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Table of contents

Body shape - Body segmentation (Primary segmentation - Fusion of segments - Supplementary divisions) - Bibliography
Larva of Anopheles
Fig. 1 - Larva of mosquito of genus Anopheles Meigen (Nematocera: Culicidae).
Author: Steffen Dietzel (Wikimedia Commons)
Resized from the original picture
(License: Public Domain)

The general appearance of dipteran larvae changes strongly with the segmentation, the ration between the size of the single segment and between the size of various segments. Further, the presence of special structures of the integument, such as the fleshy outgrowths, can sharpen some morphological traits and can help to define the shape, both from the lateral and dorsal view.

Observing the morphology and the structure of the head, the three basic types (eucephalic, hemicephalic, and acephalic) may be roughly associated with the three dipteran groups of high level, that are the paraphyletic groups of Nematocerous and brachycerous Ortorrhaphous and the monophyletic clade of brachycerous Cyclorrhapha. Entering into the shape and the segmentation of the body, instead, this simplistic association is inappropriate: among the Diptera we find a great diversity in shape which can involve even a single lineage, such as the Syrphidae, but the dipteran larvae give also several examples of similarity between forms belonging to unrelated groups. After all we have to consider that the body shape may be related to the habitat and behavior of the larva and in the evolutionary history of the flies many groups, even phylogenetically distant, had homoplasic or plesiomorphic adaptation to specific habitats. In addiction to apomorphic characters, such the habitus of the acephalic larvae of the cyclorrhaphous Schizophora, commonly known as "maggots", the dipteran larvae often manifest analogies that are clear examples of convergent evolution, derived from the indipendent secondary adaptation of distinct lineages to the same environmental and behavioral conditions.

Larva of hover fly
Fig. 2 - Predatory larva of hover fly (Brachycera: Syrphidae).
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

An example of this diversity, at first approach, can be found observing the distinction of the tagmata of the larval body: the head, the thorax, and the abdomen are usually well defined in eucephalic larvae and, sometimes, in the hemicephalic, but often, among both the Nematocerous and lower Brachycerous, the thoracic and abdominal segments have similar forms to the point that they can be distinguished only by a careful exam of the body segmentation. But the most extreme condition occurs in the acephalic larvae, with the appearant absence of the heteronomous metamerism.

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Body shape

About the body shape, a well defined type that may represent the generality of the order does not exists. In general, the dipteran larvae have an elongated shape with a transverse section more or less rounded, although the latter feature has several variants throughout the order. The differences usually involve the ratio between the length and the transverse dimensions and the uniformity of this ratio along the entire body.

Apart from extreme differentiations, typical of some small groups, the most common shapes may be attributed to the following types (Teskey, 1981a; Courtney et al., 2000):

Larva of Nephrotoma
Fig. 3 - Larva of crane fly of genus Nephrotoma Meigen (Nematocera: Tipulidae).
The head is at the left.
Author: Marcello Consolo
(Home page at Flickr)
Modified from the original picture
(License: Creative Commons BY-NC-SA 2.0)

The subcylindrical type appears as a elongated body which has an uniform section along the entire body. This condition, in general, is associated with a well formed head, not smaller than the thoracic segments, therefore this shape occurs in eucephalic larvae and is representative of many Nematocera families.

The reduction of the cephalic capsule and the morphology of the last segments cause the narrowing of the anterior and posterior ends or of one of these, so that the body may be fusiform or only anteriorly or posteriorly tapered. These shapes are widespread among the Brachycera, although not exclusive. Fusiform larvae are typical of Tabanidae and several families of Cuclorrhapha due the reduction of the pseudocephalon. Among the Nematocera, the fusiform larva is representative of the Cecidomyiidae.

Larva of Tabanus lineola
Fig. 4 - Larva of striped horse fly, Tabanus lineola Fabricius (Brachycera: Tabanidae).
The head is at the left.
Author: Sturgis McKeever, Georgia Southern University, Bugwood.org.
Resized from the original picture at Insect Images
(License: Creative Commons BY-NC)
Larva of blow fly
Fig. 5 - Larva of blow fly (Brachycera: Calliphoridae).
The head is at the left.
Author: Marcello Consolo
(Home page at Flickr)
Modified from the original picture
(License: Creative Commons BY-NC-SA 2.0)
 


The elongate or serpentine shape is caused by the development of the segments, which appear longer than wide, and this character becomes prominent. This shape recurs among the Nematocera (Ceratopogonidae) and some families of lower Brachycera (Therevidae, Scenopinidae, Pelecorhynchidae in part).

