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

Table of contents

Introduction - Pseudocephalon - Cephalopharyngeal skeleton (Mouthhooks - Intermediate sclerite - Basal sclerite) - Atrium - Cibarium - Optic depression - Bibliography
Pseudocephalon of Cochliomyia hominivorax
Fig. 1 - Pseudocephalon of Cochliomyia hominivorax (Brachycera: Calliphoridae)
In foreground the two cephalic lobes, the mouthhooks, and the first thoracic segment studded by the typical spines.
Author: John Kucharski
Readapted from the original picture
(License: Public Domain as released by the USDA Agricultural Research Service)

The larvae of Cyclorrhaphous are inappropriately named "acephalic" but they have a head strongly reduced and almost completely retracted into the thorax. This condition is an exclusive feature of flies belonging to the monophyletic clade of Cyclorrhapha.

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Introduction

The acephalic larvae have a head structure greatly differentiated cause of drastic modifications, which also make it difficult to identify the omologies in most of the internal and external elements. The reason of this difficulty is the simplification of the histology and structure of the cephalic capsule and the mouthparts. This simplification occurs together with the strong dislocation of the cephalic elements into the thorax and the appearance of new anatomical and morphologica structures. These structures can be compared with those of other larval forms by functional analogy. The main changes which occurred through the evolution of the head structure of these larvae are the following:

1. Complete desclerotization of the integument and related adaptations of "internal" elements to carrying the skeletal function.

The morphological elements analogous to the cephalic capsule of the other larval forms lost the chitinization of the integument. The vestige of the head of acephalic larvae thus appears translucent and membranous and lost one of the prominent functions of the exoskeleton, as to provide the surface for the attachment of muscles. With this involution, these larvae present an important development of and endoskeleton, derived from the close integration of the initial trunk of the stomodeum, the tentorium, and some elements of the primitive mouthparts. This endoskeleton is usually referred as "cephalopharyngeal skeleton" to point out the fusion of the pharynx and other elements of the cranium (Teskey, 1981). Similar names are widespread even in other languages: apparato or scheletro cefalofaringeo in Italian (Servadei et al., 1972; Tremblay, 1991), squelette céphalopharyngien in French (Matile, 1993), Cephalopharyngealskelett in German (Hennig, 1973), esqueleto cefalofaringeo in Spanish (see Google Scholar). A different terminological setting has given by Courney et al. (2000): based on a critical approach about the morphological and anatomical changes from both ontogenetic and phylogenetic point of views, Author prefer to use a more generic term as cephaloskeleton. In any way it must be specified that this internal structure has been referred in literature with several names (Courtney et al., 2000).

2. Invagination of parts of cephalic segments into the primitive oral cavity.

Although the evolutionary and ontogenetic steps are not yet ascertained, parts of the primary cephalic segments shift into the primitive mouth opening due the invagination of their lateral and ventral portions. That is important since a secondary structure replaces the primary and drastically alters the positions of various elements derived from the segments. Furthermore it causes the appearance of morphological and anatomical elements de novo. Clear examples are the maxillary palps close to the antennae, the cibarium shifted into the thorax and posterior to a functional oral cavity, the atrium, which has not homologies in the other larvae, the position of maxillary elements anterior to mandibular, the presence of vestigial labrum and other external structures within the functional head. To understand the relevance of these morphoanatomical chances, we can imagine the head of Cyclorrhaphous larvae as the result of a double invagination: in addition to the retraction of the head into the thorax, which occurs also in most of hemicephalic larvae, the acephalic larvae appears with a retraction of part of the head within itself, shifting further back the primitive internal structures.

3. Involution and fusion of primary morphoanatomical elements with the appearance of secondary structures which are analogous but not homologous with those of other larvae.

This is the most complex perspective because of the difficult identification of origins and homologies. A clear example is the deep change of the maxillae: a part of these mouthparts has been involved in the dislocation described above and, probably, the maxillary segment has given rise to much of the functional head in the acephalic larvae: the maxillary palpi close to the antennae at the tip of the cephalic lobes may be explained by the fusion of the antennal and the maxillary segment. On the other hand, the homology of the mouthhooks of acephalic larvae to the mandibular hooks of the hemicephalic larvae is doubtful: while the hemicephalic mandibular hooks have a mandibular origin, it is believed that the acephalic mouthhoks are both mandibular and maxillary elements, although the question is not definitively resolved, due the difficulties inherent to the different approaches (Courtney et al., 2000).

