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Diptera Celyphidae: when flies mimic Beetles

The world of insects sometimes contains surprising and mysterious species. Diptera (“flies”) Celyphidae are among them.

caption id=”attachment_3078″ align=”alignright” width=”301″]Paracelyphus hyacinthus (Source : Anthony KeiC Wong-Flickr) Paracelyphus hyacinthus (Source : Anthony KeiC Wong-Flickr)[/caption]

The name of this family comes from the Greek word “κέλυφος” which can be translated as “box” or “shell” which characterizes the most striking characteristic of these insects: that of resembling small beetles. Such in these, the abdomen is covered by a rigid cuticular structure of protection where fold the wings when the insect is at rest (see photo opposite and photo album at the bottom of page).

However, unlike beetles, this structure does not consist of the forewings but of the scutellum (element of the thorax located between the insertion points of the forewings).

This is a surprising case of evolutionary convergence.

The function of this unique adaptation in diptera remains unexplained. In 1941, the Finnish entomologist Richard Hjalmar Frey (1886-1965) put forward two hypotheses: improvement of buoyancy during flight or ornamental role. Due to lack of studies in natura, the mystery of this enlarged organ remains intact.

Zogeography

The Celyphidae family, comprising 90 species, is found mainly in eastern, Indochinese and Indomalay regions. Its range, including Pakistan, India, the Philippines, the Ryukyu Islands or Borneo, is bounded to the east by Weber’s line, to the northeast by southern China, Tsushima Island in the Korean Strait (one species) and the Solomon Islands (one species), to the northwest by Afghanistan where a species was described in 1881 (Celyphus dohrni (Bigot) and whose presence could not be confirmed since) and by Ethiopia in Africa (see mapping below).

Two species have been described outside this geographical area from a single known specimen: Celyphus inaequalis (Costa) from Australia and C. ruficollis (Macquart 1843, 1851) in Guyana.

The African region concentrates a unique diversity of 15 species all endemic, as well as two endemic genera Afrocelyphus (Vanschuytbroeck) and Chamaecelyphus (Frey).

caption id=”attachment_3060″ align=”aligncenter” width=”431″]Geographical distribution of Celyphidae (Source : Tenorio, 1972) Geographical distribution of Celyphidae (Source : Tenorio, 1972)[/caption]

A little history

The first works on these insects date back to the early 19th century:

  • Dalman J.W. (1818, 1823) : describes the genus type Celyphus (family reference) and species Celyphus obtectus (one of the most widespread species in the family)
  • Macquart J. (1843, 1851) : describes two new species including C. ruficollis (Australia) which is known from a single specimen
  • Bigot J.M.F. (1859, 1878, 1880) : describes genus Paracelyphus
  • Karsch F. (1884) : proposes a key for determining new species of the genus Celyphus
  • Van der Wulp (1884), 1886) : describes the species Paracelyphus sumatranus
  • Hendel F. (1914) : describes genus Spaniocelyphus and new species
  • De Meijeire J.C.H. (1911, 1915, 1919) : redescribes several old species, describes new species and writes the Sumatran and Java wildlife determination key
  • Malloch J.R. (1927) : proposes the key to all genera and describes two new species of Formosa (Taiwan)

It was from the end of the 1920s that more complete works appeared.

The first comprehensive study is that of the American entomologist John Russel Malloch (1875-1963), published in 1929, on the species of the Philippines where five new species and two new genera appear Acelyphus and Idiocelyphus. The author demonstrates for the first time the importance of genitalia (male genital traits) in the determination of species, especially those of the genus Spaniocelyphus.

Genre Spaniocelyphus - Cuc Phuong National Park VIETNAM (Source : Stephen gaimari)
Genre Spaniocelyphus – Cuc Phuong National Park VIETNAM (Source : Stephen gaimari)

In 1941, the Finnish entomologist Richard K.H. Frey publishes the most comprehensive study on eastern Celyphidae flies, first established as a family. He describes the genus Hemiglobus as well as six new species, proposes a key of genera and species, illustrates the genitalia of two species of the genus Spaniocelyphus and references the 37 species (including 3 from Africa) on a geographical distribution map.

In China, the works of S.H. Chen (1949) describe three new species and the genus Oocelyphus, and inventory in total in this region 11 species divided into 4 genera.