Larva of stiletto fly
Fig. 6 - Larva of stiletto fly (Brachycera: Therevidae).
The head is at the left.
Author: Anita Gould
(Home page at Flickr)
Modified from the original picture
(License: Creative Commons BY-NC 2.0)
Larva of soldier fly
Fig. 7 - Larva of soldier fly (Brachycera: Stratiomyidae).
The head is at the left.
Author: Cristophe Quintin
(Home page at Flickr)
Modified from the original picture
(License: Creative Commons BY-NC 2.0)


The dorsoventrally flattened shape is caused by another prominent character, that is the different development between the transverse dimensions, so the horizontal size is greater than the vertical. This shape is typical of most of the Stratiomyomorpha, but can be found even among Aschiza (Platypezidae and Lonchopteridae) and Schizophora groups (Fanniidae). If lateral outgrowths are presents, they may give a flattened appearance also to larvae of some Nematocera, such as the Blephariceridae.

Larva of botfly Gasterophilus
Fig. 8 - Ventral view of the larva of a botfly, Gasterophilus Leach (Brachycera: Oestridae).
The head is at the left.
Author: Alan R. Walker
(Home page)
Modified from the original picture at Wikimedia Commons
(License: Creative Commons BY-SA)

The stout shape is instead derived from a reduced development in length, so the body appears more or less ellipsoidal, ovoid, or subglobose. This form can be found expecially among the Syrphidae and brachycerous families with endoparasitoid larvae (Bombyliidae, Nemestrinoidea, Oestridae).

Other peculiarities can be added to these usual forms, but they are representative of small groups, at family or other lower level taxa. These unusual forms are often given by the adaptation to particular habitats or life habits or singular morphoanatomical features that affect the general shape. For example, the larvae of most of [[taxaCulicoidea]] have a thorax larger than the other tagmata, the larvae of Simuliidae have a body swelled at the posterior end, several endoparasitic larvae of Schizophora are pear-shaped, often as adaptation to the shape of the host, the larvae of Nymphomyiidae are laterally flattened, etc.

The most extreme difference is probably in the genus Microdon Meigen (Brachycera: Syrphidae): the larvae of these mirmecophile insects are hemisferic, ventrally flattened and dorsally dome-shaped. Their look resembles that of slugs, so much so that this genus was first described as mollusc (Courtney et al., 2000).

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Body segmentation

The final segmentation of larvae body is produced with the combination of two secondary changes: the fusion of some segments, that involves the generality of dipteran larvae, and the possible secondary segmentation, that involves the larvae of some families, both of Nematocera and Brachycera.

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Primary segmentation

The dipteran embryos have a primary suddivision into 19 segments (Teskey, 1981a; Courtney et al., 2000). The first 6 segments give rise to the head, the following 3 to the thoracic segments, the remaining 10, finally, to the abdominal. The number of apparent abdominal segments, however is less than 10 due the reduction of the tenth to a vestigial perianal segment borne by the ninth (Teskey, 1981a; Courtney et al., 2000). Only Stone (1981) has referred about the presence of ten abdominal segments in the larvae of Culicidae, by the interpretation of the perianal segment as true urite.

The presence of 9 apparent urites is an ancestral condition recurrent only in some nematocerous, but in several families the ninth and the eighth segments are fused (Teskey, 1981a). When this number is reduced, the perianal pad is borne by the last apparent segment. Further reductions of the number of the apparent segments are found only in Axymyiidae and Blephariceridae families.

In most of Brachycera, the primary segmentation includes 8 abdominal segments (Teskey, 1981a), with the only exception of the Asiloidea larvae, which have 9 segments. The author of the Manual of Nearctic Diptera and the Manual of Palaearctic Diptera did not give hypothesis about the origin of the ninth segment in these brachycerous: presuming the division into 8 segments as in the groundplan of the Brachycera, indeed, we should deduce that the ninth segment could be a secondary division derived from the eighth segment, however the embryological studies led to conflicting results (Courtney et al., 2000).

Regardless to the supplementary secondary segmentation, a summary of the number of the apparent abdominal segments is reported in Table 1 (Teskey, 1981a; Courtney et al., 2000). The asterisk indicates, within each group, the presence of possible supplemetary divisions as described in the next paragraph.

Tabella 1: primary segmentation of the abdomen in dipteran larvae
Number
of segments
Nematocera Brachycera
9 - Bibionomorpha:
     Bibionoidea
     Pachyneuroidea
     Mycetophiliformia in general (*)
- Blephariceromorpha:
     Nymphomyiidae
- Culicomorpha:
     Ceratopogonidae (*)
     Chironomidae
     Thaumaleidae
     Culicoidea in general
- Psychodomorpha:
     Scatopsoidea
- Ptychopteromorpha:
     Tanyderidae
- Muscomorpha:
     Asiloidea (*)
8 - Blephariceromorpha:
     Deuterophlebiidae
- Culicomorpha:
     Simuliidae
- Psychodomorpha:
     Anisopodoidea (*)
     Psychodoidea (*)
     Trichoceridae (*)
- Ptychopteromorpha:
     Ptychopteridae
- Tipulomorpha:
     Tipuloidea (*)
- Stratiomyomorpha:
     Stratiomyoidea
- Tabanomorpha:
     Tabanoidea
     Vermileonoidea (*)
- Xylophagomorpha:
     Xylophagoidea
- Muscomorpha:
     Nemestrinoidea
     Empidoidea
     Cyclorrhapha in general
7 - Axymyiomorpha:
     Axymyiidae
6 - Blephariceromorpha:
     Blephariceridae

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Fusion of segments

The further fusion of the segments in the Nematocera larvae recurs only in some families of the Culicomorpha, in Axymyiidae, and in Blephariceridae.