Head of a calliphorid larva in frontal view
Fig. 2 - Diagrammatic representation of the head of a calliphorid larva . Frontal view
I, II: thoracic segments; a: antenna; lc: cephalic lobes; p: thoracic fold; pm: maxillary palpus; ps: pseudocephalon; s: thoracic spines; sp: anterior spiracles; u: mouthhooks.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

The complexity of these morphoanatomical adaptations and the difficulty in ascertaining the homologies has strengthened a composite specific terminology in literature. In many cases, authors adopted names which refer to analogies or supposed homologies, in other case new terms was conied ad hoc referring to structures de novo or with uncertain derivation from the primitive condition.

Most of the questions derives from the apparent discrepancies between different approaches in the studi (Courtney et al., 2000). The anatomic structure of the head varies throghout the tree larval instars. An ontogenetic approach, which refers to the morphoanatomical adaptations during the embryonic development, gives results that might seem incompatible with the conclusions achieved by approaches based on the comparative anatomy, especially trying to find the homologies. These problems were highlighted in the Manual of Palaearctic Diptera by Courtney et al. (2000). Authors gave a critical discussion using both the morphogenetic and comparative morphological approaches. However, their considerations remain in part unresolved due the poor documentation in literature: the works about the morphology and anatomy usually refer only to the third instar larvae, while those about the morphogenesis are limited to very few species. Then a substantial difficulty remains when trying to find a general agreement among the published works. The dissertations in the Manual of Nearctic Diptera (Teskey, 1981) and in the Manual of Palaearctic Diptera (Courtney et al., 2000) differ considerably also in the terminology, because the Authors followed different approaches, so Courtney et al. (2000) have often proposed more generic terms which refers to latest acquisitions or doubts on the real homologies of the cephalic structures of these larvae.

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Pseudocephalon

The pseudocephalon is the external portion of the head which comes out from the thorax. In literature it has often referred as "cephalic segment", even by Teskey (1981), but this name is inappropriate. The name "cephalic segment", in effect, should imply that the head originates from a single segment of the primary metamerism, whereas it derives by the fusion of at least six segments. For this reason, Courtney et al. (2000) prefer "cephalic region" or "pseudocephalon". The second name is more widespread in recent works.

Head of a calliphorid larva in lateral view
Fig. 3 - Diagrammatic representation of the head of a calliphorid larva . Lateral view
a: antenna; cr: oral ridges; lb: cephalic lobe; lc: labial lobe; p: thoracic fold; pm: maxillary palpus; ps: pseudocephalon; s: thoracic spines; sk: cephalopharyngeal skeleton; t: thorax; u: mouthhook.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

The pseudocephalon appears as a small cilindrical or conical region, translucent and completely membranous, that lies anterior to the thorax. The boundary between the pseudocephalon and the thorax is identified by a marked increase of the diameter in the dorsal side, corresponding to the fold which has caused the retraction of the head. The anterior part of the pseudocephalon is clearly bilobate and presents a depression which separates the cephalic lobes ("antennomaxillary lobes" sensu Teskey (1981)).

Each cephalic lobe carries at the tip two papilliform sensory organs. The dorsal, more internal with respect to the sagittal plane, is the antenna, the ventral and more lateral is the maxillary palpus. The antenna appears as bisegmented, with small dome over a basal ring. The maxillary palpus is button-like bearing sensorial papillae. Embryological and neuroanatomica studies pointed out that the most of papillae has a maxillary origin, but at least one or two have derived from the antennal segment (Courtney et al., 2000).

The prominent ventral morphological elements are the cephalic lobes and the facial mask, from which two mouthhooks protude.

The facial mask is a ventral depression, enterely membranous, which slopes until the functional mouth opening, making a preoral cavity. The mouth opening is a secondary structure derived from the invagination of part of cephalic somites. The anterolateral walls of the preoral cavity are the ventral and medial sides of the cephalic lobes whereas posteriorly is delimited by a membranous median lobe, named labial lobe, which protracts forward. The mouthhooks are closely associated to the facial mask; they lie in the membranous walls and come out anteriorly. The appearance, shape, and size of the facial mask vary widely in function of the species or the systematic groups or the larval instar. The most relevant feature occurs in sapropaghous larvae: the surface of the preoral cavity is covered by oral ridges which converge to the functional mouth opening and mark several grooves. These grooves facilitate the flow of liquid food toward the mouth opening (Teskey, 1981; Courtney et al., 2000). In addition to the oral ridges, the facial mask may bear other structures similar to combs, called cirri, whose function is not well clear at least in most of Cyclorrhaphous larvae (Courtney et al., 2000). Unlike the saprophagous type, the facial mask of phytophagous, predatory and parasitoid larvae is small and lacks the oral ridges.

