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Table of contents

Morphology of the cephalic capsule - Mouthparts (Labrum - Mandibles - Maxillae - Labial-hypopharyngeal complex - Cibarium and pharynx) - Tentorium - Antennae - Eye - Bibliography
Fig. 1 - Ventral view of a head of a chironomid, subfamily Orthocladiinae. Some elements are evident: labrum, mandibles, labium, stemmata and postoccipital carina of the genae.
Author: Andreas Plank (Home page)
Modified from the original picture at Wikimedia Commons
(License: Public Domain)
Head of chironomid

Eucephalic larvae are the primitive condition and recur in most Nematoceri. The head is feactured by a well developed cephalic capsule distinct from the thorax and it has mouthparts turned forward, with opposing mandibles which move on a horizontal plane. The anatomy of the cranium can show several specializations, however it has not significant structural mutations in the mouthparts and the endoskeleton.

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Morphology of the cephalic capsule

According to the original structure, the exoskeleton of the heas is composed by three sclerites: the medial dorsal frontoclypeal apotome and two lateral genae. These sclerites are separated by lines where the integument is less resistance and it cuts during the moulting (ecdysial lines).

The apotome is a trapezoidal o triangular region, narrow posteriorly. In some larvae it extends to the occiput and it narrows gradually between the genae (e.g. Ceratopogonidae). The ecdysial lines converge backwards without meeting in a midline, thus they resemble a U more or less closed. In other larvae, the apotome does not reach the occiput and the genae meet along the vertex (e.g. Bibionidae). In such larvae, le ecdysial lines have a posterior common stem that extends to the occiput, resembling a Y.

The homology of the frontoclypeal apotome to the frontoclypeal region of adults is not clear, because the ecdysia line are not true sutures (Teskey, 1981). The separation of the clypeus from the apotome is rarely evident, with a presence of a frontoclypeal suture, but usually the bound between these regions is not well defined in appearance. They can be distingueshed based on the insertion of the muscles which act on the cibarium and the pharynx.

The genae are the remnant of the cephalic head. With the frontoclypeal apotome they contribute to border the opening of the mouth. They are dorsally bounded by the ecdysial lines, whereas ventrally they join to each other or remain separated by a ventral ecdysial line. Due to lack of a ventral gular sclerite, the genae surround entirely the occipital foramen, except when the frontoclypeal apotome reaches it.

In the groundplan of Diptera, the genae form sclerotize thickening of the integument along the posterior and the ventral anterior edges. The thickening of the posterior margin is called postoccipital carina and is ofte juxtaposed by a slight groove called postoccipital sulcus. The posterior tentorial pits may be present at the lateral ends of the postoccipital sulcus. From the ventral view the anterior margin can appear with a similar thickening, called subgenal margin, where the mouthparts are articulated. The appearance of this region is various due to morphological and anatomical adaptation throghout nematocerous larvae.

The most prominent changes from the basic structure consist of simplification of the ventral part, which may have a reduced chitinization in transitional forms toward the hemicephalic condition, as a head more or less retracted within the thorax recurrent in many groups of Nematocera. The desclerotization involves mainly the posterior and ventral margins of the genae; it reaches its greatest evidence in larvae of Tipuloidea, where the postoccipital margin is incised in the dorsolateral portion, with the loss of the cuticle thickening and the reduction of the back of cephalic capsule in three or four sclerotized rods. Intermediate mutations which appear as reductions of the posterior portion of genae recur even in other families, as Tanyderidae, Axymyiidae, Cecidomyiidae (Teskey, 1981; Courtney et al., 2000).

A singular condition of eucephalic larvae is found in Blephariceridae, whose larvae have the cephalic capsule, the thorax, and the first addominal segment coalesced in a structure whitout apparent division, called cephalothorax.

