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Table of contents

Structure and Morphology - Aristate antenna - Other forms - Antennomeres - Bibliography - Web resources

The antennae are segmented, paired, and symmetrical appendages of the head, aimed to a sensorial function. The presence of these appendages is an apomorphic condition of clade composed by taxa of Hexapoda and Myriapoda and called, according to different Authors, Uniramia, Tracheata, or Antennata. The possible absence of antennae, in these arthropods, is a derived character that rarely occurs as condition which is related with particular behavioral adaptations.

The Insects (Arthropoda: Hexapoda) usually have the antennae, inserted on the frontal region, and their absence or reduction to vestigial remnants occurs in regressed forms, ofted adapted to a parasitic life. In adults of Dipters, which etology strictly depends from external relations, the antennae are always present and are on the top of the face (Cyclorrhapha) or the transition zone between the face and the frons (Nematocera and Orthorrhapha), inserted into membranous acetabula called antennal sockets. In most Dipters, these sockets are barely visible or are borne by a flashy stalk, as syrphids of genus Ceriana Rafinesque.

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Structure and morphology

Like other appendages, the antennae of Insects, including Flies, are segmented and consist of a variable number of segments named antennomeres. In the flies and most of Insects, the antenna is morphologically and functionally differentiated into three parts:

Under the anatomical aspect, the antenna of Diptera (and Insects in general) is of annulate type, so has intrinsic muscles only in the scape and pedicel. The number of antennomeres depends on the number of flagellomeres and is an important character from a phylogenetic point of view. This number varies widely in several taxa of high level. Think that the traditional distinction between the suborders Nematocera and Brachycera is based on the development and appearance of antennae: the Nematocerous have usually long and slender antennae, composed at least by 7 segments similar in appearance, while the Brachycerous have short antennae, composed by 3-6 segments (Servadei et al., 1972). This conventional distinction is reflected in the etymology of names: "nematocerous" comes from Greek by the combination of nēmatos ("thread") and kèras ("horn"), referring to antennae long and slender; "brachycerous" comes from the combination of brakhýs ("short") and kèras and it refers to short antennae.

The simplistic distinction based on the number of antennomeres (at least seven in Nematocerous, less than seven in Brachycera) is ambiguous when concerns some brachycerous of section Orthorrhapha. McAlpine (1981), specifying the usual conditions in Nematocera, Orthorrhapha, and Cyclorrhapha:

The ground plan of Diptera is the antenna composed of 16 segments, distinct into 2 basal and 14 flagellomeres (Hennig, 1973). This number may exceptionally increase, in alcuni Nematoceri, as derived character, or decreases due the fusion of distal antennomeres during the differentiation of Brachycerous.

Under the morphological aspect, the level of differentiation among the antennomeres causes the occurrence of several types of antennae, heterogeneous in Nematocerous. The most frequent forms are the following: aristate sensu lato, filiform, moniliform, verticillate or plumose, pectinate or serrate.

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Aristate antenna

Aristate antenna
Fig. 1 - Antenna aristate on a blow-fly (Brachycera: Calliphoridae).
a: arista; f: first flagellomere; p: pedicel; s: scape; sa: antennal seam; st: bristle.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

This is the most common type among the Diptera, because occurs in great part of Brachycera. The morphology and structure of this type has a particular importance in cladistics because it would allow to define the relationships between the lower and upper Brachycerous. Approaches about that was developed by Hennig and next time by McAlpine (McAlpine, 1989), but not gave an established knowledge that goes beyond the hypothesis.

