The stalk-eyed fly Achias rothschildi belongs to the family Platystomatidae, which are sometimes known as signal flies, and currently contains about 1,200 described species. Although Platystomatidae are found in most parts of the world, the highest numbers of species are in the Old World tropics, especially Australasia.
The genus Achias is almost entirely restricted to New Guinea and northern Australia, but contains nearly 100 described species, and in New Guinea it is the most diverse platystomatid genus. Male Achias are usually stalk-eyed, but the eye-stalks are lacking in a few species but even in these cases the eyes are widely separated and usually distinctly protrude from the sides of the head.
This species was described in 1910 by Ernest Edward Austen, an entomologist at the British Museum, from specimens in Lord Rothschild’s collection, some of which were then donated to the museum by Rothschild after whom Austen named the species.
There are other families of acalyptrate Diptera containing stalk-eyed flies, this character having evolved independently in different groups. The best known of these are Diopsidae as most adults in this family are stalk-eyed, but these are smaller than Achias and they show much less variation in eye-stalk length within each species and between sexes.
This is the largest species of Achias, the most similar being A. gjellerupi, which is possibly conspecific but differs by having a slightly different wing pattern, a dark marking on the underside of each femur and the males have a pale yellow marking on the eye-stalk. A. latividens is also similar but has a median facial stripe, a humeral bristle, and broader front tarsi.
The genus Achias can be confidently placed in the subfamily Platystomatinae, as it shows the diagnostic features of that group including the structure of the male genitalia. Within this subfamily, Achias seems to be most closely related to Lamprogaster, but further studies on this group are needed to determine its phylogeny with more certainty.