Sawfly


Sawflies are wasp-like insects that are in the suborder Symphyta within the order Hymenoptera. The common name comes from the saw-like appearance of the ovipositor, which females use to cut into the plants where they lay their eggs. The name is associated especially with the Tenthredinoidea, by far the largest superfamily in the suborder, with about 7,000 known species in the entire suborder. There are 8,000 described species in more than 800 genera.
Despite their given taxonomic rank of suborder, Symphyta is a paraphyletic group, consisting of several basal groups within the order Hymenoptera, with each one rooted inside the previous group; the various superfamilies of insects called "sawfly" do form a natural group or clade, but this clade also includes the Apocrita – the ants, bees, and wasps – which are not considered sawflies. The primary distinction between sawflies and the Apocrita is that the adult sawflies lack a "wasp waist" or petiole; the segments connecting the abdomen and the thorax smoothly transition between the two. Sawflies first appeared 250 million years ago in the Triassic. The oldest superfamily, the Xyeloidea, is still extant today. Over 200 million years ago, a lineage of sawflies evolved a parasitoid lifestyle, with carnivorous larvae that ate the eggs or larvae of other insects. Sawflies are distributed globally, though they are more diverse in the northernmost hemispheres.
Some sawflies are Batesian mimics of wasps and bees, and the ovipositor can be mistaken for a stinger. Sawflies vary in length, most measuring ; the largest known sawfly measured. The great majority of sawflies are herbivorous, though the members of the superfamily Orussoidea are parasitoids. Their predators include birds, small mammals such as shrews, and other insects, such as Dipterans and other Hymenopterans, which themselves may be parasitoids of sawflies. The larvae of some species have anti-predator adaptations such as regurgitating irritating liquid and clustering together for safety in numbers.
The herbivorous larvae are often confused with caterpillars, but can be distinguished from these by the number of prolegs and the absence of crochets. Sawflies go through a complete metamorphosis with four distinct life stages – egg, larva, pupa and adult. Adult sawflies are short-lived, with a life expectancy of 7–9 days, though the larval stage can last from months to years, depending on the species. Parthenogenetic females, which do not need to mate to produce fertilised eggs, are common in the suborder, though many species have males and practice sexual reproduction. The adults feed on pollen, nectar, honeydew, sap, other insects, including hemolymph of the larvae hosts; they have mouth pieces adapted to these types of feeding. The female uses her ovipositor to drill into plant material and then lays eggs in groups called rafts or pods. As the larvae approach adulthood, they seek a protected spot to pupate, typically in bark or the soil.
Large populations of species such as the pine sawfly can cause substantial damage to economic forestry, while others such as the iris sawfly are major pests in horticulture. Outbreaks of sawfly larvae can defoliate trees and may cause dieback, stunting or death. Sawflies can be controlled through the use of insecticides, natural predators and parasitoids, or mechanical methods.

Etymology

The suborder name "Symphyta" derives from the Greek word, meaning 'grown together', referring to the group's distinctive lack of a wasp waist between prostomium and peristomium. Its common name, "sawfly", derives from the saw-like ovipositor that is used for egg-laying, in which a female makes a slit in either a stem or plant leaf to deposit the eggs. The first known use of this name was in 1773. Sawflies are also known as "wood-wasps".

Phylogeny

In his original description of Hymenoptera in 1863, German zoologist Carl Gerstaecker divided them into three groups, Hymenoptera aculeata, Hymenoptera apocrita and Hymenoptera phytophaga. However, four years later in 1867, he described just two groups, H. apocrita syn. genuina and H. symphyta syn. phytophaga. Consequently, the name Symphyta is given to Gerstaecker as the zoological authority. In his description, Gerstaecker distinguished the two groups by the transfer of the first abdominal segment to the thorax in the Apocrita, compared to the Symphyta. Consequently, there are only eight dorsal half segments in the Apocrita, against nine in the Symphyta. The larvae are distinguished in a similar way.
The Symphyta have therefore traditionally been considered, alongside the Apocrita, to form one of two suborders of Hymenoptera. Symphyta are the more primitive group, with comparatively complete venation, larvae that are largely phytophagous, and without a "wasp-waist", a symplesiomorphic feature. Together, the Symphyta make up less than 10% of hymenopteran species. While the terms sawfly and Symphyta have been used synonymously, the Symphyta have also been divided into three groups, true sawflies, woodwasps or xylophaga, and Orussidae. The three groupings have been distinguished by the true sawflies' ventral serrated or saw-like ovipositor for sawing holes in vegetation to deposit eggs, while the woodwasp ovipositor penetrates wood and the Orussidae behave as external parasitoids of wood-boring beetles. The woodwasps themselves are a paraphyletic ancestral grade. Despite these limitations, the terms have utility and are common in the literature.
While most hymenopteran superfamilies are monophyletic, as is Hymenoptera, the Symphyta has long been seen to be paraphyletic. Cladistic methods and molecular phylogenetics are improving the understanding of relationships between the superfamilies, resulting in revisions at the level of superfamily and family. The Symphyta are the most primitive taxa within the Hymenoptera, and one of the taxa within the Symphyta gave rise to the monophyletic suborder Apocrita. In cladistic analyses the Orussoidea are consistently the sister group to the Apocrita.
The oldest unambiguous sawfly fossils date back to the Middle or Late Triassic. These fossils, from the family Xyelidae, are the oldest of all Hymenoptera. One fossil, Archexyela ipswichensis from Queensland is between 205.6 and 221.5million years of age, making it among the oldest of all sawfly fossils. More Xyelid fossils have been discovered from the Middle Jurassic and the Cretaceous, but the family was less diverse then than during the Mesozoic and Tertiary. The subfamily Xyelinae were plentiful during these time periods, in which Tertiary faunas were dominated by the tribe Xyelini; these are indicative of a humid and warm climate.
The cladogram is based on Schulmeister 2003.

