Morphology of Diptera
Dipteran morphology differs in some significant ways from the broader morphology of insects. 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 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, or holometabolous development. The larvae are legless, and have head capsules with mandibulate mouthparts in the Nematocera. The larvae of "higher flies" are however headless and wormlike, and display only three instars. Pupae are obtect in the Nematocera, or coarcate in Brachycera.
Adult
Adults are small to medium-sized insects. Larger Diptera are rare, only certain families of Diptera Mydidae and Pantophthalmidae reach wingspan while tropical species of Tipulidae have been recorded at over. 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.External
Head
The head is distinct from the thorax, with a marked narrowing at the neck. In "lower flies", it is prognathous, in "higher flies" it is hypognathous.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, 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. ; 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 proposed the name "postocellar bristle" the adopted term in the Manual of Nearctic Diptera and the Manual of Palaearctic Diptera 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 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 of large seta 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
- acrostical bristles adjacent to the median longitudinal axis of the scutum. They may be irregular or align in two or more rows. The number of rows, the number of setae in each row, the size and the thickness are significant. In many groups, the acrostical setae are replaced by setulae or hairs.
- prescutellar two acrostical bristles, more developed than the other acrosticals, inserted in front of the scutoscutellar suture.
- dorsocentral these bristles are aligned along two rows adjacent to and outside the acrosticals.
- posthumeral bristles aligned in the presutural area and parallel to the suture which separates the scutum from the humeral calli.
- humeral sometimes called postpronotal bristles are on the humeral calli.
- presutural
- notopleural bristles on the notopleuron
- intralar more or less regularly aligned bristles near the dorsocentral series. The position is not well defined.
- supralar these setae are limited in number, and located from the prealar callus to the supralar area.
- postalar limited in number, they are located on the postalar callus near the lateral margin of the scutum behind the insertion of the wing.
- scutellar bristles on the scutellum. They may be marginal or on the dorsal side of the scutellum.
- anepisternum or mesopleuron – anepisternal or mesopleural bristles
- katepisternum or sternopleuron – katepisternal or sternopleural bristles
- proepisternum and proepimeron – proepisternal and proepimeral bristles, or propleural bristles
- anepimeron – anepimeral or pteropleural bristles
- meron – meral or hypopleural bristles.
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. 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.
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 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 separated from the wing by an indentation called the alular incision. Aluli are a newly acquired feature of the Diptera and aluli are usually absent or poorly developed in the Nematocera 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. The proximal lobe is called the lower calypter. 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, folds sharply over it. The upper calypter is usually larger than the lower calypter, but in some groups, 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 is the most proximal plate at the base of the costal margin. Next to it 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.