on the thorax. These latch onto scaly areas on the underside of the forewings and help to hold the wings in place when the insect is not in flight (i.e. at rest). It is thought the the cephids (family Cephidae) form a link between the primitive Symphyta and the more advanced Apocrita (all of which lack cenchri). The cephids also have a very slight waisting between the thorax and the abdomen which suggests that they are the most advanced of the sawflies and lie between then and the more evolutionary advanced wasps of the Hymenoptera.
In most female sawflies the genitalia include the 'saw' from which the animals get their common/English name. The females use these saws to cut through plant tissues in order to lay their eggs/ova. However, in the Wood-wasps (Siricidae) the females must bore through the bark layer of woody plants (e.g. trees) in order to lay her eggs into the soft sapwood. As a consequence of this egg laying behaviour and due to bark being a very strong material the females have a needle-like ovipositor similar to that used by parasitic wasps in the family Orussidae.
The larvae of all sawflies (at least in Britain) feed on plant material - and the majority feed openly on leaves. Some may feed singly when they are usually cryptically coloured as a means of camouflage) whilst others are distinctly gregarious when they are often brightly coloured and have group movements (e.g. body lashing) as a means of deterring predators and parasites. There is also some evidence that the larvae may produce chemical defence secretions as handling large numbers of Diprion larvae can cause skin irritation.
Some sawfly larvae have developed more specialised methods of feeding such as leaf-mining, leaf rolling and even gall formation in their foodplants. Wood-wasp larvae live inside tree and bush sapwood, whilst several other sawfly families have larvae which live within the stems of plants such as grasses (including cultivated cereals) and non-woody herbs.
Most sawfly species are quite specific in terms of the host plant upon which they feed with very few being polyphagous. However, as a group sawflies feed on a large range of plant species ranging from primitive plants such as horsetails and ferns, through grasses and herbaceous plants to woody plants and trees such as conifers, fruit and other deciduous trees. However, the foodplants, habits and biology of several British sawflies are still unknown and much work remains to be done both on sawflies in general, but particularly with sawfly larvae.
The free living larvae of sawflies are very similar in appearance to the larvae (caterpillars) of the Lepidoptera (butterflies and moths) though they have only one pair of eyes rather than the several eye pairs found in lepidopterous larvae. Sawfly larvae have 6 or more pairs of abdominal pro-legs whilst lepidoptera never have more than 5 pairs. However, due to constraints brought on by specific habitat requirements those sawfly larvae which mine leaves or bore in stems and trunks may have the legs reduced in number and/or size and the larval morphology of these species may vary considerably from species to species.
Most of the adult sawflies, particularly the larger species, are carnivorous, eating flies etc. but many will also take nectar at flowers, especially at umbellifer flowers (e.g. Hogweed, Fennel and Angelica). The larvae, many of which resemble moth caterpillars, are herbivorous, feeding on leaves and other plant matter.