Head of a calliphorid larva in ventral view
Fig. 4 - Diagrammatic representation of the head of a calliphorid larva . Ventral view
a: antenna; cr: oral ridges; lb: cephalic lobes; lc: labial lobe; m: facial mask with preoral cavity; pm: maxillary palpus; ps: pseudocephalon; s: thoracic spines; sk: cephalopharyngeal skeleton; t: thorax; u: mouthhook.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

In addition to the antennae and maxillary palpi, other sensory receptors are present. A pair of small sensory organs, named ventral organs, lies in the facial mask between the oral ridges and they are innervated by the maxillary nerve. Other sensilla lie on the labial lobe and are called labial organs.

As said above, there are not enough facts to well ascertain the homologies of these external. Formerly, some authors supposed that the pseudocephalon would partially originate from thoracic elements, but according to Courtney et al. (2000) there are not evidences which support this hypothesis. Therefore the pseudocephalon would be derived from these parts of the cephalic segments that are not involved by the invagination of the primary mouth opening. The dorsal portion of the pseudocephalon would be mainly derived by the maxillary segment, with the contribute of the acron, near the thoracic fold, and the antennal and mandibular somites (Courtney et al., 2000). These latter, according to Authors, would be involved marginally: the antennal segment has produced the antenna, the mandibular a narrow strip between the antenna and the maxillary palpus. All the rest would be of maxillary origin. Regarding to the wall and the oral ridges of the facial mask, Teskey (1981) and formerly other authors believed that these elements originate from the mandibular segment, but Courtney et al. (2000) do not agree because this hypothesis would be incompatible with the innervation of the ventral organs. They support the hypothesis that the facial mask is entirely maxillary according to embryologic and neuroanatomic evidences. Finally, studies about the morphogenesis and the innervation suggest that the labial organs may be homologised to the labial palpi and this would imply that the labial lobe could be homologous to the primitive prementum (Courtney et al., 2000).

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Cephalopharyngeal skeleton

The cephalopharyngeal skeleton is the ensemble of the scleorized structures of the Cyclorrhapha larvae. As the name implies, this skeleton is derived from the morphoanatomic and functional integration of generic cephalic elements and the wall of part of the stomodeum, in this case the pharynx. However, due to the complex structure and the composite origin, Courtney et al. (2000) proposed the name cephaloskeleton instead of "cephalopharyngeal skeleton". The adoption of "cephaloskeleton", although more appropriate, is not widespread and also in recent works, authors usually prefer the classic "cephalopharyngeal skeleton".

Cephalopharyngeal skeleton in lateral view
Fig. 5 - Diagrammatic representation of structure of the cephalopharyngeal skeleton in Schizophora larvae. Lateral view
a: atrium; ad: dorsal apodeme; av: ventral apodeme; b: parastomal bar; c: cibarium; cd: dorsal cornu; cv: ventral cornu; do: optic depression; e: epistomal sclerite; pd: dorsal bridge; ps: pseudocephalon; pv: vertical plate; sb: basal sclerite; sd: dental sclerite; si: intermediate sclerite; sl: labial sclerites; t: thorax; u: mouthhook.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

Except for the distal end of the mouthhooks, the cephalopharyngeal skeleton is internal and lies entirely within the pseudocephalon and, mostly, the thorax. Due to its dark pigmentation, that contrasts with the pale color of other parts, the skeleton may be perceived as a small dark spot near the anterior end of the body, specially in larvae with a translucent integument. With a light magnification it is possible to see the mouthhooks which protruded from the ventral side of the cephalic lobes.

In anterior-posterior order, the cephalopharyngeal skeleton is composed of a pair of mouthhooks, some intermediate sclerites, where an intermediate sclerite sensu stricto is the prominent, and finally the basal sclerite. With exception of the mouthhooks, which retain a closely anatomic relation with the wall of the preoral cavity, all the parts of the cephalopharyngeal skeleton are not joined with the integument, so they form a true internal skeleton.

