Platystomatidae (Signal flies) is a distinctive family of flies (Diptera). Signal flies are worldwide in distribution predominating in the tropics. It is one of the larger families of acalyptrate Diptera with around 1200 species in 119 genera.
Adults are found on tree trunks and foliage and are attracted to flowers, decaying fruit, excrement, sweat, and decomposing snails. Larvae are found on fresh and in decaying vegetation, carrion, human corpses, and root nodules. Most larvae are either phytophagous (eating plant material) or saprophagous (eating decomposing organic matter). Some are predatory on other insects and others have been found in human lesions, while others are of minor agricultural significance.
Signal flies are very variable in external appearance, ranging from small (2.5 mm), slender species to large (20 mm), robust individuals, often with body colours having a distinctive metallic lustre and with face and wings usually patterned with dark spots or bands.
Many bizarre forms of morphology occur in this family. Heads and legs (fore legs especially) may be oddly shaped, extended in various ways or with adornments, all of which serve to supplement agonistic behaviour. Such behaviour underlies social and sexual interaction between individuals of the same species of Signal flies, first researched in Australian species of the genera Euprosopia and Pogonortalis 
In males of Pogonortalis, the length and degree of development of hairs (setae) on the lower facial area, together with widening of the head, facilitates territorial dominance  by head-butting and rearing-up behaviours. Head-butting is taken to the extreme in the Australasian genus Achias, in which species have the fronto-orbital plates expanded laterally to produce eyestalks.
Development of eyestalks in flies is also prevalent in the family Diopsidae, through lateral development of the frontal plate, with the result that the antennae are situated on the stalk near the compound eye. The process of development in signal flies is different, however, in that the fronto-orbital plates expanded laterally to produce eyestalks and consequently the antennae remain in a central position. The development of eyestalks reaches its extreme in the species Achias rothschildi Austen, 1910 from New Guinea, in which males have an eye-span of up to 55 mm.
See also 
Adults are sometimes amongst the most morphologically bizarre forms of all the Diptera.
- McAlpine, D.K.1973. Observations on sexual behaviour in some Australian Platystomatidae (Diptera, Schizophora). Records of the Australian Museum 29(1): 1-10.
- McAlpine, D.K.1975. Combat between males of Pogonortalis doclea (Diptera, Platystomatidae) and its relation to structural modification. Australian Entomological Magazine 2(5): 104-107.
- McAlpine, D.K.1979. Agonistic behavior in Achias australis (Diptera, Platystomatidae) and the significance of eyestalks. In: Blum, M. S. and Blum, N. A. (eds). Sexual selection and reproductive competition in insects. Academic Press, New York.
- McAlpine, D.K.1994. Review of the species of Achias (Diptera: Platystomatidae). Invert. Taxon. 8(1): 117-281.
- Whittington, A.E. 2003. Taxonomic revision of the Afrotropical Plastotephritinae (Diptera; Platystomatidae). Studia dipterologica Supplement 12: 1-300.
- Whittington, A.E. 2006. Extreme head morphology in Plastotephritinae (Diptera, Platystomatidae), with a proposition of classification of head structures in Acalyptrate Diptera. Instrumenta Biodiversitatis VII: 61-83.
- Arnauld, P. H. Jr. 1994. Frontispieces: Achias rothschildi Austen (Diptera: Platystomatidae). Myia 5: iv.
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