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Table of contents

Morphology of the head capsule - Mouthparts (Labrum - Mandibles - Maxillae - Maxillar-mandibular complex - Labium and hypopharynx - Cibarium and pharynx) - Tentorium - Antennae - Eye - Bibliography
Neopachygaster maculicornis larval head
Fig. 1 - Head of the larva of Neopachygaster maculicornis (Hine, 1902) (Brachycera: Stratiomyidae). Dorsal view.
Author: Charles T. Greene (1917)
Modified from the original plate
(License: Public Domain)

The larvae which have a cephalic capsule partially retracted within the thorax are called hemicephalic. The retraction is usually not permanent, thus the capsule can be protuded or retracted by muscles.

Although their presence also in nematocerous groups, more or less prominent, the hemicephalic larvae are widespread in all lower Brachycera, in which the head capsule is strongly reduced and more or less retracted within the thorax. For this reason the hemicephalic larvae may be called cryptocephalic also (Tremblay, 1991).

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Morphology of the head capsule

The exposed portion of the head has usually a conical shape, moro or less thin and tapered forward.

A transition between the eucephalic and hemicephalic types, in lower Brachycerous can be found in the Stratiomyidae family. In this group, the larvae have not a retractile head, but this is permanently embedded within the thorax for a third or half (James, 1981b; Rozkošný, 1997). The cephalic capsule is well developed and sclerotized, specially the genae, which appear elongated and extended forward (Courtney et al., 2000). The occipital foramen is rather broad (Teskey, 1981a; James, 1981b) and this feature distinguishes the cranium of Stratiomyidae larvae from the eucephalic type.

The larvae of Xylophagidae are closer to the hemicephalic condition than Stratiomyidae: as the larvae of Stratiomyidae, they have a head embedded within the thorax (James, 1981a; Nagatomi & Rozkošný, 1997) with the exposed portion well sclerotized and genae extended forward (Courtney et al., 2000), but the posterior part is reduced to two metacephalic sclerites. These sclerites are not homologous to those of other lower Brachycera.

Tabanoidea larval cephalic capsule
Fig. 2 - Diagrammatic representation of the cephalic capsule of a Tabanoid larva (Brachycera: Tabanoidea). Frontolateral view.
a: antennae; ap: frontoclypeal apotome; g: gena; i: hypopharynx; lb: labrum; m: maxilla; p: maxillary palpi; sp: mandibular brushes; t: thorax; u: mandibular hooks.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

In larvae of Tabanidae the desclerotization involves the posterior-lateral part of the cranium and keeps small sclerites apparently isolated but joined by a transparent cuticle (Teskey, 1981a).

In larvae of Rhagionidae and Vermileonidae the head capsule is reduced to a convex dorsal plate widely open in the posterior and ventral sides (Teskey, 1981a).

However, in many families of lower brachycerous closely related to cyclorrhaphous Brachycera the trend to the reduction of the cephalic capsule appears with a desclerotization along the medial dorsal part, which is usually divided into two sclerites more or less thin and elongated and joined by a transparent medial cuticle. This structures are often called metacephalic rods because of their shape. The widespread condition, with two rods joined by a medial transparent cuticle, recurs among various families of Asiloidea (Apioceridae, Mydidae, Asilidae) and Empidoidea (Empididae, Dolichopodidae) (Teskey, 1981a). Among the Asiloidea, in Therevidae and Scenopinidae larvae the two metacephalic rods are reduced to an single undivided sclerite (Teskey, 1981a).Another morphological featur is found in Dolichopodidae larvae, Un'altra caratteristica morfologica ricorre invece nelle larve dei Dolichopodidae, whose metacephalic rods have the wide posterior ends (Teskey, 1981a; Courtney et al., 2000; Tremblay, 2005).

The ventral side of the cranium shows genae clearly separated and usually with the interposition of a flattened sclerite referred by authors as submentum or ventral plate. Teskey (1981a) identified this structure as submentum, whereas Courtney et al. (2000) called it ventral plate, although they believe it has a labial origin. The presence of the ventral plate is referred for Stratiomyidae (Teskey, 1981a) and, at least some families, for Tabanoidea and Asiloidea (Teskey, 1981a; Courtney et al., 2000). In Xylophagidae larvae, the ventral plate is fused with the dorsal part of the cranium forming a conical capsule with the mouthparts extruded from a narrow anterior opening (Courtney et al., 2000).

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Mouthparts

La struttura dell'apparato boccale risente delle profonde modificazioni anatomiche e funzionali associate alla rotazione delle mandibole. È possibile ancora riconoscere l'origine di alcuni elementi, ma la morfologia e la struttura anatomica sono marcatamente differenti rispetto a quelle descritte per le larve eucefale.