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Cephalopharyngeal skeleton in dorsal view
Fig. 6 - Diagrammatic representation of structure of the cephalopharyngeal skeleton in Schizophora larvae. Dorsal view
a: atrium; bp: parastomal bar; c: cibarium; cd: dorsal cornua; cv: ventral cornua; f: pharynx; pd: dorsal bridge; ps: pseudocephalon; sb: basal sclerite; se: epistomal sclerite; si: intermediate sclerite; t: thorax; u: mouthhooks.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

Mouthhooks

The mouthhooks are sometimes referred also with alternative but inappropriate names (i.e. mandibles). They are the portion more visible of the cephalopharyngeal skeleton, cause of their shape and the external position of the distal ends. Strongly sclerotized, these organs are responsible of the myiasis, which are diseases caused by epizootic parasite larvae to vertebrates, men included.

The distal part is sickle-shaped, sharp and curved downward and more or less outwards. The ventral edge may be toothed (Teskey, 1981). At rest the mouthooks are almost completely retracted into two membranous cavities of the pseudocephalon, lateral to the facial mask, slightly inclined with respect to the axis of the pseudocephalon and more or less divergent from the sagittal plane. In some saprophagous sirphid larvae, the mouthhooks have lost the primitive function: in these larvae, the invagination of the cephalic segments into the pseudocephalon is more deep and the facial mask becomes a suction dut where the oral ridges and the cirri have a filtering function, whereas the mouthhooks act as support of the lateral walls (Teskey, 1981; Courtney et al., 2000).

The basal part appears as more stout, it enlarges and takes a triangular or quadrangular shape to the lateral view. The posterior margin of each mouthhook articulates with the corresponding anterior arm of the intermediate sclerite, forming a joint similar to that of the epicondile with the tentorial fragmata in the hemicephalic larvae. In fact the abductor and adductor muscles act on corresponding apodemes arranged at the basal end of the mouthhooks, dorsal and ventral with respect the joint. The abductor muscle is directly attached to the dorsal apodeme, whereas the adductor, in many Schizophora, is attached to a small ventral accessory sclerite, named dental sclerite, firmly joined to the ventral apodeme. The dental sclerite would be primitively missing in some lineages of the paraphyletic Aschiza group (Courtney et al., 2000). The posterior ends of the abductor and adductor muscles are attached to the external sides of the ventral cornua of the basal sclerite. The entire skeletal-muscular complex works therefore as a robust motor system which gives to the mouthhooks abduction and adduction movements in a vertical plane. The fulcrum of this lever is the horizontal axis that joins the anterior ends of the intermediate sclerite.

The structure as described above has several variations, among the Cyclorrhapha, in shape, size, and anatomic structure (Courtney et al., 2000). In general, all the first instar larvae have small and weakly sclerotized mouthhoks, but the mouthhooks may vary also in function of the systematic group. For example, the mouthhooks may be totally or partially fused one to each other; one of two may atrophy reducing the number to a single mouthhook (some Agromyzidae); in viviparous forms both the mouthhooks are atrophic. On the contrary, predacious larvae may have slender and well developed mouthhooks, sometimes composed of accessory sclerites. Finally, the mouthhooks may lose the primitive function: in addition to the syrphid larvae described above, larvae of some lower Cyclorrhaphous (i.e. Lonchopteridae) have mouthhooks strongly reduced and their function is surrogated by labial sclerites.

A curious feature of the mouthhooks is the use as rough movement organs: the larva bends and clasps the posterior end with the mouthhooks, then sharply contracts the muscles releasing the grab. So the reaction on the support allows to jump up. This behaviour occurs when the larva is disturbed or brought out from their habitat. For example, the larvae of the cheese skipper, Piophila casei Linnaeus, 1578 (Brachycera: Piophilidae), are able to perform jumps of few centimeters.

The homology of the mouthhooks to the mandibles of the orthorrhaphous larvae is doubtful, cause of a possible double maxillary and mandibular origin in the acephalic larvae (Courtney et al., 2000). The embryological studies on Drosophila melanogaster Meigen, 1830, support the hypothesis that the mouthhooks have derived from the maxillary segment. However, the classic compared anatomical approach shows the similarity of the basal part between the mouthhooks of acephalic larvae and the mandibles of the orthorrhaphous. Only a presumed mixed origin can explain the apparent discordance between the embryological and morphological approaches (Courtney et al., 2000).

