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General - Tracheal spiracles - Preabdomen - Bibliography - Web resources

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Abdomen of Culicidae female
Fig. 1 - Diagram of morphology of the female abdomen in Culicidae.
c: cerci; mp: pleural membrane; sbp: subanal plate or postgenital lobe; spm: spermathecae; spp: supranal plate; spt: spiracles; s1-8: sternites 1-8; t1-8: tergites 1-8.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

The abdomen is the third morphological region, primitively composed of 11 segments, called urites, the latest of which are more or less reduced and differentiated in morphological structures related to the reproduction. A single urite appears ad a ring composed of a dorsal sclerite, called tergite or tergum, and a ventral one, called sternite or sternum. They are connected by a pleural membrane. Each urite is joined to adjacent by a intersegmental membrane.

In all Diptera, the urite 11, called proctiger, is vestigial and reduced a small terminal region bearing a pair of cerci and the anal opening. However, within the order there is a trend to the reduction in the number of segments that keep the primary structure, due the coalescence of adjacent sclerites or the morphological and structural modification of latest urites. Due these changes, that may hide the primary metameric division, the urites are often referred as apparent. The most conspicuous changes of terminal urites are, in the male, the reduction and possible coalescence of adjacent sclerites and, in several groups, the axial rotation combined with a ventral flexion of 180°. In the female, however, the terminal urites are often transformed into tubules within the previous and telescopically evertable. The description of the abdomen of Diptera, particularly the abdomen of males, discerns between two distinct morphological regions:

Abdomen of Asilidae female
Fig. 2 - Diagram of morphology of the female abdomen in Asilidae, subfamily Asilinae: the ovipositor is usually tapered, darker than the preabdomen, composed of the urite 8 and following, and retractile into the urite 7.
c: cerci; ov: ovipositor; s1-8: sternites 1-8; t1-10: tergites 1-10.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

In the female terminalia, the genital opening is placed between the sternites 8 and 9. Unlike other orders of insects, the female gonapophysesin the Diptera are often simplified because the oviposition is remitted to an organ derived from the morphological and structural adaptation of the latest urites. For this reason, the female postabdomen is often called ovipositor.

In the male terminalia, the genital opening is placed into the copulatory organ, called also aedeagus or penis, located between the sternites 9 and 10. The male hypopygium, in most Diptera, has strong adaptations which make uncertain and disputed the interpretation of some homologies and may lead to incongruences in the terminology. The specificity of these changes make the male terminalia one the most important elements in the taxonomic diagnosis and the phylogenetic studies.

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Tracheal spiracles

Female adbomen of Tephritoidea
Fig. 3 - Diagram of morphology of the female abdomen in most Tephritoidea.
c: fused cerci; g: retractile sheath of the ovipositor; mi: intersegmental membrane; mp: pleural membrane; os: oviscape; spm: spermathecae; st: spiracles; s1-8: sternites 1-8; t1+2: syntergite 1+2; t3-8: tergites 3-8; u7: urite 7.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

The primitive condition within the Diptera is the presence of eight pairs of abdominal spiracles. This condition occurs only on females of species or groups which belong to some nematocerous and orthorrhaphous families. In females of most Diptera there are a maximum of seven pairs of spiracles. In the males, however, there are no more than seven pairs and this maximum is referred by Hennig (1973) as apomorphic condition of the Diptera (McAlpine, 1981).

Compared with the general condition, in several groups the number of spiracles decreases to 5 or 6 pairs due the loss of the seventh and possibly the sixth pair, but exceptionally there may be fewer than 5 pairs until the complete absence in some nematocerous (McAlpine, 1981).

The spiracles are positioned on the pleural membrane or the lateral edge of tergites.

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As mentioned above, the preabdomen is usually composed of first 5 or 6 urites and it does not show a wide differentiation in structure, except for the possible fusione of some adjacent sclerites.

In many Diptera, the first segment is reduced and it may merge with the second by the coalescence of the tergites or the sternites or both. In Cyclorrhapha, a general condition is a reduced first urite and the consequent fusion of first and second tergite into a single sclerite called syntergite. The first sternite, however, is usually reduced or fused with the second or, less frequently, missing (McAlpine, 1981).

Abdomen of Muscidae female
Fig. 4 - Diagram of morphology of the female abdomen in most Muscidae.
c: cerci; mi: intersegmental membrane; mp: pleural membrane; pr: proctiger; st: spiracles; s1-8: sternites 1-8; t1+2: syntergite 1+2; t3-8: tergites 3-8.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

The possible fusion of tergites of following segments is not frequent and occurs only in a few groups: the tergites 2 and 3 are coalescent in Ptychopteridae and more than two tergites may be fused in Tachinidae and Cryptochetidae.

An important feature is the wide lateral expansion of preabdominal tergites in Calyptratae. In these flies, the tergite is strongly convex and extends laterally and ventrally covering the pleural region and part of the ventral region. The sternites are consequently reduced.

Elements useful for taxonomic diagnosis are also the distribution of the pigmentation, which may be uniform or striped, and the presence and the position of bristles and setae. The chaetotaxy of the preabdomen, however, is less important than the one of head, thorax, legs and postabdomen.

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Web resources

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Last update of this page: 5 January 2017
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