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Tabanoidea larval cephalic capsule dorsal view
Fig. 3 - Diagrammatic representation of the cephalic capsule of a Tabanoid larva (Brachycera: Tabanoidea). Dorsal view.
a: antenna; ap: frontoclypeal apotome; g: gena; lb: labrum; m: maxilla; p: maxillary palpus; sp: mandibular brush; t: thorax; u: mandibular hook.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

Labrum

Compared with the labrum of eucephalic larvae, that of hemicephalic is clearly reduced in size and shape, has a different dislocation and loses in mobility. The rotation of mandibles causes the shift of the mandibular articulations which affects the proportions of the dorsal parts of the head capsule: the clypeal region appears generally reduced in witdh and the base of the labrum is narrow.

The labrum of most lower Brachycera is stiff and strongly sclerotized. It appears as a narrow and elongated rostrum bent downwards. It remains enclosed between the mandibles and usually it is visible at the lateral view after the removal of maxillae and mandibles. It is dorsally fused with the frontoclypeal apotome, of which it appears as a prolongation, and it is internally fused with the tentorial phragmata. In some groups, the labrum bears cuticular elements as teeth and spines.

Its ventral and posterior surface is sclerotized and forms a structure called epipharyngeal plate. Before the epipharyngeal plate there is a hollow contiguous with the cibarium.

A condition of all Brachyera is the lack of the additional sclerites associated with the labrum widespread in the eucephalic larvae, as the premandibles and the tormae (Teskey, 1981a; Courtney et al., 2000).

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Mandibles

As said above, the mandibular and maxillary appendages are strongly modified due the reduction of the head capsule and the shift of mandibular articulation. These anatomical changes are combined with the functional adaptation due the feeding habits of orthorrhaphous larvae. The diet of Nematocera larvae is usually scavenger, often combined with the mycophagy or the microvory, or phytophagy as secondary adaptation. The zoophagy is sporadeic because it recurs in few taxa. Instead, the zoophagy is the most widespread habit among the larvae of orthorrhaphous Brachycera, through the parasitoidism and especially predation, usually versus other arthropods. It is possible that the zoophagy was derived by the transition from aquatic to terrestrial habitats and the increased complexity of terrestrial biocenosis during the Jurassic, the period when the radiation of most orthorrhaphous clades occurred. Regardless of the phylogenetic relationships, in larvae of most of lower Brachycera we can find the anatomical and functional adaptation of mandibles to zoophagy and homoplasies, if not homologies, are prominent.

Asilidae larval head
Fig. 4 - Diagrammatic representation of the head of a asilid larva (Brachycera: Asilidae). Lateral view.
a: antenna; g: gena; lb: labrum; m: maxilla; sb: basal mandibular sclerite; t: thorax; u: mandibular hook; pv: ventral plate.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

In this general trend of lower Brachycera, the clade of Stratiomyomorpha is the exception. because it includes families whose larvae are scavenger (Stratiomyidae), xylophagous or xylosaprophagous (Pantophthalmidae), or mainly saprophagous (Xylomyidae). The larvae of this group show many analogies in the anatomical structure of the head with the other lower Brachycera, but they differ for the maxilla coalesced with the mandible. This feature recurs also in some families of other lower Brachycera, whose larvae have maxillae closely associated with the mandibles, but the coalescence is incomplete and allows the distinction of the mouthparts (Teskey, 1981a).

In most of zoophagous hemicephalic larvae of lower Brachycera, the mandible is divided into two segments. The proximal is called basal mandibular sclerite, the distal is the mandibular hook.

The basal mandibular sclerite is a internal structure U-shaped and is essential to the functionality of the mandible. The posterior margin of its aboral arm bears two apodemes. The dorsal is the abductor apodema, the ventral is the adductor (Teskey, 1981a; Courtney et al., 2000). These apodemes offer the insertion of the abductor and adductor mandibular muscles. The adductor apodeme is also connected to the base of the mandibular hook. The basal mandibular sclerite also bears the mandibular articular processes: the epicondyle is placed on the posterior margin of the internal arm, between the junctions of the apodemes, and its articulates with the tentorial phragma; the hypocondyle is placed on the posterior margin of the external arm and it articulates with the anterior margin of the cranium.