In general, the aristate antenna sensu lato is composed of three basal segments morphologically differentiated: scape, pedicel and first flagellomere, this called also postpedicel. The scape is very small and often is not easy to detect; the pedicel is usually larger than the scape, but remains isodiametric in many groups. Finally, the postpedicel is well developed and is larger than the scape and pedicel combined. The shape of postpedicel varies considerably so this character is useful to taxonomic determination. Usually it is oblong, but in many groups may be subglobose. From a structural point of view, the postpedicel is conventionally identified as first flagellomere, but relatively recent studies suggest that is derived from the fusion of first six flagellomeres, compared with the pesiomorphic condition of Nematocera (Stuckenberg, 1996; Merz & Haenni, 2000). On the postpedicel there is an articulated appendage that may appear very thin, like a long and thick bristle (arista), or may be more or less swollen and variously shaped (stylus). The morphological and structural differences between the arista and the stylus underlies the formal distinction between the aristate type sensu stricto and the one called "stylate" by McAlpine (1981). This distinction enables to easily discern between the upper Brachycerous (especially Schizophora) and some primitive groups placed among the Orthorrhapha, where the anatomical differences are clearly evident. By other hand the lower Cyclorrhapha (Aschiza) and, above all, the Orthorrhapha of Nemestrinoidea, Asiloidea, and Empidoidea superfamilies, show intermediary types that make uncertain and confused the distinction between the antenna cyliate and the true aristate.

The aristate antenna sensu stricto have a clear structure trisegmented composed of scape, pedicel, and postpedicel. The postpedicel bears a filiform appendage, more or less pubescent, called arista. The arista is usually composed of three segments, ground plan of Cyclorrhapha (McAlpine, 1989), but may be also bisegmented (e.g. Syrphidae) or vestigial or absent (e.g. Braulidae, Cryptochetidae, etc.). Another character which has phylogenetic and systematic significance is the insertion of the arista on the postpedicel. The primitive condition is the insertion on the apex of the postpedicel, that occurs in many Aschiza and the Orthorrhapha, but in most Schizophora and some groups of Aschiza, the insertion move to dorsal side of basal portion of postpedicel. Between the apical and the dorsobasal position there are several transitional forms.

Antenna stylate
Fig. 2 - Antenna stylate of Tabanomorpha.
(Haematopota pluvialis Meigen)
sc: scape; pe: pedicel; pp: first flagellomere; st: stylus
Author: Christine Lambkin (Morphbank Biological Imaging)
Modified from the original picture
(License: Creative Commons BY-NC-SA)

The stylate antenna sensu McAlpine, that occurs in flies generally included into Tabanomorpha, Xylophagomorpha, and Stratiomyomorpha infraorders, shows the trisegmented structure again, composed by scape, pedicel, and first flagellomere. However, the appendage is composed by a variable number of flagellomeres more or less differentiated and is inserted at the apex of postpedicel. These flagellomeres are larger than the aristomeres of the true arista, so the appendage appears relatively thick and is usually called stylus. The primitive condition would be that of a flagellum composed of 8 segments, including the postpedicel (McAlpine, 1981, 1989), but this number is often reduced due the fusion of some flagellomeres. It follows a marked morphological heterogeneity, which does not enable to identify a really representative model for the lower Brachycera. In the more primitive groups, the antennae are clearly "stylate" and can be various in shape, from moniliform to club-like to fusiform, according to he conformation of various flagellomeres. In more advanced groups, represented by clades related to Cyclorrhaphous the antenna have a transitional structure between the stylate and the aristate types.

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Other forms

Other forms of antennae occur mostly into the heterogeneous assemblage of Nematocera substantially as plesiomorph characters. More types of antennae may occur within the same family and sometimes in the same species, due the sexual dimorphism.

The filiform antenna is present in some nematocerous families (e.g. Dixidae, Deuterophlebiidae, Pachyneuridae, Mycetophilidae), but are frequent transitional forms between the filiform and the plumose or the moniliform types. This antenna is composed of differentiated segments more or less elongated; only the scape and pedicel can be often more expanded transversely.

The moniliform antenna occurs in several families (Axymyiidae, Pachyneuridae, Cecidomyiidae, Ceratopogonidae, etc.) and it is composed of globose antennomeres poorly differentiated. The length can vary depending upon the number of antennomeres.