A new brief revision of the family was conducted by P. Vanschuytbroeck in 1952, integrating three new species, two of which were synonymous with C. obtectus. The author will describe several new species later: two for Nepal in 1965 (one synonymous with C. obtectus and the other, represented by a single specimen, remaining unclassifiable), one in the Philippines and one in Southeast Asia in 1967.

The last major study is published in 1969 by JoAnn M. Tenerio (University of Honolulu-Hawaii) who reviews the fauna of the Philippines and describes new species, bringing to 26 the number of known species of this archipelago. The author also illustrates the genitalia of several of them, serving as a basis for the constitution of a determination key.

Morphological characterization

The singular habitus of the Celyphidae has made their classification indecisive for nearly a century.

caption id=”attachment_3085″ align=”alignleft” width=”300″]Celyphidae (not determined) (Source : Nicky Bay-Flickr) Celyphidae (not determined) (Source : Nicky BayFlickr)[/caption]

The morphology of the head, the structure of the buccal parts and the nervation of the wings presents strong analogies and similarities with that of the Lauxaniidae, having led to consider previously the Celyphidae as a subfamily of the Lauxaniidae. Today, there is no doubt that these two families are distinct while being very phylogenetically close.

Although Celyphidae scutellum hypertrophy is extreme, it occurs on a smaller scale in other families such as Chloropidae and Ephydridae. The definition of the Celyphidae family is essentially based on the unique structure of genitalia (presence of gonapophyses : protuberances surrounding the sexual orifices and varying strongly between taxa) which has no similar structure in Diptera. This is one of the rare cases where the characterization of a family is based on a single morphological criterion.

Definitions

Clypeus : element of the head of insects covering the mouth parts as a lip (lire this article)

Arista : sensory silk located on the 3rd antennary article of the Diptera (lire th article)

Cellule : wing membrane space bounded by ribs (lire cet article)

The main morphological characters of Celyphidae are:

  • Clypeus* enlarged and projecting ( see figure 1)
  • Fusion of cells* disc and basal wing (distinct cells in Lauxaniidae) (see figure 2)
  • Arista* enlarged at its base and subapically implanted on the 3rd antennary segment (basal implantation in Lauxaniidae) ( see figure 3)
  • Scutellum strongly enlarged and often convex on top
  • Pregenital and genital segments located under the abdomen ( see figure 4)
  • Variable colors according to species and between individuals : brilliant blue, green or metallic violet (see photo album at the bottom of page). It was observed in C. obtectus that yellowish colored individuals present a smooth appearance scutellum while those of bluish coloration, the scutellum presented an either rough or smooth appearance.

caption id=”attachment_3073″ align=”aligncenter” width=”825″]Main morphological characters of Celyphidae flies (Source : Tenorio 1972 - modified by Benoît GILLES) Main morphological characters of Celyphidae flies (Source : Tenorio 1972 – modified by Benoît GILLES)[/caption]

Biology

In 2016, knowledge and publications on biology and ecology of Celyphidae remain sparse and rare.

The only data come from S.K. Sen (1929) on Celyphus obtectus and Spaniocelyphus scutatus.

  • C. obtectus : 70 eggs are laid on the underside of decaying (yellowing) leaves. Larvae feed on the surface of the laying leaves. According to laboratory studies, the development cycle takes about 23 days: eggs 4-5 days, larvae 7-10 days, pupa 9-11 days. The development time should certainly depend on the temperature and humidity conditions.
  • S. scutatus : the eggs are laid on the margins of grass blades on decaying leaves of Duranta repens (Vaniller de Cayenne – Verbenaceae). The development cycle and morphology are similar to those of C. obtectus.

caption id=”attachment_3082″ align=”alignleft” width=”301″]Celyphidae egg (not determined) (Source : Celyphidae egg (not determined) (Source : Anthony KeiC Wong-Flickr)[/caption]

Little information exists on the type of biotope that Celyphidae like. The study of the sampling sites reveals that these flies were mainly caught in grassy areas and along river banks and ponds. In 1878, Lucas indicates to have seen several times individuals of Paracelyphus hyacinthus floating around leaves of trees like banana trees.

Photo album
Video

Source

Tenorio J.M (1972) : A revision of the Celyphidae (Diptera)of the Oriental Region. Royal Entomological Society of London 123(4):359-453

Recommendations for works on the theme

 



Benoît GILLES
Chargé de recherche – Entomologiste chez Cycle Farms