The presence of two mandibular articulation, widespread among the nematocerous larvae, in the lower Brachycera it is a transitional condition. In some taxa related to cyclorrhaphous Brachycera, especially many Asiloidea and Empidoidea families, the larvae lose the hypocondyle, that is the groundplan of higher Diptera. According to Teskey (1981a), the reduction to a single articulation would result from the close anatomical association of the maxillae and mandibles: due the evolutionary changes, the mandibles are enclosed between the labrum and the maxillae and by the connection of the maxillae to the cranium, the movements of the mandibles are bound to a vertical plane. This adaptation would remove the need for a double articulation of the mandible.

Asilidae larval head dorsal view
Fig. 5 - Diagrammatic representation of the head of a asilid larva (Brachycera: Asilidae). Dorsal view.
a: antenna; g: gena; lb: labrum; m: maxilla; sb: basal mandibular sclerite; t: thorax; u: mandibular hook.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

The mandibular hook appears as a blade bent downwards, weakly sclerotized, more or less pointed at the end, and usually not longer than the maxilla. Dorsally it articulates with the basal mandibular sclerite. Its shape clearly denotes the piercing function combined with predation. The posterior margin is often toothed, but the prominent and widespread feature is a longitudinal groove along the aboral side: when the hooks are juxtaposed, their grooves form a duct which acts as sucking food channel. According to Courtney et al. (2000), due its wide spread, this feature is in the groundplan of Brachycera.

Regardless of the strong differences that recurs in larvae of Stratiomyomorpha, the structure of mandibular hooks shows some variants. The main recur in larvae of Tabanomorpha.

The longitudinal groove on the internal side is lack in some families of Tabanoidea (Courtney et al., 2000). Larvae of these families have an anatomical adaptation which partially surrogates the function of the food canal: the hook is hollow because it includes a internal duct which opens on the anterior margin near the tip. This canal, called poison duct, is connected at the base to a poison gland which releases a substance that paralyzes the prey when the hooks pierce its cuticle (Courtney et al., 2000).

Another feature unique of Tabanomorpha is the mandibular brush. This element is composed by a tuft of strong spines, placed on the dorsal side of the internal arm of the basal mandibular sclerite, near the attack of the abductor apodeme, usually at the end of a short sclerite. When the mandible is at rest, the spines are reclined and covered by a fold of the integument. During the adduction, the brush slips from the cuticular fold and extends forward with the spines pricked. The function of the brush was observed in larvae of Rhagio (Rhagionidae): the spines straighten during the piercing with the adduction and probably help the penetration of the capsule head and its anchorage within the prey (Teskey, 1981a; Courtney et al., 2000).

Another changes among the lower Brachycera may be the subdivision of the mandibular sclerites into further segments or, instead, their fusion. In Empidoidea, the basal sclerite and the hook are connected by two small sclerites or four in some Empididae (Courtney et al., 2000). A different division is referred by Teskey (1981a) also in Tabanidae larvae: the basal sclerite appears divided into two elements closely joined, on which the sclerite that bear the mandibular brush is articulated. The fusion of the mandibular sclerites is found in Acroceridae larvae and, perhaps, in the related Nemestrinidae (Courtney et al., 2000).

The movements of mandibles are given by abductor and adductor muscles and the system acts as a lever which has the fulcrum on the epicondyle and, if present, the hypocondyle (Fig. 6). The adductor muscles work on the ventral apodemes and cause the apposition and the movement downwards of the hooks, making them as piercing organ (adduction). The abductor muscles, instead, work on dorsal apodemes and cause the uplift of the hooks with a forward rotation (abduction).

Mandible movements
Fig. 6 - Diagrammatic representation of the mandibles movements in hemicefalic larvae: Abd: abduction; Add: adduction.
e: epicondyle; fr: tentorial phragma; m ab: mandibular abductor muscle; m ad: mandibular adductor muscle; sb: internal arm of the basal mandibular sclerite; u: mandibular hook.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

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Maxillae

The homology of the maxilla is clearly defined by its distal part, whereas the basal is hardly recognizable cause of the close association with the basal mandibular sclerite. In the hemicephalic larvae, the cardo loses its identity so in literature it has sometimes confused with mandibular elements (Courtney et al., 2000). The stipes is recognizable by the insertion of the maxillary palpus and the presence of the distal lobes (galea and lacinia). As a whole, the maxilla appears as a lateral appendage, usually more or less as long as the mandibular hook. It is largely membranous except sometimes the distal lobes. The basal portion is closely joined to the external arm of the basal mandibular sclerite. This connection allows the maxilla and mandible to move together in abduction under the action of the abductor muscle (Teskey, 1981a; Courtney et al., 2000). The unique maxillary muscle retained by the orthorrhaphous larvae is the adductor, which acts on a specific apodeme attached to the base of the lacinia.

