Disclaimer: This is a poor translation from the Italian version. Sorry for my bad English!

Table of contents

Coxa - Trochanter - Femur - Tibia - Tarsus - Pretarsus - Bibliography - Web resources
Lef og muscoid fly
Fig. 1 - Leg of a muscoid fly (Brachycera: Schizophora).
1: coxa; 2: trochanter; 3: femur; 4: tibia; 5: basitarsus or first tarsomer; 6: second, third, fourth, and fifth tarsomeres; 7: acropod; 8: pulvillus; 9: claw; 10: empodium.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

The legs of insects are appendages consisting of three pairs, one for each thoracic segment. The forelegs are called also prothoracic, the midlegs mesothoracic, and the hindlegs metathoracic. These pairs can sharply differ in morphology, specially in relation to particular functions performed, while the metameric structure remains unchanged. In proximal-distal order, each leg consists of following segments: coxa, trochanter, femur, tibia, tarsus, and acropod. In turn, the tarsus is composed of several articles called tarsomeres.

The presence and the position of morphological details of the integument are important in taxonomic diagnosis, so the surface is divided into four faces (McAlpine, 1981): the anterior and posterior faces are ideally perpendicular to he median sagittal plane, the dorsal and ventral faces are parallel, outside or lateral the first one, internal or medial the second one. Other transitional surfaces are added to these as following: anterodorsal, anteroventral, posterodorsal, and posteroventral.

About the structure and function, the legs of Diptera are ambulatory or cursorial, with specializations, in some groups, which may involve a single pair, usually the prothoracic. The most significant adaptation is related to raptorial function, recurrent in some families or groups at lower rank including predatory species. Another specialization occurs in the males of some groups which use the legs to hold the female during the mating; in this cases, the modifications, usually of tarsi, are a secondary sexual feature.

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The coxa is the proximal segment of the leg, usually short and stout, but in some groups it is greatly elongated in all legs (Mycetophiliformia]] in part) or in the forelegs only (Empididae). Each coxa connects proximally to the lateroventral region of thorax by two joints: the dorsal one is a process of the pleuron (coxifer), the ventral is the prosternum (forecoxa) or the furcasternum (mid- and hindcoxa).

The midcoxa is divided into two parts, the anterior one is called eucoxa, the posterior meron. The meron is strongly flattened and becomes a ventral sclerite of the mesopleuron, placed under the epimeron. The eucoxa divides further into two segments, called basicosta and disticoxa.

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The trochanter is a short segment joint to the femur and articulated to the coxa. Usually it does not show significant features from the taxonomic point of view.

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Acropod of Empididae
Fig. 2 - Scanning Electronic Microscope (SEM) of the acropod of a dance fly (Brachycera: Empididae).
Author: Louisa Howard (Dartmouth College)
Resized from the original picture
(License: Public Domain)

With the tibia, the femur is the largest segment of the leg and appears elongated and more or less enlarged. Usually there are integumental appendages as bristles or setulae, spines, processes, whose number and position are treated in taxonomic diagnosis.

Specific adaptations in the form, development, and integumental arms occur in the fore fomera of certain groups with adult predators, where they are transformed in raptorial legs. Other adaptations occur in males of various groups, that use the fore legs to grap the female during the mating. Finally, in some groups there are structures used to emit sounds.

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The tibia is the second largest segment, usually more slender of the femur and more or less elongated. Like the femur, in the tibia there are integumental appendages of taxonomic interest related to specific functions. The taxonomic descriptions refer widely to the presence and features of setulae and bristles near the distal end.

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The tarsus consists of several segments, called tarsomeres, and joins the distal end of the tibia. In the ground-plan there are five tarsomeres, with the proximal (basitarsus or metatarsus) is longer than the other distal. Only a few groups of Diptera have less than five tarsomeres. The males of some groups may have specific adaptation of the basitarsus as a secondary sexual character.

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Acropod of Drosophilidae
Fig. 3 - Scanning Electronic Microscope (SEM) of the acropod of a fruit fly (Brachycera: Drosophilidae).
Author: Louisa Howard (Dartmouth College)
Resized from the original picture
(License: Public Domain)

The acropod is the distal segment of the leg. It is structurally distinct from the tarsus but is morphologically associated and closed to fifth tarsomere. Due the distal position, the term pretarsus is not appropriate, whereas acropod or posttarsus are names more correct (McAlpine, 1981, Tremblay, 1982). The name "pretarsus" is still widely used in literature.

The structure and conformation vary throughout the order and are closely related to the functionality of the leg, because they are finalized to facilitate the movement or the standing on various surfaces. It is structurally articulated to fifth tarsomere by three sclerites: one unpaired and median, called unguitractor, and two lateral and symmetrical, called basipulvilli. These plates bear three organs usually present in most of flies and, more generally, of insects: pulvilli, claws, and empodium.

The pulvilli are two membranous lobes, more or less e expanded, and are connected to the basipulvilli. Instead, the claws are two processes more or less bent and connected by a membranous base to the unguitractor. The distal end of unguitractor bear a more or less membranous lobe, called arolium, which may extend with a median process called empodium. The empodium, not even present, is usually setaceous or, in primitive groups, lobe-shaped.

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Web resources

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Last update of this page: 8 January 2017
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