Morphology of Diptera

The Diptera is a very large and diverse order of mostly small to medium-sized insects. They have prominent compound eyes on a mobile head, and (at most) one pair of functional, membraneous wings, which are attached to a complex mesothorax. The second pair of wings, on the metathorax, are reduced to halteres. The order's fundamental peculiarity is its remarkable specialization in terms of wing shape and the morpho-anatomical adaptation of the thorax – features which lend particular agility to its flying forms. The filiform, stylate or aristate antennae correlate with the Nematocera, Brachycera and Cyclorrhapha taxa respectively. It displays substantial morphological uniformity in lower taxa, especially at the level of genus or species. The configuration of integumental bristles is of fundamental importance in their taxonomy, as is wing venation. It displays a complete metamorphosis (egg, larva, pupa, adult), or holometabolous development. The larvae are legless, and have head capsules with mandibulate mouthparts in the Nematocera. The larvae of "higher flies" (Brachycera) are however headless and wormlike, and display only three instars. Pupae are obtect in the Nematocera, or coarcate in Brachycera.
Adult.
Adults are small (< 2mm) to medium-sized insects (- < 10mm), larger Diptera are rare, only certain families of Diptera Mydidae and Pantophthalmidae reach 95–100 mm wingspan while tropical species of Tipulidae have been recorded at over 100 millimetres. They have dull or bright colors, uniform or variegated and are sometimes mimetic such as in Syrphidae . Of fundamental importance, for taxonomy, is the presence and distribution of the attached integumental bristles.
Head.
The head is distinct from the thorax, with a marked narrowing at the neck. In "lower flies" (Nematocera), it is prognathous (head horizontally oriented with the mouth anterior), in "higher flies" (Brachycera) it is hypognathous (head vertically oriented with the mouth ventrad).
The shape of the cranial capsule also varies. In the Nematocera, the dorsal–ventral part of the head extends forward from the eyes due to the development in length of the clypeus and subgenal area (subgena), the distal end of the extension is the 'mouthparts'.
In the "higher" Diptera the head has a subglobose shape and the fronto-clypeus is an area bounded superiorly by the eyes and the vertex. In Cyclorrhapha Schizophora, a morphological element of particular importance is the presence of the ptilinal suture formed by the resorption of the ptilinum after emergence from the pupa. The suture separates two regions: 1. the upper one is the frontal region, which has continuity with the apex, the orbital region and the gena, 2. the lower one, the face or clypeus, contains the insertion of the antennae and ends with the epistomal edge which comprises the upper lip.
The eyes are usually very obvious, but reach a remarkable development in the Brachycera. In this suborder the eyes are markedly convex and have grown to occupy most of the side of the head. The space between the two eyes can sometimes be reduced to a narrow strip running from the front of the occipital region, or disappear altogether because of the direct contact between the eyes or their margins. The morphology of the compound eye is characterized by a significant number of ommatidia, of the order of thousands in muscoids. The ocelli, when present, are located in the top of the head, arranged at the corners of a triangle in an area called stemmaticum or ocellar triangle.
For the purpose of systematics, the presence, the arrangement, and the conformation of the cephalic bristles is important and they have a specific terminology. Bristles on the head are: frontal bristles, sometimes named lower orbital bristles are located on the frontal plates of the frons resembling a small alley extending from the base of the antennae toward the vertex and edging the median frontal stripe laterally. Sometimes they are situated lower, along the frontalia below the antennal attachment and over a greater or lesser distance. Orbital bristles are located on the vertex plates of the frons and usually restricted to its upper half. They may be arranged in longitudinal rows named inner and outerorbital bristles. (The frontal and vertex plates of the frons can be visualised on the basis of the arrangement of the frontal and orbital bristles); ocellar bristles are located on the vertex between the ocelli; outer and inner vertical bristles are located on the border between the vertex and the occiput and near the upper corner of the eyes; postvertical bristles are located behind the ocelli on the occiput near the median line of the head; vibrissae usually arrayed in small numbers along the facial sections of the arcuate suture, near the margin of the oral cavity; sometimes they ascend along the suture over a greater or lesser distance, occasionally almost to the place of antennal attachment; false vibrissae-bristles are placed along the margin of the oral cavity.