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Cephalopharyngeal skeleton in ventral view
Fig. 7 - Diagrammatic representation of structure of the cephalopharyngeal skeleton in Schizophora larvae. Ventral view
a: atrium; c: cibarium; cd: dorsal cornua; cv: ventral cornua; f: pharynx; ps: pseudocephalon; sb: basal sclerite; sd: dental sclerites; si: intermediate sclerite; sl: labial sclerites; t: thorax; u: mouthhooks.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

Intermediate sclerite

As primitive condition of the Cyclorrhaphous larvae, a single sclerite composes the part of the cephalopharyngeal skeleton posterior to the mouthhooks. This feature occurs in all larvae of the lower Cyclorrhapha (the paraphyletic Aschiza group) and in the first instar in most of Cyclorrhapha (Courtney et al., 2000).

Throughout the Schizophora, the second and third instar larvae have a posterior cephalopharyngeal skeleton divided into two distinct sclerites. The anterior is referred by Courtney et al. (2000) as intermediate sclerite or H-shaped sclerite. This sclerite was formerly referred with various names, for example "hypostomal sclerite" (Teskey, 1981), but these names were inappropriate because presume uncertain homologies.

From the dorsal and ventral views, the shape of the intermediate sclerite resembles a H, with two anterior longitudinal arms, two posterior, and a transverse bar. The anterior arms articulates with the mouthhooks, the posterior articulates with the anteroventral part of the basal sclerite. Behind the transverse bar there is the salivary duct opening.

The intermediate sclerite gives also lateral and ventral support to the atrium, but in this function it is helped by small ventral accessory sclerites, named labial sclerites. These sclerites are ventral and anterior to the intermediate sclerite and vary in number and shape. They gives support to the anterior part of the atrium. Dorsal support is given by other sclerites, which lie above the intermediate sclerite and are called parastomal bars and epistomal sclerite. The parastomal bars are two longitudinal rods, posteriorly contiguous with the basal sclerite, the epistomal sclerite is a transverse bar that lies in front to the parastomal bars and usually is fused with their tips. In some larvae of Syrphidae and many first instar larvae, in front to the parastomal bar there is a vestigial labrum.

Studies about the morphogenesis in Drosophila melanogaster and the exam of the neural connections highlight that these sclerites have a complex origin. Courtney et al. (2000) presume that they are de novo structures, without homologies in hemicephalic larvae. The origin is mixed because they would be derived from the three postoral somites (mandibular, maxillary, and labial segments).

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Basal sclerite

Totally retracted into the thorax, the basal sclerite is the most prominent part of the cephalopharyngeal skeleton, posterior to the other. Teskey (1981) named it "tentoropharyngeal sclerite", but this and other former names are rejected by Courtney et al. (2000), because of presumed pharyngeal derivation. Based on embryological, neuroanatomical and morphoanatomical evidences, these authors chose a neutral denomination and proposed in the Manual of Palaearctic Diptera the name "basal sclerite".

The basal sclerite is a considerable internal element composed of two symmetrical sclerotized structures, more or less closely joined, which combined with dilator muscles acts as suction pump. Furthermore, the basal sclerite give the surface for the attachment of the mandibular and the dilator muscles of the cibiarium. It is an anatomical adaptation which includes elements derived from the primitive mouthparts, tentorium, and the wall of the initial part of the alimentary canal. The latter is identified as the pharynx (Teskey, 1981) or as the cibarium (Courtney et al., 2000).

Although it varies in shape and size, the basal sclerite is composed of two dorsal arms ventrally fused with two ventral arms. From the lateral view, this sclerotized structure resembles a U rotated by 90 degrees clockwise, with a deep incision of the posterior profile.

The dorsal arms appears as two vertical parallel plates, posteriorly expanded and divergent from the sagittal plane. They are named dorsal cornua (Teskey, 1981, Courtney et al., 2000). At the anterior end the dorsal cornua are joined by a weakly sclerotized bar called dorsal bridge. The morphology of the ventral arms, named ventral cornua (Teskey, 1981, Courtney et al., 2000), is more complex and variable among the different systematic groups. According to Courtney et al. (2000), based on morphoanatomical and embryologica evidences, the ventral cornua enclose the cibarium. In Schizophora larvae, the anterior end articulates with the intermediate sclerite and is contiguous with the parastomal bars, whereas in larvae of lower Cyclorrhaphous and all first instar larvae, the intermediate e and basal sclerites are fused.

Each dorsal cornu is joined to the corresponding ventral cornu by a vertical plate (Courtney et al., 2000) derived from the tentorium by a strong adaptation of the structure. During the embryonic development, the anterior tentorial arms shift backwards under the posterior arms and they blend with the ventral cornua, whereas the posterior arms are fused with the dorsal cornua (Courtney et al., 2000).