Antenna plumose of Culicomorpha
Fig. 3 - Antenna plumose of Culicomorpha. Note the size of pedicel, containing the Johnston's organ.
s: scape; p: pedicel.
Author: Louisa Howard (Dartmouth College)
Modified from the original picture
(License: Public Domain)

The plumose or verticillate antenna is very common and occurs in various families (Chaoboridae, Chironomidae, Culicidae, Psychodidae, Tipulidae, ecc.). It is composed of cylindrical segments that bear hairs or setulae more or less long, arranged in whorls. The appearance of antennae may be various, sometimes also in the same species comparing the male's and females' antennae. For example, in mosquitoes, males have plumose antennae well visible to the naked eye, due the whorls of long hairs, while in females this form is less visible because the whorles are composed by short and scattered hairs.

The pectinate or serrate antenna is formed by antennomeres that have a lateral expansion or a double and symmetrical expansion (bipectinate). This type occurs, for example, in Tipulidae of subfamily Ctenophorinae and Limoniidae of genus Limonia Meigen or in the Keroplatidae family.

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Antennomeres

Over the total legnth, the number of segments, and the appearance of the entire antenna, other characters used by taxonomic keys are the shape and the size of single antennomeres, in particular referring to three basal segments, the properties of flagellum or the stylum or the arista, the presence and appearance of specific details as setulae, hairs, ridges, grooves, pits, etc.

The scape is usually a small segment, compared to pedicel and postpedicel. Sometimes is rudimentary or almost invisible (e.g. Culicidae). Not significant particular are usually present, except eventual bristles.

The pedicel is the second basal segment. Usually it has the same size of the scape or is slightly larger, but in most Culicomorpha is the largest segment of the antenna, distinct from both the scape and flagellomeres. This segment bears the Johnston's organ, a sensory receptor composed of several scolopidia, with a complex sensory function (mechanoreceptive and propriocetive). Among the morphological character there are:

The morphological features of the flagellum represent the main element of systematic determination concerning the antennae.

The morphology of first flagellomere is particularly important in most Brachycerous. In Nematocerous, in fact, it has not a significant differentiation from other flagellomeres, while in Brachycerous it differ more or less according to the conformation of aristate antennae. In this suborder, the postpedicel is usually the largest segment, articulated at the basis to the pedicel and bears, with various positions, the style or the arista. The most common form is oblong and occurs generally among the Schizophora. In the primitive groups of Muscomorpha, traditionally included in the Aschiza and Orthorrhapha, the postpedicel has frequently a subglobose shape, with an apical or subapical arista. Among the primitive Orthorrhapha, various types occurs. Some genera of Xylophagidae and Stratiomyidae have a primitive structure of antennae, with the postpedicel poorly differentiated from the style, so it appears the middle segment of a linear appendage. This is a transitional form between the aristate antenna and one of plesiomorphic types of Nematocerous (e.g. the antenna club-shaped of some Stratiomyidae). In other groups (e.g. Tabanidae) the postpedicel is well differentiated in shape and size, so it appears as a distal segment bearing the style as an appendage.

The apical flagellomeres, from second to distal, are strongly differentiated from the postpedicel in the aristate antenna. The shape of aristomeres is various and we can distinguish between a structure called "stylus", more or less swollen, and another, identified as "arista", thin and acute, similar to a strong bristle or a plumose appendage. The distinction between the stylus and the arista is not clearly defined and among the Orthorrhapha occur several transitional forms (McAlpine, 1981, Merz & Haenni, 2000).

The style is usually present in lower Brachycera more primitive, all included in the Orthorrhapha assemblage. This appendage is inserted at the apex of first flagellomere. Rarely occurs also in Cyclorrhaphous dipters, such as Physocephala Schiner (Schizophora: Conopidae). The arista is present in most Cyclorrhapha and in the Orthorrhapha more evoluted, that are included, by different Authors, in the Asilomorpha or Muscomorpha infraorders. An important character is the position where the arista is inserted onto postpedicel: the primitive condition is the apical position, which occurs in most Orthorrhapha ad Aschiza, while the derived condition is the dorsobasal position, which occurs in most Schizophora and some Aschiza. Between these conditions there are several transitional forms, with the arista inserted in subapical or dorsomedian position.

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Bibliography

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Web resources

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