''Craesus septentrionalis'', the flat-legged tenthred or birch sawfly, is a

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

of insect in the order

Hymenoptera Hymenoptera is a large order of insects, comprising the sawflies, wasps, bees, and ants. Over 150,000 living species of Hymenoptera have been described, in addition to over 2,000 extinct ones. Many of the species are parasitic. Females typi ...

, the suborder

Symphyta Sawflies are wasp-like insects that are in the suborder Symphyta within the order Hymenoptera, alongside ants, bees, and wasps. The common name comes from the saw-like appearance of the ovipositor, which the females use to cut into the plants ...

and the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Tenthredinidae Tenthredinidae is the largest family of sawflies, with well over 7,500 species worldwide, divided into 430 genera. Larvae are herbivores and typically feed on the foliage of trees and shrubs, with occasional exceptions that are leaf miners, stem ...

. It was first described by

Carl Linnaeus Carl Linnaeus (23 May 1707 – 10 January 1778), also known after ennoblement in 1761 as Carl von Linné,#Blunt, Blunt (2004), p. 171. was a Swedish biologist and physician who formalised binomial nomenclature, the modern system of naming o ...

in 1758. The adult sawflies are black and brown with transparent wings and the

larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...

e are yellowish-green and resemble

caterpillar Caterpillars ( ) are the larval stage of members of the order Lepidoptera (the insect order comprising butterflies and moths). As with most common names, the application of the word is arbitrary, since the larvae of sawflies (suborder ...

s. The larvae feed on the leaves of various species of deciduous tree.

Description

The adult ''Craesus septentrionalis'' has a black head, thorax and anterior part of the abdomen while the posterior part of the abdomen is chestnut brown. The wings are clear. The legs are broader near the feet than they are close to the body. The larva has a black head, yellow collar and greyish-green slender body with longitudinal rows of black spots.

Ecology

This sawfly lays its eggs on a number of different host trees including

alder Alders are trees of the genus ''Alnus'' in the birch family Betulaceae. The genus includes about 35 species of monoecious trees and shrubs, a few reaching a large size, distributed throughout the north temperate zone with a few species ex ...

, ash,

birch A birch is a thin-leaved deciduous hardwood tree of the genus ''Betula'' (), in the family Betulaceae, which also includes alders, hazels, and hornbeams. It is closely related to the beech- oak family Fagaceae. The genus ''Betula'' contains 3 ...

, hazel,

hornbeam Hornbeams are hardwood trees in the plant genus ''Carpinus'' in the family Betulaceae. Its species occur across much of the temperateness, temperate regions of the Northern Hemisphere. Common names The common English name ''hornbeam'' derives ...

maple ''Acer'' is a genus of trees and shrubs commonly known as maples. The genus is placed in the soapberry family Sapindaceae.Stevens, P. F. (2001 onwards). Angiosperm Phylogeny Website. Version 9, June 2008 nd more or less continuously updated si ...

, poplar,

rowan The rowans ( or ) or mountain-ashes are shrubs or trees in the genus ''Sorbus'' of the rose family, Rosaceae. They are native throughout the cool temperate regions of the Northern Hemisphere, with the highest species diversity in the Himalaya ...

, wild service tree, whitebeam and

willow Willows, also called sallows and osiers, of the genus ''Salix'', comprise around 350 species (plus numerous hybrids) of typically deciduous trees and shrubs, found primarily on moist soils in cold and temperate regions. Most species are known ...

. The larvae stay together in groups and feed on the margins of leaves. When disturbed, they characteristically cling to the leaf with the front three pairs of legs and curl their bodies upward in a "S" shape. When the larvae are numerous, young trees may be completely defoliated. When mature, the larvae drop to the ground and

pupa A pupa (; : pupae) is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in their life cycle, the stages th ...

te in the soil. There may be several generations of larvae between spring and autumn and the insect overwinters underground in the pupal state. The integument of the larvae is easily damaged and oozes hemolymph when it is injured. It is hypothesized that this contains harmful substances, possibly derived from the plant on which the larvae are feeding. The hemolymph was found to repel '' Myrmica rubra'' ants, however many vertebrate and invertebrate predators feed on the larvae.

References

{{Taxonbar, from=Q14582530 Tenthredinidae Sawflies described in 1758 Taxa named by Carl Linnaeus