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Table of contents

General morphology of adult - Relative positions and orientation - Metamerism - Tegument - Bibliography

Textbooks of general entomology are usually referring to insects characterized by a primitive morphology and little specialized. For example, it is proposed often as a morphological model the grasshopper (Orthoptera: Caelifera), because this insect lacks special skills and is easily observable for the large size. This model, as valid in the essentials, especially when aimed at a general knowledge of the morphology of insects, does not lend itself to details of a systematic group that is phylogenetically distant or, otherwise, including highly specialized forms. In the discussions of individual orders, therefore, Authors develop a more specific terminology, usually adopted in the specialized areas, especially in the description of types for taxonomic purposes.

The Dipters considered here, both for the strong morphologic specialization and for depth studies about them. The strong differences in front to insects simpler and more generic, caused the development of a wide and complex nomenclature. In the approach with the terminology adopted in dipterology, the main problem encountered is the standard: different Authors, throughout the history of dipterology, have not adopted the same terminology and case of different meanings of same term are frequent. The problem is accentuated whe the incongruences have relation with disputes about the homology of certain morphologica characters, for example such as names of specific elements of the chaetotaxy or the wing venation.

So, a general confusion or ambiguity generated from these incongruences or controversies in the literature about Dipters. Attempts at standardization of terminology were introduced in the 80s, with the publishing of the Manual of Nearctic Diptera, and late 90s, with the publishing of the Manual of Palaearctic Diptera. These works, developed with the contribute of a large number of dipterologists, represent the milestones in the dissemination of a shared terminology. On other hand, the need for a standard extended to the entire order, also caused choices that could differ from previous literature, more or less shared, or were questionable according to dissertations related to homology. Therefore, certain choices may not have found support from some Authors in their works about specific groups. It should also be considered tha the Manual of Nearctic Diptera has more tha 20 years of history: while it is still regarded as a fundamental reference, it may be also suprpassed by newer works in specific areas, according to new acquisitions by the phylogeny.

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General morphology of adult

The adults of Diptera are fairly uniform in the general morphology and do not show strong differences, within the order, about the basic morphological regions. For example, a general description of Hemiptera, inevitably falls into considerable differences between the females of scales and a pentatomiid, because they are organisms very different. Almost all adults of Diptera are basically similar, despite the differential growth of individual regions, that gives differences in the global appearance. For example, although they appear very different, a housefly and a mosquito are united by evidence of a head, thorax, abdomen, three pairs of legs, a pair of wings, a pair of antennae, etc. This substantial convergence lacks when one examines the details of morphology, such as the structure and the features of the mouthparts or the terminalia.

The body of Diptera have the usual heteronomous metamerism of many insects, with the subdivision into three main morphological regions: head, thorax, and abdomen. These regions bear some articulated appendages, also present in most insects:

Except for some wingless forms, which require more detailed examination, most Dipters are easily recognizable by the presence of only the mesothoracic wings, a pair of halteres, and mouthparts forming a proboscid able to suck or lick fluids. There are few insects that have characters that may confuse them with flies. Presence of only two wings occurs only in Ephemeroptera order, in males of Strepsiptera, and in males of scales (Hemiptera: Coccoidea). The Ephemeroptera are easily distinguished by the absence of halteres and the presence of long cerci. The males of Strepsiptera are distinguished by the different position of wing and halteres: the hind wings are normally developed, while the mesothoracic wings are reduced to halteres, so they are placed before of wings. Yhe males of the cochineals, like Dipters, have mesothoracic wings and metathoracic halteres, so require a closer examination. They can be distinguished by the different morphology of mouthparts and the presence, in general, of long caudal appendages.

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Relative positions and orientation

Directions and planes dorsal view
Fig. 1 - Directions and planes (dorsal view)
P.M.: median sagittal plane; P.T.: transverse plane; ant: anterior or cephalic; ap: apical or distal; bas: basal or proximal; lat: lateral; med: medial; post: posterior or caudal.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

The terminology that refers to axis and planes is the same adopted in the general entomology. Referring to the usual direction of progressing and the posture of insect on a horizontal plane, we can distinguish, in the insect body, an axis and three planes. The median longitudinal axis extends from head to end of abdomen. The three planes are:

Directions and planes lateral view
Fig. 2 - Directions and planes (lateral view)
P.O.: horizontal plane; P.T.: transverse plane; ant: anterior or cephalic; ap: apical or distal; bas: basal or proximal; dors: dorsal; post: posterior or caudal; vent: ventral.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

The body of Dipters has bilateral symmetry with reference to the median sagittal plan. The relative position and the orientation of a morphological structure is defined with terminology based on these axis and planes:

These concepts can be better understood using two examples. The basal or proximal part of the wing is the one articulated to the thorax, while the apical or distal is the farthest. Assuming the cross section of a leg, we can find four sides: the anterior is the side oriented with the direction of the insect progressing, the posterior is the opposite; the dorsal is the side upwards and outward, the ventral is the opposite.

