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Prolegs - Creeping welts - Suckers - Bibliography

Fleshy outgrowths are evident mobile or flexible protrusion, that work as locomotory or anchorage organs. Regardless of their variety, these outgrowths may be identified as one of three patterns: prolegs, creeping welts, and suckers.

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Larva of Dicranota
Fig. 1 - Larva of Dicranota Zettersted (Nematocera: Pediciidae), lateral view.
This larva has five pairs of prolegs on abdominal segments 3-7.
Author: Erin Hayes-Pontius (Wikimedia Commons)
Reduced from the original photo
(Licenza: Creative Commons BY-SA 3.0)

Prolegs

The groundplan of Diptera is the lacking of thoracic legs and abdominal prolegs in the larvae. However in several families there are structures similar to prolegs of eruciform larvae of other orders (p.a. Lepidoptera, Hymenoptera) which appears as fleshy and not articulate. The presence of these structures occurs with various features in groups phylogenetically not related. This conditions suggests that the prolegs are derived from a evolutionary convergence (Teskey, 1981) and anyway without relation with analogous organs of eruciform larvae of other insects.

In several species, these organs appear as pairs of ventral fleshy tubercles. They are rounded or oblong, retractable, and bear one or more spines near the tip. Depending on the systematic group, the number and position of prolegs vary and these differences may be useful to the taxonomic determination. Based on the position the prolegs can be identified as thoracic, abdominal, and anal.

The thoracic prolegs are absent throughout the order except the larvae of some sirfidi and the entirety of Chironomoidea superfamily. According to several Authors, the recurrence in all families suggests that this feature may be a synapomorphy of the Chironomoidea (Courtney et al., 2000). When they are present, these prolegs are borne by the first thoracic segment. Larvae of Thaumaleidae and Simuliidae have a single median proleg, probably derived from the fusion of a primitive pair of appendages (Teskey, 1981).

The abdominal prolegs are locomotory appendages present I to VIII segments. Only the prolegs of the last segment (anal) of some Nematocera are distinct from the others (Teskey, 1981; Courtney et al., 2000). Cause of the variety of their number and position and the recurrence in several families throughout the order, Authors deduces that the prolegs are a secondary character independently developed, during the evolution, in not related lineages. Therefore it would be an homoplasy derived from evolutionary convergence in larvae that have aquatic habitats (Courtney et al., 2000).

Among the Nematocera, the abdominal prolegs occur in larvae of Dixidae, Ptychopteridae, Blephariceromorpha, and Dicranota Zettersted genus (Tipulomorpha: Pediciidae).

Among the lower Brachycera these appendages are present in larvae of Athericidae and Tabanidae, some genera of Asilidae and several genera of Empididae.

Among the upper Brachycera Teskey (1981) reported the abdominal prolegs in larvae of Syrphidae ando those of Ephydra Fallén (Schizophora: Ephydridae) and Limnophora Robineau-Desvoidy and Graphomyia Agassiz (Schizophora: Muscidae), but, according to Courtney et al. (2000), these appendages should be creeping welts apparently transformed in prolegs.

The anal prolegs are present on the terminal abdominal segment. The univocal identification of these prolegs is not well clear, perhaps because in the secondary segmentation the terminal segment of brachycerous larvae is derived from the fusion of 8th and 9th segments. In effect Courtney et al. (2000) refer to these prolegs in the Nematocera, while they merely mention the presence of prolegs on the last apparent segment in some Brachycera.

Prolegs on the 9th segment in nematocerous larvae (Teskey, 1981; Courtney et al., 2000) are found in all Chironomoidea and in Nymphomyiidae and Tanyderidae families. Among the Chironomoidea, this feature is combined with the absence of abdominal prolegs and the presence of thoracic prolegs, while, in larvae of Tanyderidae only anal prolegs are present. Finally, in Nymphomyiidae, this character is accompanied by the presence of prolegs in all abdominal segments except to 8th. Thoracic and abdominal segments work as true locomotory structures, whereas the anal prolegs would perform as anchoring organs (Courtney et al., 2000).

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Creeping welts

Usually the creeping welts are fleshy transverse ridges along the anteroventral edge of the first seve abdominal segments, bearing setae or spines variously arranged, often aligned in transverse rows. In some groups, these bulges extend up to lateral and dorsal sides and in several larvae of Cyclorrhapha may involve the posterior margin of the adjacent segment, assuming the appearance of intersegmental ridges. Their function is to assist the movement of creeping apod larvae by pressing the surface of substrate.

The creeping welts are commons among the apod larvae of Schizophora, but they occur also in many larvae of lower Diptera (Teskey, 1981; Courtney et al., 2000), both among the Nematocera, as Dixidae, Tipulidae, Bibionomorpha, and among the Orthorrhapha, as Rhagionidae, Empidoidea, Xylophagidae, and Acroceridae.

Larva of Ceratitis capitata
Fig. 2 - Larva of medfly (Ceratitis capitata Wiedemann [Brachycera: Tephritidae]), lateral view.
Focus on the abdominal creeping welts.
Author: Florida Division of Plant Industry, Florida Department of Agriculture and Consumer Services, Bugwood.org
Modified from the original photo
(License: Creative Commons BY 3.0)

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Suckers

The suckers, also called suctorial discs or suction discs, are a curious feature of Blephariceridae, developed as evolutionary adaptation to the habitats of thei larvae. These structures have been extensively studied and described by Authors. The suckers are six organs aligned along the median axis on the ventral side of the cephalothorax and the first five apparent segments (2nd to 6th segments of the primary segmentation). Each sucker has a circular shape and consists of a depression surrounded by a fleshy margin that is plated by an elastic cuticle with a complex structure. At the centre of the depression there is a cavity that contains a piston moved by muscles. When the edge of the sucker adheres to a smooth surface, the lifting of the piston creates a negative pressure, allowing the attachment of the body just like an idraulic suction cup. On the anterior side of the margin there is a notch that allows the release of the sucker and the separation from the surface. The homology of these organs is unknown (Teskey, 1981; Courtney et al., 2000).

Blepharicerid larva
Fig. 3 - Larva of Blephariceridae, ventral view. The six ventral suckers and the six pairs of ventrolateral prolegs are visible.
Author: Allen Gathman (Home page on Flickr)
Modified from the original photo
(License: Creative Commons BY-NC-SA 2.0)

The perfection of this apparatus allows the blepharicerid larvae to adhere to the rocks hit by the impetuous flow of the streams. Usually, the movements of these curious larvae is slow and predictable due the working of these suckers, but when disturbed they are able to move quickly sideways rotating the body using the adhesion of the terminal sucker as fulcrum (Hogue, 1981; Courtney, 2000; Courtney et al., 2000).

Similar but not homologous structures have been described for some genera of Psychodidae (Teskey, 1981; Quate & Vockeroth, 1981; Wagner, 1997). The larvae of Maruina Müller genus have eight sucker along the ventral side of thorax and segments 1 to 7. The larvae of Horaiella Tonnoir genus are instead provided with a single large sucker.

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Bibliografia

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