The Nemestrinoidea are a small, monophyletic superfamily of flies, whose relationship to the remaining Brachycera

Brachycera

Brachycera is a suborder of Diptera. It is a major suborder consisting of around 120 families. The most distinguishing characteristic of the suborder is reduced antenna segmentation...

 is uncertain; they are sometimes grouped with the Tabanomorpha

Tabanomorpha

The Brachyceran infraorder Tabanomorpha is a small group that consists primarily of two large families, the Tabanidae and Rhagionidae , and an assortment of very small affiliated families, most of which have been included within the Rhagionidae...

 rather than the Asilomorpha

Asilomorpha

The Brachyceran infraorder Asilomorpha is a large and diverse group of flies, containing the bulk of the non-muscoid Brachycera.The larvae of asilomorphs are extremely diverse in habits, as well.-Classification:...

. They are presently considered to be the sister taxon to the Asiloidea

Asiloidea

Asiloidea is a very large cosmopolitan superfamily of Diptera. The former infraorder Asilomorpha is paraphyletic and thus not a natural group; as its type taxon the Asiloidea would still belong there in any case, but current knowledge of brachyceran relationships would make it monotypic: the...

. The group contains two recent very small families, the Acroceridae

Acroceridae

Acroceridae is a small family of odd looking flies most closely related to Nemestrinidae. There are about 520 species in 50 genera. They are characterized by a humpbacked appearance and a small head, sometimes with a long proboscis for nectar. As such, acrocerids are commonly known as small-headed...

 and Nemestrinidae

Nemestrinidae

Nemestrinidae, or tangle-veined flies is a family of flies in the superfamily Nemestrinoidea, closely related to Acroceridae. The family is small but distributed worldwide, with about 300 species in 34 genera. Larvae are endoparasitoids of either grasshoppers or scarab beetles...

, both of which occur worldwide but contain only small numbers of rare species. One extinct family, Rhagionemestriidae, is also included in Nemestrinoidea.

These insects are parasitoid

Parasitoid

A parasitoid is an organism that spends a significant portion of its life history attached to or within a single host organism in a relationship that is in essence parasitic; unlike a true parasite, however, it ultimately sterilises or kills, and sometimes consumes, the host...

s, with Acroceridae attacking spider

Spider

Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...

s, and Nemestrinidae typically attacking Orthoptera

Orthoptera

Orthoptera is an order of insects with paurometabolous or incomplete metamorphosis, including the grasshoppers, crickets and locusts.Many insects in this order produce sound by rubbing their wings against each other or their legs, the wings or legs containing rows of corrugated bumps...

. Both families have unusual and distinctive wing venation by which they can be easily recognized, in addition to other features.