Orussidae
The Orussidae or the parasitic wood wasps represent a small family of sawflies ("Symphyta"). Currently, about 85 extant and four fossil species are known.[2] They take a key position in phylogenetic analyses of Hymenoptera, because they form the sister taxon of the megadiverse apocritan wasps, and the common ancestor of Orussidae + Apocrita evolved parasitism for the first time in course of the evolution of the Hymenoptera. They are also the only sawflies with carnivorous larvae.
The fully winged wasps are 2−23 mm long. They are predominantly black but species of Chalinus, Mocsarya and Orussobaius are more or less metallic. Some species have a red thorax or abdomen and conspicuous white or golden pilosity. Many Orussus species bear white spots on the legs. The antennae of males are composed of 11, those of females of 10 articles. The modified distal antennal articles of females (article 9 enlarged, article 10 very small) are involved in vibrational sounding to detect suitable oviposition sites and the host larvae living concealed inside wood.[3] Contrary to other "Symphyta", the antennae insert near the lower edge of the compound eyes and close to the mandible. The mandibles are orthognathous and lack evident teeth. The number of palpomeres of the maxilla and the labium varies and is used as a taxonomic character. On the wings, some cross-veins are reduced in comparison with the more complete venation of other basal Hymenoptera. Similar as in most other sawflies, the wings are held at rest with a device called "cenchri". The complete body is strongly sclerotized and bears a species-specific microstructure, which is relevant for species identification. The ovipositor is several times as long as the body, and at rest it extends inside the body from the abdomen to the prothorax, where it is coiled, and back towards the tip of the abdomen again.
Orussidae and Stephanidae are the sole Hymenoptera, in which the head bears a corona of erected teeth around the frontal ocellus. Contrary to Orussidae, the Stephanidae lack cenchri, and their mesosoma and metasoma are separated by a wasp waist, which is absent in Orussidae.
Similar to the larvae of apocritan Hymenoptera, larvae of Orussidae have reduced some morphological features as a result of their parasitic life style inside the tunnels of wood-boring insects. They are white, subcylindrical, weakly sclerotized with a distinct head capsule. The mouthparts are hypognathous. Eyes and legs are reduced or completely absent. The mandibles are well developed and strongly sclerotized. Palps of maxilla and labium lack. The surface of the body is subdivided into distinct segments, each bearing a transverse row of 8−10 backward pointing spines.[4]
Orussid wasps have been collected only rarely. Over the years, their abundance has been subject to strong fluctuations.[5] The species are thermophilous and wasps are active during the hottest hours of the day. Therefore, they are rarely found by the entomologists specialized in sawflies.