The maxillary palpus is usually the element most visible, variously shaped throghout the group. It may be composed of three segments (e.g. Tabanidae and Rhagionidae) or, in many families, a single segment. Also shape and size may vary: from thin and elongated in Athericidae, to wide and stubby in Vermileonidae, or reduced to a small conical protuberance in Empidoidea (Teskey, 1981b; Courtney et al., 2000).

The distal lobes may be more or less distinct, with various appearance in different groups (Teskey, 1981a; Courtney et al., 2000). The internal lobe, the lacinia, can have the internal side adherent to the mandibular hook and forms a sheath or a groove to protect it. The tip may bear setulae or spines otherwise it is strongly sclerotized and shovel-shaped. The external lobe, the galea, is usually directed forward and may bear setulae or spines at the apex. As the case, these features give different functions of the distal lobes: protect the mandibular hooks and clean them sometimes, work as fossorial organs that help the movements in the soil, work as sensory organs which allow to detect the prey and find the point of attack, etc.

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Maxillar-mandibular complex

The maxillar-mandibular complex is exclusive of the larvae of Stratiomyomorpha. In these larvae, the mandible and maxilla are fused in a single anatomical and functional apparatus, in which some elements are still recognizable. The only significant difference, which occurs moreover among the Xylophagidae only, is the shift of the hypocondyle in a more internal position.

The distal part is composed by a structure derived from maxillary elements cause the lost of the mandibular hook. In the distal part bears the maxillary palpus and a membranous organ which ends with a set of hairs. This structure works as a brushe which carries the food into the mouth and is a clear adaptation to a not zoophagous habit.

The movements are given together by mandibular and maxillary muscles. The mandibular muscles retain the same insertions with the basal mandibular sclerite as described above, so acts as abductor and adductor muscles of the maxillary-mandibular complex, with the fulcrum formed by the horizontal axis which connects the mandibular condyles. The third muscle acts with a special adaptation: the maxillary apodema passes into the basal sclerite and works as a pulley. The contraction of the maxillary adductor muscles makes therefore a traction of the distal part towards the base of the maxillary-mandibular complex (Courtney et al., 2000).

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Labium and hypopharynx

Except only the Empidoidea, the larvae of all lower Brachycera have a membranous and very reduced labium, of which only the labial palpi are retained. These are poorly developed and hidden by the mouthparts (Courtney et al., 2000).

In most of hemicephalic larvae, the hypopharynx appears as elongated sclerite which extends forward beyond the tentorial phragmata and forms the floor of the oral cavity.

The larvae of Empidoidea have a sclerotized bacilliform complex derived from the union of the hypopharynx and prementum bearing external labial palpi.

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Cibarium and pharynx

In hemicephalic larvae, the cibarium and pharynx together form an elongated chamber which extends from the oral opening to the esophagous. The distinction between the cibarium and the pharynx is not well defined and different interpretations have been made about, based on the dislocation of the dorsal dilator muscles. Teskey (1981a) adopted a criterion analogous to that used to identify the cibarium in eucephalic larvae of Nematocera. According to this criterion, the cibarium is the cavity bounded by the epipharynx and the hypopharynx and associated with dilator muscles inserted on the ventral surface of the frontoclypeal apotome. A different criterion was adopted by Courtney et al. (2000) and formerly followed by some Authors. This criterion is based of dislocation of muscles relative to the frontal ganglion: the cibarium would be the cavity associated with dilator muscles placed before the ganglio. This statement makes the cibarium longer than the Teskey interpretation, because it extends posteriorly beyond the base of the hypopharynx.

Regardless of these interpretation, the cibarium-pharyngeal complex is formed by a long cavity with a sclerotized floor and an elastic dorsal wall, on which the dilator muscles acts. This structure allows the apparatus to function as a sucking pump which takes a fluid food, as in the eucephalic larvae of Nematocera. However the hemicephalic larvae of lower Brachycera show some anatomical and functional differences.

First of all, the lower Brachycera differ in the food filtration. In most of orthorrhaphous larvae, the zoophagy does not require a filter, being the food itself concentrated. An exception is the group of Stratiomyomorpha, whose larvae are generally saprophagous or xylophagous. These larvae have a cibarial filter which is analogous to pharynceal filter of nematocerous larvae. It is likely that the two filter are not homologous (Courtney et al., 2000) cause of their different dislocations. The cibarial filter of Stratiomyomorpha larvae is composed by two longitudinal ridges that extend along the floor and divide it into three draining channels. The ridges are covered by setulae which works as filtering combs, holding back the food particles when the excess liquid is pushed out by the cibarial-pharyngeal pump. Another anatomical feature in larvae of this group is a grinder system with a complex conformation, called grinding mill: the floor of the pharynx is shaped as a mortar, with a rigid wall strengthen by a chitinized thickening; the dorsal wall form a process similar to a pestle which acts on the mortar (Teskey, 1981a; Courtney et al., 2000).