Taxonomy

There are approximately 8,000 species of sawfly in more than 800 genera, although new species continue to be discovered. However, earlier studies indicated that 10,000 species grouped into about 1,000 genera were known. Early phylogenies such as that of Alexandr Rasnitsyn, based on morphology and behaviour, identified nine clades which did not reflect the historical superfamilies. Such classifications were replaced by those using molecular methods, starting with Dowton and Austin. As of 2013, the Symphyta are treated as nine superfamilies and 25 families. Most sawflies belong to the Tenthredinoidea superfamily, with about 7,000 species worldwide. Tenthredinoidea has six families, of which Tenthredinidae is by far the largest with some 5,500 species.
Extinct taxa are indicated by a dagger.

Superfamilies and families

Many species of sawfly have retained their ancestral attributes throughout time, specifically their plant-eating habits, wing veins and the unmodified abdomen, where the first two segments appear like the succeeding segments. The absence of the narrow wasp waist distinguishes sawflies from other members of hymenoptera, although some are Batesian mimics with similar to wasps and bees, and the ovipositor can be mistaken for a stinger. Most sawflies are stubby and soft-bodied, and fly weakly. Sawflies vary in length: Urocerus gigas, which can be mistaken as a wasp due to its black-and-yellow striped body, can grow up to in length, but among the largest sawflies ever discovered was Hoplitolyda duolunica from the Mesozoic, with a body length of and a wingspan of. The smaller species only reach lengths of.
Heads of sawflies vary in size, shape and sturdiness, as well as the positions of the eyes and antennae. They are characterised in four head types: open head, maxapontal head, closed head and genapontal head. The open head is simplistic, whereas all the other heads are derived. The head is also hypognathous, meaning that the lower mouthparts are directed downwards. When in use, the mouthparts may be directed forwards, but this is only caused when the sawfly swings its entire head forward in a pendulum motion. Unlike most primitive insects, the sutures and sclerites are obsolescent or absent. The clypeus is not divided into a pre- and postclypeus, but rather separated from the front. The antennal sclerites are fused with the surrounding head capsule, but these are sometimes separated by a suture. The number of segments in the antennae vary from six in the Accorduleceridae to 30 or more in the Pamphiliidae. The compound eyes are large with a number of facets, and there are three ocelli between the dorsal portions of the compound eyes. The tentorium comprises the whole inner skeleton of the head.
Three segments make up the thorax: the mesothorax, metathorax and prothorax, as well as the exoskeletal plates that connect with these segments. The legs have spurs on their fourth segments, the tibiae. Sawflies have two pairs of translucent wings. The fore and hind wings are locked together with hooks. Parallel development in sawfly wings is most frequent in the anal veins. In all sawflies, 2A and 3A tend to fuse with the first anal vein. This occurs in several families including Argidae, Diprionidae and Cimbicidae.
File:Abia sericea 2.jpg|thumb|right|Aposematically coloured caterpillar-like larva of Abia sericea
The larvae of sawflies are easily mistaken for lepidopteran larvae. However, several morphological differences can distinguish the two: while both larvae share three pairs of thoracic legs and an apical pair of abdominal prolegs, lepidopteran caterpillars have four pairs of prolegs on abdominal segments 3–6 while sawfly larvae have five pairs of prolegs located on abdominal segments 2–6; crochets are present on lepidopteran larvae, whereas on sawfly larvae they are not; the prolegs of both larvae gradually disappear by the time they burrow into the ground, therefore making it difficult to distinguish the two; and sawfly larvae only have a single pair of minute eyes, whereas lepidopteran larvae have four to six eyes on each side of the head. Sawfly larvae behave like lepidopteran larvae, walking about and eating foliage. Some groups have larvae that are eyeless and almost legless; these larvae make tunnels in plant tissues including wood. Many species of sawfly larvae are strikingly coloured, exhibiting colour combinations such as black and white while others are black and yellow. This is a warning colouration because some larvae can secrete irritating fluids from glands located on their undersides.