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As mentioned, the metamerism of adults' body is the heteronomous type: the body is divided into regions morphologically differentiated, each subdivided into segments more or less distinguished. The primitive condition of Insects is the subdivision of the body into 20 segments, including six cephalic segments, fused and changed, three thoracic, and eleven abdominal.

In Dipters, the number of abdominal segments is reduces by the atrophy of segment 11 and the strong reduction of segments 8 to 10, that are arranged to form the terminalia; segment 7 can undergo strong modification and sometimes the segment 6 also. It follows that the number of apparent segments is reduced to 5-7, while others take the conformation of caudal appendages more or less visible. The complex of terminal segments is frequently named as postabdomen or terminalia, thus bringing attention to that structural modification.

Strong changes in metameric structure are also observed in the thorax. The improvement in the function of flight, that is particularly sophisticated in this order, meant that during the evolution, Dipters have developed a complex thoracic structure, where the three segments are not easily distiguishable. Most of thorax is composed by the second segment (mesothorax), while the other are reduced to parts more or less confused and placed in front and rear to mesothorax. These segments are mostly visible from the lateral view.

The metameric organization is also recognizable by the segmentation of main appendages, such as legs and antennae. The structure of segmentation of the legs is similar to the generality of insects, with the division into four basal segments (coxa, trochanter, femur, and tibia) and an distal organ which is composed by more segments (tarsus). Instead of legs, the metamerism of antennae is less evident in most of the order, due to the reduction in number of segments (antennomeres) and the modification of terminal antennomeres. The metamerism of mouthparts is almost totally disappeared, since the structural simplification and the functional specialization have led to atrophy of many segments.

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In general, the exoskeleton of Dipters is slightly sclerotized, compared with that of other insect orders. Nevertheless, it is composed of sclerotic plaques, named sclerites, connected to other by lines of minor thickening (sutures) or portions of membranous integument, more or less wide. These are particularly found among the regions that require a marked flexibility of movements, such as the neck and the connections between the abdominal sclerites. In contrast, the extension of membranous areas is significantly reduced in regions where high mechanical stress occur and the flexibility of movements of sclerites is less essential, such as in the thorax. For this reason, the thorax of flies has a robust appearance that contrasts with the more delicate abdomen.

Sclerites of two adjacent segments are connected by an intersegmental membrane, that can undergo also sclerotizations more or less intense, determining the appearance of a secondary metamerism. The presence of these sclerotizations is the basis of the emergence of a more complex metamerism.

The primitive condition of organization of exoskeleton of Insects, in the thorax and abdomen, is the conformation of each segment like a ring, through the articulation of four sclerotized plaques: one dorsal (tergum or tergite), one ventral (sternum or sternite), and two lateral and symmetric (pleura). In the abdomen of most insects, the pleural sclerites disappear to make place for a membrane able to allow more or less extensive size of the abdomen. The Dipters show the condition of lacking of lateral sclerites and their function is often subrogated by lateral expansions of tergite, which can extend and bend to almost completely cover the sides also.

In contrast, the exoscheleton of thorax show a high complexity due the subdivision of tergite and pleura in several sclerites, separed by suture lines. The terminology related to these sclerites is fundamental for taxonomic purposes.

The chaetotaxy is a morphological character very important, because has large use in the systematic determinations, specially within the Brachycerous. The trichoid structures are distinguished in microtrichia and macrotrichia. The first ones are simple protrusions of tegument not articuled; they are similar to very short and tiny hairs and are hardly visible without a microscope except the whole appearance: the vestiture of microtrichia, in fact, gives the pruinescence of the sclerites in many dipters. The macrotrichia are real hairs, more or less stout, articulated to tegument, with the basis hollowed in an alveolar groove and connected with nerves. The terminology applied in the distinction between several types of microtrichia and macrotrichia is not well defined, so there are some incongruences, in the literature, that make uncertain the exact correspondence of therms used by different authors. The Manual of Nearctic Diptera identifies three type of macrotrichia or setae, without definition (McAlpine, 1981): bristles, setulae, and hairs. The bristles are long and thick macrotrichia, while hairs should correspond to the common meaning of the word: long and thin. Setulae is a term more uncertain; this should be a macrotrichium thin and short. The Manual of Palaearctic Diptera makes a distinction significantly different (Merz & Haenni, 2000): macrotrichia are distinct into setae, setulae, and spurs. The definitions related to these terms suggest a divergence between the two cited works: seta, which McAlpine uses as synonym for macrotrichium, is used by Merz & Haenni as synonym of bristle sensu McAlpine, while setula refers to a short and thin hair. More precisely, the setulae are defined as scattered hairs, without ordered placements and without specific names. Finally, Merz & Haenni cite as macrotrichia the tibial spur, while they do not refer to hairs sensu McAlpine.

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