The ventral portion of the cephalic capsule show strong differences among various nematocerous groups. The homologies of these specializations may be controversial. The ancestral condition could be similar to the ventral structure of the cranium of bibionid larvae (Teskey, 1981). In these larvae the ventral portion of the postoccipital margin extends forward involving almost the entire ventral side. Therefore the genae of Bibionidae larvae are nearly completely separed and joined by a narrow ventral connection called hypostomal bridge. The hypostome

In most eucephalic nematocerous larvae a broad desclerotization of the ventral side of the cranium occurs and it appears nearly membranous. The strong modifications do not allow a sure interpretatin of homology of a sclerotized plate, with the anterior margin toothed, present in the ventral part of the subgenal region of various nematocerous larvae (Teskey, 1981; Courtney et al, 2000). This element, referred with various names, was called hypostoma by Teskey (1981) and it should not be confused with the hypostoma bridge referred above. The hypostoma would be originated from the postmentum, whereas the hypostomal bridge (or ventral bridge or subgenal bridge sensu Courtney et al. (2000)) would have a subgenal origin, but those are only hypothesis.

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Mouthparts

Larval head of a bibionid
Fig. 2 - Diagrammatic representation of a bibionid larval head (dorsal view).
an: antenna; ap: frontoclipeal apotome; cl: clypeus; cp: postoccipital carina; ecd: ecdysial line; gn: gena; lbr: labrum; mnd: mandible; msc: maxilla; sp: postoccipital sulcus.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

Mouthparts of eucephalic larvae resemble the morphological and anatomical structure of the ancestral chewing type, with strong modification in single elements related both to feeding habit and habitat of larvae.

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Labrum

The labrum o upper lip is the anterior sclerite, visible both the dorsal and ventral view of the cephalic capsule. It is dorsally contiguous to the frontoclypeal apotome and in most Nematocera appears as a more or less broad and dorsoventrally flattened plate. The expansion in width is related to the functionality of mandibles, which move on a horizontal plane in most of nematocerous larvae. However in some groups it appears quite narrow when the mandible move on a oblique or vertical plane (Courtney et al., 2000).

The ventral side of the labrum is the epipharynx, which closes dorsally the mouth opening. Sometimes accessory structure are associated to epipharynx and they integrate the functionality of the labrum in some feeding habits. The labrum contributes in the taking of food or cleaning of the mandibles; the epipharynx is often covered by setae or hairs which are scattered or arranged in comb-like or brush-like structures. It can be also bear additional sclerites which offer the attachment point to special muscles of the labrum and epipharynx elements. Some of these sclerites have a primary origin and they are ancestral, whereas other are derived. The prominent are the following:

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Mandibles

With the labrum, the paired mandibles are the elements more evident of mouthparts of the larvae. In the eucephalic larvae they are placed behind and at the sides of the labrum and often they are visible both from the dorsal and ventral view. In their ancestral condition, the mandibles are 1-segmented sclerites, but they may be 2-segmented as derived condition (Teskey, 1981; Courtney et al., 2000). Cause of their mainly involvement in feeding, the mandibles vary in form and structure also considerably according to the ontogenic development, the feeding habits, and phylogenetic relationships.

About the feeding habits, the primary condition is the adaptation of mandibles both to piercing and to chewing (Teskey, 1981; Courtney et al., 2000). In nematocerous larvae the mandibles are thus stout and strongly sclerotized. They bear two lobes: the distal lobe have toothed margins and is adapted to incising and to piercing, whereas the proximal has a grinding function. The predatory larvae may loss this form and the mandible is usually slender and sickle-shaped (Courtney et al., 2000).

Each mandible articulates with the cephalic capsule by two condyles, the posterior is called hypocondyle, the anterior epicondyle. The position of the mandibular articulations may be imortant about the phylogeny, because it could have influenced the transition from the horizontal movements of the mandibles to the vertical, as all high Diptera.