Sometimes the terminology is conflicting. For instance in the Acalyptratae there are usually two bristles, more or less strong, positioned along the posterior margin of the ocellar triangle. These bristles are called "postvertical bristles" in old literature, since the nineteenth century, and the term is used sometimes in the recent literature. Steyskal (1976) proposed the name "postocellar bristle" the adopted term in the "Manual of Nearctic Diptera" (McAlpine, J.F., 1981) and the "Manual of Palaearctic Diptera" (Bernhard Merz, Jean-Paul Haenni, 2000) and, therefore, this term occurs widely in the literature that refers to these two fundamental works. Two other bristles, present only in some families of Acalyptratae, are located posteriorly and laterally to the ocellar triangle, and are called "internal occipital" in old literature. Steyskal (1976) uses the name "paravertical bristles" and the same name is used in the basic nomenclature of the two manuals cited. In Russian the lateral parts of the frons are termed 'orbits'. In English this part is most commonly termed 'frontalia', 'parafrontalia', or 'frontal orbit', while the simple term 'orbit' refers to the margin of the compound eye. The median part of the lower head, or face, often bears in its lower corners a pair (or a few pairs) of large seta (bristles) called 'vibrissae' and sometimes several or even a complete series along a ridge extending upward from the vibrissae. The latter setae are in Russian texts also called vibrissal bristles, not facial bristles.
The antennae are divided into two basic morphological types that are the basis of the distinction between the two suborders. All fly antennae consist of three sections: the scape, the pedicel, and the flagellum. Intrinsic muscles are only found in the scape and pedicel. In Nematocera, they are pluriarticulate, threadlike or of feathery type, composed of 7-15 undifferentiated items. In Brachycera the antennae consist of up to six segments, of which the first three are well-developed. In most of the Brachycera families, the third segment is enlarged and the more apical segments are reduced to an appendage — called a stylus when rigid and an arista when bristle-like.
The insect mouthparts show, according to the systematic group, a variety of conformations. Mouthparts are modified and combined into a sucking proboscis, which is highly variable in structure. The ancestral condition is the piercing and sucking type proboscis, more modified proboscis forms variously rasp or sponge fluids. The labellum is one such sponging organ. Some species have non-functional adult mouthparts.
Thorax.
Taxonomically important bristles on the thorax.
The chaetotaxy of the pleura is also of taxonomic significance. The characters taken into consideration are presence or absence, the number, and the position of setae and groups of hairs on the 
The fundamental peculiarity of the Diptera is the remarkable evolutionary specialization achieved in the shape of the wings and the morpho-anatomical adaptation of the thorax. Except for infrequent wingless forms, the Diptera are usually winged and use the wings as the principal means of locomotion.
The wings.
The level of specialization – anatomical, functional and morphological – is such that in general these insects fly, often exceptionally, well, with particular reference to agility. All Diptera are equipped with only one pair of functional wings, which are on the mesothorax (front). The wings on the metathorax are transformed into the halteres or rocker arms. From this characteristic comes the name of the order, from the Greek "dipteros", which means "two wings". In consequence of this morphological structure, the mesothorax represents the segment of greater development and complexity, while the prothorax and metathorax are considerably reduced.
The halteres are club-shaped organs, used to balance the insect in flight, consisting of a proximal portion connected to a mechano-sensory organ. The homology between the wings and halteres is demonstrated by the four-winged mutant of the fruit fly "Drosophila melanogaster". The development of the halteres varies according to the systematic group: in the Tipulidae are they are thin but long and clearly visible, but are usually hidden by the wings in most other groups. In Calyptratae which includes the most advanced Diptera, the halteres are protected by calyptrae (small membranes above the halteres).
The mesothoracic wing is entirely membranous, completely transparent and colourless, or bearing zonal pigmentation useful for recognition. Its surface is divided into three regions: the most developed is the alar (main flight) region, supported by robust wing veins; posteriorly is the anal region; and, finally, in the rear section-proximal, there is an expanded lobiform alula. The alula also termed the axillary lobe is a broad lobe at the proximal posterior margin of the wing stalk. It is continuous with the upper calypter and distally it is (usually) separated from the wing by an indentation called the alular incision. Aluli are a newly acquired feature of the Diptera (Hennig, 1973) and aluli are usually absent or poorly developed in the Nematocera (excepting Anisopodidae) but present and relatively large in the Brachycera. In higher Diptera between the alula and the thorax is the upper calyptra, also the tegula. The calyptra are just below the junction of the wing with the thorax and are part of the axillary membrane of the wings of some Diptera – the two basal lobes are called the calypteres (also termed squamae, squamulae). The proximal lobe is called the lower calypter (or basicalypter or squamula thoracica). It arises from the furrow between the scutellum and the postnotum as a narrow, membranous ligament and ends where the more distal lobe, termed the upper calypter (or disticalypter or squamula alaris), folds sharply over it (calyptral fold). The upper calypter is usually larger than the lower calypter, but in some groups (Tabanidae, Acroceridae, and many Calyptratae), the lower calypter is larger than the upper one. The calyptral fringe is a fringe of hairs along the posterior margin of each calypter. The tegula (Shown here ) is the most proximal plate at the base of the costal margin (also termed the costal plate or epaulet). Next to it (distal) is the basicosta.