A second difference between nematocerous and some lineages of lower Brachycera is the close anatomic relationship between the endoskeleton and the pharynx. In larvae of Stratiomyidae, Vermileonidae, Empididae, and Dolichopodidae families, the wall of the pharynx is fused with the tentorial phragmata. According to Teskey (1981), this adaptation could be the prelude to the full integration of the cephalofaryngeal skeleton, that is the apomophic condition of the cyclorrhaphous Brachycera.

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Tentorium

In lower Brachycera, the cephalic endoskeleton of hemicephalic larvae is formed by the tentorium sensu stricto and two dorsal extensions of its structure called tentorial phragmata. This structure is important because allows functional and anatomic connections to mouthparts in larvae which have a reduced exoskeleton. About the evolutionary aspect, this close connection between the endoskeleton and the mouthparts reaches the highest expression with the cephalopharyngeal skeleton of acephalic larvae in higher Diptera.

The tentorial phragmate are internal structures not exclusive of the hemicephalic larvae, because similar structures would also be present in eucephalic larvae of some Nematocera: Teskey (1981) has referred about invaginations of the integument from the clypeal margins in some nematocerous larvae, called with various names in literature (e.g. "paraclypeal folds"). According Teskey, these could be assumed as possible precursors of the tentorial phragmata of brachycerous larvae, but the homology has not been ascertained.

In orthorrhaphous Brachycera larvae, the tentorial phragmata are real arms originated by the invagination of the anterior tentorial pits and the adjacent sutures and posteriorly contiguous with the anterior arms of the tentorium. In most of larvae they appears as vertical plates anteriorly fused with the labium and posteriorly connected with the tentorium. Their shape and connections change among the different lineages (Teskey, 1981; Courtney et al., 2000). In Stratiomyomorpha and Tabanomorpha, the tentorial phragmata are usually fused with the anterior tentorial arms, but in some larvae of Rhagionidae and Pelecorhynchidae they seem only weakly fused (Courtney et al., 2000). In taxa nearest to higher Dipera, at least in most of Asiloidea and Empidoidea, the tentorial phragmata are instead jointed to tentorial arms by real articulations (Courtney et al., 2000). In the general chapters of the Manual of Nearctic Diptera and the Manual of Palaearctic Dipter no descriptions about the tentorial phragmata was given referring to Nemestrinoidea and Bombyliidae with related families.

As regards the tentorium sensu stricto, most of the orthorrhaphous shows a cospicuous development at least of the anterior arms, unlike the eucephalic larvae, where the reduction of the tentorium is often compensated by the presence of equivalent structures of the cephalic capsule. In the hemicephalic larvae, the tentorial arms play a prominent role giving the attack of muscles and the support to the pharyngeal pump. Furthermore, in the Stratiomyidae, the tentorium is fused with the wall of the cibarium making a single structure which perhaps strengthens the grinding apparatus of the pharynx faringe (Courtney et al., 2000).

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Antennae

The antennae of the hemicephalic larvae are usually placed on the dorsal-anterior margin of the head capsule, Le antenne delle larve emicefale sono generalmente posizionate sul margine anteriore-dorsale della capsula cefalica, near the hypondyle or, in Empidoidea larvae, forward, anterior to the hypocondyle. Usually they appears as small knobs or bulges.

The number of segments varies from one to three (Courtney et al., 2000). Antennae 3-segmented are usually recurrent in larvae of Tabanidae and Nemestrinidae, 2-segmented in larvae of Stratiomyidae, Pelecorhynchidae, Athericidae, and some Bombyliidae, but in most of lower Brachycera the antennae have a single segment.

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Eye

As in the eucephalic larvae, the eyes of hemicephalic are stemmata composed by photoreceptors which provide the perception of the direction of the light. The presence of stemmata in ortorrhaphous larvae is referred by various authors in some families and these sensory organs would be present in most of families althought they are apparently absent (Courtney et al., 2000). In general they are poorly visible because ventral to lateral margin of the head capsule and protected by layers of transparent cuticle.

Compared with stemmata of nematocerous larvae, in those of orthorrhaphous the lens is missing except the larvae of Stratiomyidae and Xylophagidae (Courtney et al., 2000).

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Bibliografia

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