Orussidae

Family of wasps

The Orussidae or the parasitic wood wasps represent a small family of sawflies ("Symphyta"). Currently, about 93 extant and four fossil species are known. 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 sawflies 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. 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 erect teeth around the frontal ocelli. 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.

Orussid sawflies have been collected only rarely. Over the years, their abundance has been subject to strong fluctuations. 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. Only for few species the larval biology is known. Orussidae are parasitoids of xylobiontic larvae of beetles or Hymenoptera, particularly of the larvae of jewel beetles (Buprestidae), long horned beetles (Cerambycidae), and wood wasps (Siricidae, Xiphydriidae). Orussids can be observed running around quickly on dead tree trunks. The females locate the host larvae living concealed inside wood by generating vibrations by tapping the tips of their antennae against the wood surface. The vibrations are picked up by the modified fore legs. After locating the host, the female drills into the wood with the very long ovipositor and lays the egg. The egg is very elongate with a small expansion on the anterior end and a long expansion on the posterior end. The egg is coiled on the host. Accordingly, parasitic life style has not evolved first in Apocrita but in the common ancestor of Orussidae + Apocrita. Earlier, the Orussidae were sometimes put into a separate suborder, Idiogastra, Orussidae are demonstrably monophyletic.

The oldest members of Orussoidea, the group containing Orussidae and its close relatives, are known from the Middle Jurassic Karabastau Formation of Kazakhstan. The oldest known members of modern Orussidae are from the Late Cretaceous. Ophrynopus peritus Engel, 2008 was described from Dominican Amber, Baltorussus velteni Schedl, 2011 from Baltic Amber, Mesorussus taimyrensis Rasnitsyn, 1977 from the Late Cretaceous of Taimyr, Siberia und Minyorussus luzzii Basibuyuk, Quicke & Rasnitsyn, 2000 from the Late Cretaceous of New Jersey.