The system of venation is simplified but is representative of the Comstock–Needham system, which was conceived in the late nineteenth century to define precisely the terminology of the wing morphology of insects. In Diptera are the wing-veins are costa, subcosta, radial, medial and cubital. In addition, there are two anal veins, of which the second, also called the axillary, separates the anal region from the alula. The details of the wing veins, the transverse veins and the shape of the cells, are important characteristics for determining taxonomic groups including at species level.
Taxonomically important wing venation terms.
The most encountered terms used in Diptera identification keys are:–
The scutellum is nearly always distinct, but much smaller than (and immediately posterior to) the mesoscutum. The scutellum macrochaetae are important in taxonomy.
The relatively thin legs have precisely arranged bristles which also function in chaetotaxy. The femora and tibia may bear combinations of dorsal, anterodorsal, posterodorsal, ventral, anteroventral and posteroventral bristles. The position, number, size and inclination of these bristles is important in the taxonomy of higher flies. The leg flexes (tibia on femur) in the dorsal ventral plane. The dorsum of the tibia (especially) and the femur is often identified by a double line of very small bristles. Another important bristle is the preapical on the tibia (presence or absence is important at family level).
Abdomen.
The morphology of the abdomen is substantially determined by morphoanatomic adaptation, in both sexes, as a function of the reproduction. In general, the 10 urites (one of the segments of the abdomen or post-abdomen) are reduced to a lower number of urites because of structural modifications of the first urite and the last. Typically there is atrophying of the first urite and the merging of 2° and 3° urotergites. Tergites and sternites can be well distinguished from each other, but often there is a differential development, with the tergites overlapping the sternites; the extreme case is when the expansions of tergite ventrally merge, forming a tube structure or ring. In females, the last urites become thinner and stretch forming a flexible telescopic ovipositor. This morphological adaptation is often accompanied by sclerotisation of the terminal eighth urite, so that the ovipositor is able to penetrate through the tissues of the organism which will accommodate the eggs and larvae. In the male, the last urites undergo a complex transformation to form a device, integrated with the genitalia called the hypopygium. The degree and nature of structural change varies according to the systematic group, but usually involves the development of the lobes of the ninth urotergite into forceps (epandrium) and IX urosterno (hypandrium). There is sometimes a twist along the axis of the abdomen, resulting in reversal of the positions of the epandrium and the hypandrium.
Larva.
Most of the larvae of Diptera live in an aquatic environment, in decaying organic substrates, and in other organisms (fungi, animals, plants). Their morphological structure therefore has a substantial simplification.
The Diptera larva is apodous (with no legs), but sometimes, especially in aquatic larvae, has appendages similar to pseudopodia. The head is usually devoid of eyes, has chewing mouthparts, modified antennae with up to six segments, more or less developed or reduced to papillae. The head can be: clearly distinguished from the thorax (eucephalic larvae), indistinct from the rest of the body (microcephalic), or sunken in the thorax (cryptocephalic).
Depending on the number and position of the tracheal spiracles, the following types of breathing apparatus can be distinguished. 
The most frequent type, found in the generality of Brachycera, is amphipneustic, while other types appear mostly in aquatic larvae.
The larvae of Brachycera Cyclorrhapha have a wormlike appearance, with little differentiation of body regions (head, thorax, abdomen) to the point that they are commonly called, improperly, worms. These larvae have a cylindro-conical form, wider in the abdomen. Simplified mouthparts, represented by two jaws shaped like a hook and a series of internal cephalic sclerites, which form in the complex cephalo-pharyngeal apparatus unlike other chewing mouthparts, the hooks of the cephalo-pharyngeal apparatus are equipped with movements along a vertical plane.
Special morphological adaptations are observed in larvae adapted to live in an aquatic environment or as endoparasitoids: for example, sapropagous aquatic larvae of "Eristalis" which have a long respiratory siphon, which allows them to live immersed in slushy or in putrid waters, while those of Tachinidae have breathing tubes that lead into tracheae of the host or outside of the host's body.
Pupa.
The pupae of Diptera can be obtect, exarate or coarctate. Obtect pupae have the outlines of the wings and legs visible but pressed close to the rest of the body, as the whole is wrapped by a single cuticle; exarate pupae have the appendages enveloped by a cuticle of their own and are therefore detachable from the rest of the insect. Coarctate pupae develop inside the larval skin.
Pupae of Cyclorrhapha, suborder Brachycera, have coarctate pupae in a puparium (a case formed by the hardening of the larval skin), formed by a modification of morphological and biochemical exuvia of the last larval stage. The way in which the opening of the puparium, at the time of adult emergence, distinguishes between two large systematic groups, the Aschiza and Schizophora.
Obtect pupae are generally free and unprotected, with the exception of those of Simuliidae, which are protected by bozzoletti constructed with debris cemented together by silk.
In the last phase of their lives, the pupae of Diptera become mobile.
References and further reading.
This article is largely based on a translation of the Italian page on .