In most eucephalic larvae, these articulations are in two levels, one dorsal, the other ventral. The dorsal point, with which the epicondyle articulates, is on the anterior margin of the gena, near the insertion of the anterior tentorial arm. From the outside it is placed where the front corner of the gena meets the base of the frontoclypeal apotome. The ventral articulation is on a lower level, on the subgenal margin. These positions form a hinge on a vertical axis, so the mandibles move on a horizontal plane opposing to each other. As well as other panorpoid orders, this feature recurs in various nematocerous groups, e.g. throughout Tipulomorpha, Bibionomorpha, and Axymyiomorpha infraorders. For this reason the movement of mandibles on a horizontal plane should be a plesiomorphic character of Nematocera (Wood & Borkent, 1989).

In some groups of Nematocera, more or less wide, an evolutionary trend, called rotation of the mandibles occurs (Teskey, 1981; Courtney et al., 2000) albeit with various degrees: the articulation of the epicondyle move inwards, whereas the articulation of the hypocondyle shift laterally, outwards and upwards. Because of that, the mandibular articulations are quite aligned at the same level and they form a hinge on a horizontal or oblique axis. The mandibles therefore move on a vertical or oblique plane. This structural condition reaches the highest expression in various groups of orthorrhaphous Brachycera and all cyclorrhaphous Brachycera. If the rotation of the mandibles may be assumed as apomorphic condition throughout the Brachycera, this interpretation is difficult for the Nematocera. The movement on a vertical plane occurs in Pyschodomorpha and the movement on an oblique plane occurs in more or less primitive lineages, as Ptychopteromorpha, Culicomorpha, and Blephariceromorpha. By studies of the nineties it can be deduced that the rotation of the mandibles should be plesiomorphic throughout the Diptera (Courney et al., 2000). Therefore this feature appears unreliable to find relationships of single lineages of Nematocera with the Brachycera.

Secondary elements associated with the mandibles are setae or hairs arranged in various combinations as rows, combs, and brushes. Among larvae that have opposing mandibles, a tuft of long hairs called prostheca is widespread. The prostheca is placed on a medial side of each mandible, near the molar surface. Its appearance suggests that the prostheca works a brush to clean the epipharynx (Teskey, 1981).

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Maxillae

Larval head of a bibionid
Fig. 3 - Diagrammatic representation of a bibionid larval head (ventral view).
cl: cardo; cp: postoccipital carina; e: epipharynx; ft: posterior tentorial pit; gn: gena; i: hypopharynx; ipo: posterior mandibular articulation (hypocondyle); mnd: mandible; msc: maxilla; msgn: subgenal carina; p: premandibles; pm: maxillary palpus; pn: hypostomal bridge; pre: prementum; pst: postmentum; sp: postoccipital sulcus; st: stype; t: torma.
Author: Giancarlo Dessì
(Licence: Creative Commons BY-NC-SA)

The maxillae are paired appendages less developed than the mandibles and having mainly secondary functions in feeding. In larvae of some groups they have the primitive structure of the chewing type, with a proximal cardo and a distal stype. The stype bears a maxillary palpus 1-segmented and sometimes the two distal lobes, the lacinia and the galea. However, in most Nematocera, the galea and the lacina are lost or fused in a single lobe.

The secondary function, generally, consists of a passive action expressed by the ventrolaterally closure of the mouth opening (Teskey, 1981). Furthermore the maxillae have a sensory function; this is poorly known in details, but maybe it is related to finding food substrate (Courtney et al., 2000). Usually the maxillae are almost entirely membranous, except the larvae of Sciaroidea, which have well sclerotized stypes with toothed medial margins. This allows these larvae to use the maxillary stypes as scaping organs to separate the food particles from the substrate (Teskey, 1981).

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Labial-hypopharyngeal complex

The labium or lower lip is the appendage with the wide range of variability. It is also the most changed compared to the ancestral condition of the chewing mouthparts. Often the interpretation of homologies it is very difficult and controversial, so a general description is almost impossible without considering the differences and the specializations among the various taxa.