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Atrium

As mentioned above, one of the prominent features of the acephalic larvae is the invagination of part of cephalic elements and this causes the dislocation of the primitive oral cavity into the thorax. The facial mask is contiguous with a functional oral cavity named atrium, which has not homologies in the larvae of Nematocera and lower Brachycera.

The atrium is a cavity with membranous wall that extends, within the pseudocephalon, from the functional mouth opening, in the facial mask, to the primitive mouth opening, which corresponds to the beginning of the cibarium. Its ventral wall is anteriorly supported by the labial sclerites and posteriorly by the transverse bar of the intermediate sclerite. The dorsal wall is supported by the epistomal sclerite and parastomal bars. These sclerites give also the attachment to the muscles of the atrium, that act as dilator of the cavity. The negative pressure allows the suction of food liquid carried by the oral ridges into the mouth opening. The flow of nutrients arrived in the atrium is then aspirated by the cibarial pump.

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Cibarium

No boundary between the cibarium and the pharynx is well defined. These cavities are contiguous and the anatomical distinction is arduous. Moreover authors gave discordant references about the position of the cibarium and the pharynx. Teskey (1981) and former authors stated that the pharynx would lie within the cephalopharyngeal skeleton due the fusion of its wall with the ventral cornua of the basal sclerite. This explains the classic name given to the skeleton. Courtney et al (2000), referring to interpretation of some former authors and the embryological and anatomical evidences, have identified the cavity within the basal sclerite as the cibarium. This claim is particularly based on the position of the frontal ganglion and the morphogenetic origin of some ventral elements of the basal sclerite (Courtney et al., 2000):

The cibarium is the true oral cavity, homologous to the mouth of the other larvae. According to Courtney et al. (2000), this cavity has totally shifted into the thorax and its wall is fused with the ventral cornua. The floor and roof must be identified respectively as the hypopharynx and the epipharynx, that in the acephalic larvae become interal structures.

Cibarial filter
Fig. 8 - Diagrammatic representation of the cibarial filter in saprophagous Cyclorrapha larvae.
CD: dorsal chamber; EP: epipharynx; IP: hypopharynx; VC: ventral cornua; c: ventral canals; cr: ventral cibarial ridges.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

As the other dipteran larvae, le cibrium works as suction pump due a stout extrinsic muscle which causes its dilatation. The dilator muscle extends from the epipharynx to medial surfaces of the dorsal cornua (Courtney et al. (2000)).

In saprophagous larvae, the dorsal surface of the hypopharynx bears several parallel ridges, called cibarial ridges or T-ribs, which protrude towards the lumen of the cibarium. The tip of each ridge bears thin lateral expansions which give to the section a profile that resembles a T or a Y and touch the expansion of the adjacent ridges. Thus the cibarial cavity is divided into two spaces: a dorsal chamber functions as alimentary canal and several ventral grooves, delimited by the ridges, serves as draining ducts. This structure forms a filtering apparatus similar in function to the cibarial filter of the Stratiomorpha hemicephalic larvae and the pharyngeal filter of the Nematocera eucephalic larvae: due the dilator muscle, the dorsal chamber expands and the negative pressure draw the food liquid from the atrium, then the constrictor muscles push the excess liquid into the ventral canals whereas th T-ribs hold the food particles.

The cibarial filter is absent in phytophagous, preacious, and parasitoid larvae (Courtney et al., 2000).

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Optic depression

The stemmata of the acephalic larvae differ from those of other larvae in both the position and structure. The photoreceptors lie into the optic depressions, that are two sockets of the anterior-lateral portion of the basal sclerite. The anterior margin of the basal sclerite has a concave profile, below the dorsal bridge, corresponding to the optic depressions. Therefore the "eyes" of the acephalic larvae are internal sensorial organs within the thorax and they functions due the membranous and translucent integument.

The photoreceptors of the stemmata of acephalic larvae lack the retinula pigment, however direction of the light is allowed by the dark background given by the sclerotized wall of the optic depression (Courtney et al., 2000).

The stemmata of Cyclorrhapha are usually referred as visual cells (Teskey, 1981; Courtney et al., 2000), but those of Schizophora are differently named Bolwig's organs, because their structure is different from those of the ommatid, due the missing of the rabdome (Courtney et al., 2000).

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Bibliografia

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