The structure and the morphology that best resemble the ancestral condition occur in larvae of Olbiogaster genus (Anisopodidae). In these larvae, the labium is well developed and distinct from the other cephalic eleents. It is composed of a proximal segment, called postmentum, and a distal called prementum. The postmentum is divided in a basal submentum and a distal mentum. The prementum bears two distal lobes, the glossae, and two labial palpi. The larvae of other nematocerous groups, as Ptychopteromorpha and Bibionidae, have still a well developed labium, but with the submentum fused with the mentum to form the postmentum. In most Nematocera, however, the labium shows strong modifications related to morphological and structural adaptation of the ventral part of the cephalic capsule. The most prominent are the following:

These trends simplify the structure of the posterior mouthparts and make difficult the recognization of the original elements. Both these conditions in some groups, e.g. Culicomorpha and Tipulidae, whereas in other groups the prementum and the hypopharynx retain their morphological identity (e.g. Ptychopteromorpha and part of Blephariceromorpha and Bibionomorpha).

The postmentum, free or fused with the subgenal margin, closes the opening mouth posteriorly. Its anterior margin is often toothed, as for example in larvae of Culicomorpha.

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Cibarium and pharynx

The beginning of the digestive channel is composed of the preoral mouth cavity, usually called cibarium, the pharynx, and the esophagus. The structure of the cibarium and the pharynx is the same in all Nematocera, with some slight adaptations in various groups. A complex of muscles connects the dorsal walls of both cavities to the ventral surface of the frontoclypeal apotome. This complex consists of dilator muscles that by their action allow both the cibarium and the pharynx (or only the pharynx) to work as pump (Teskey, 1981; Courtney et al., 2000).

In many nematocerous larvae with aquatic habitat, the food consists of organic particles suspended in the water. The simple sucking would introduce too much water in the digestive trait, therefore the preoral-pharyngeal complex is integrated by a filtering system, called pharyngeal filter. The presence of a pharyngeal filter is a frequent feature, but it is not a general condition of Nematocera and it is not related to phylogenetic patterns (Courtney et al., 2000). Indeed, this structure recurs in almost all infraorders but it is not representative of their generality: the pharyngeal filter is present in all Ptychopteromorpha and Axymyiomorpha and in some families of Culicomorpha (Culicidae, Dixidae, and Thaumaleidae) and of Blephariceromorpha (Blephariceridae). It is also present sometimes in some families of Pyschodomorpha (Scatopsidae, Trichoceridae, Anisopodidae and part of Psychodidae). It lacks in all Bibionomorpha and Tipulidae, except the only Ula tipulid genus (Courtney et al., 2000).

The lack of structures similar to the pharyngeal filter in other panorpoid orders and the wide distribution among the Nematocera suggest a possible ancestral condition of this feature among the Diptera (Courtney et al., 2000). Therefore its presence would be a plesiomorphic condition at least in some clades (Wood & Borkent, 1989). The lack would be a secondary adaptation (Courtney et al., 2000), probably related to different contexts, as the transfer from aquatic to other habitats (e.g. all Bibionomorpha), the transition to a zoophagous diet (e.g. Corethrellidae and Chaoboridae), or the acquisition of a selective action in taking the food particles from a liquid (e.g. Ceratopogonidae and Chironomidae).

The pharyngeal filter consists of a set of setae arranged to form a filtering barrier. Its structure has been described, for example, by Wood (1981) referring to Axymyiidae: the apparatus comprises two long and narrow opposing plates, each of them bearing about 20 longitudinal ribs; each rib bear a row of parallel setae. Before Wood, other author described variants of the pharyngeal filter from other nematocerous groups, but the basic structure of this organ is the same in all Nematocera (Teskey, 1981; Courtney et al., 2000). The functioning is conceptually simple. First, with the esophagus closed, the contraction of the dilator muscles causes the expansion of the cibarium and the pharynx and induces a negative pressure which aspirates the water and the food particles suspended. In a second step, the dilator muscles stretch and, with the esophagus closed, the intrinsic pharyngeal muscles causes the contraction of the pharynx. The pressure forces the expulsion of the water from the mouth, but in the passage through the filtering barrier, the filter retains the food particles and allows their concentration.

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Tentorium

The tentorium is the prominent part of the endoskeleton of Insects. It is formed by an internal chitinized structure which gives rigidity to the cranium and the surface for the insertion of the muscles. It is present as plesiomorphic character even in eucephalic larvae of Nematocera, however it is usually reduced and simplified in various taxa, contextually with other adaptations of the structure of the cephalic capsule.

In the ancestral condition of the Insects, this structure consists of two pairs of rods, called tentorial arms, which arise from the oral ad occipital margins of the cephalic capsule:

The tentorium, generally resemble a X or H shape, more or less dorsally convex, with a possible presence of two dorsal processes beared by the anterior arms.

Beyond any hypothesis, the ancestral structure of the tentorium of Diptera is not known, due the wide distribution of its reduction and simplification (Courtney et al., 2000). A complex tentorium recurs only in first instar larvae of Psychodinae subfamily (Psychodidae) (Courtney et al., 2000) and in some larvae of Anisopodidae (Teskey, 1981), whereas its structure and conformatin are more or less simplified in all taxa and successive larval stages. A well developed tentorium, but lacking the dorsal processes, it is still present only in some families of Bibionomorpha and Pyschodomorpha (Courtney et al., 2000).

In most Nematocera, the tentorium is reduced to the paired arms only and the tentorial bridge is lost. In some groups it is reduced to the anterior pair and in extreme cases, the entire apparatus is vestigial and reduced to residues associated with the tentorial pits.

Authors presume, at least in many cases, that the reduction of the tentorium is related to the development of a reinforcement of the cephalic capsule identified by the expansion of the subgenal margin and the consequent formation of the hypostoma and the hypostomal bridge (Courtney et al., 2000). This trend would represent the transfer of the rigidity and the insertion of muscles to a different structure of the cephalic capsule, making the function of the tentorium less essential.

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Antennae

The insertion of the antennae is on the anterior angle of the genae, near the anterior mandibular articulation. In most nematocerous larvae, the antennae consists of simple appendage composed of 1-3 short segments and they are thus little prominent. The most representative of this reduction is the antenna of Bibionidae larvae, which appears as a small conical excrescence, similar to a pimple. Similar feature recur even in other familis of Bibionomorpha and in Anisopodidae, Ptychopteridae, and some Psychodidae.

For against, relatively long antennae are found in some Chironomidae larvae, where up to six segments may be present, and in some other families, where the segments are long and the antennae can appear well developed with a singular structure. Among the latter the larvae of Deuterophlebiidae, Chaoboridae, and Corethrellidae are the most prominent.

In Deuterophlebiidae larvae, the antenna is clearly 2-branched, with a basal elongate segment which ends in two branches, one longer than the other.

In larvae of Chaoboridae and Corethrella, the antennae are adapted to predatory habit and are transmormed into prehensile organs: they are elongated and bear a tuft of long curved bristles so they can used to retain a small prey.

Regardless of specific functional features in the two culicoid families, the antennae have usually a sensory function.

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Eye

The "eye" term is usually recurrent but it is inappropriate for insects larvae, because sensory organs similar to eyes of the adults are lack. The name stemmata (sing. stemma) is more correct.

The stemmata of nematocerous larvae have been described referring to a few families but they are widespread among the lower Diptera (Teskey, 1981; Courtney et al., 2000). They appear as small pigmented spots and are usually composed of a crystalline lens on the surface of the genae. Under this lens a group of sensory cells are present and their base is shielded by dark pigment granules, so the organ perceives the direction of the light.

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Bibliography

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