The larvae of three Culicoidea families (Culicidae, Chaoboridae, and Corethrellidae) have the 3 thoracic segments fused into an only one apparent segment (Teskey, 1981; Stone, 1981; Minář, 2000). This feature, with the shape and the size of the thorax, make easily recognizable the larvae of these families, cause of the stout thorax, well distinct from the other tagmata. In the Simuliidae, another family of Culicomorpha but belonging to Chironomoidea, the thoracic segments are apparently fused, as referred by Teskey (1981a), but this feature could be a condition associated with the poor evidence of the segmentation in the body of these larvae, because this character was not reported by other authors (Peterson, 1981b; Jedlička & Stloukalová, 1997). In any case, the larvae of Simuliidae are clearly distinguishable from those of the Culicoidea, due the particular form of the body.

The larvae of Axymyiidae have a well defined segmentation including 3 thoracic and 7 abdominal segments (Wood, 1981; Krivosheina M.G., 2000). The seventh segment bears the anal papillae and a long respiratory siphon that Wood (1981) identified as the eighth segment.

The larvae of Blephariceridae, finally, are a further example of diversity within the Diptera. In these larvae, the entire body seems composed of only 7 segments cause of the fusion of anterior and posterior segments (Teskey, 1981a; Hogues, 1981; Courtney, 2000; Courtney et al., 2000). The head, the thorax, and the first abdominal segment are fused into an only one apparent segment, called cephalothorax (Courtney_bis2000|Courtney, 2000; Courtney et al., 2000). The next 5 originate from segments 2-6, well distinct. The last apparent urite is derived from the fusione of the segments 7-10.

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Supplementary division

In several groups, belonging both to Nematocera and Brachycera, the primary segmentation is modified by the supplementary divisions which sometimes are not discernible from the primary divisions. These divisions are often called annuli (singular annulus, "ring", Quate & Vockeroth, 1981; Krivosheina N.P., 1997a, 1997b; Wagner, 1997).

In Anisopodidae and related Mycetobiidae, the primary segmentation includes 3 thoracic and 8 abdominal segments, but supplementary divisions are presents, as short intercalary rings that lie in the anterior part of the main segment. The reports about this pseudosegmentation, in the Manual of Nearctic Diptera and the Manual of Palaearctic Diptera, are different because in the first book these groups was debated as a single family. Teskey (1981a), in the general section, and Peterson (1981a), in the chapter about the Anisopodidae, referred to the larvae of both groups. According to the Authors, the secondary suddivision includes a brief intercalary ring before the prothoracic segment and each abdominal segments. Moreover, Peterson (1981a) stated that the eighth abdominal segment may have 2-5 supplementary divisions. In the Manual of Palaearctic Diptera, these groups was described as separated families by Krivosheina (Krivosheina N.P., 1997a, 1997b). About the Anisopodidae sensu stricto, Krivosheina confirms the general description given by Teskey (1981a) and Peterson (1981a) (Krivosheina N.P., 1997a). About the Mycetobiidae, instead, she affirms that the intercalary ring lies only on the prothoracic and the seventh abdominal segment, whereas the eighth is divided into 4 rings with different sizes (Krivosheina N.P., 1997b). The apparent disagreement should be caused by the different interpretations about the intercalary rings on the segments 1-6, that are not less defined in larvae of Mycetobiidae. Definitively, the larvae of these two families seem to have a body divided into 20-23 postcephalic pseudosegments.

Even in the Psychodidae, the larvae have the body divided into 3 thoracic and 8 abdominal segments, but in some subfamilies the larval body is further divided in pseudosegments. The supplemetary division in larvae of the Psychodinae subfamily is well described by Teskey (1981a), Quate & Vockeroth (1981), and Wagner (1997): each thoracic segments and the first abdominal are divided into two annuli, the following 6 abdominal segments (from segment 2 to 7) are each divided into three annuli, whereas the last (segment 7) is undivided. The body of these larvae appears so divided into 27 postcephalic pseudosegments. Each annulus can be often identified by a dorsal sclerotized plates, but some of these plates may be fused, with a consequent reduction of the number of the apparent segments (Quate & Vockeroth, 1981). In other subfamilies the pseudosegmentation is not clear than the Psychodinae (Teskey, 1981a; Wagner, 1997), however the supplementary division into annuli is more or less evident, although the primary segmentation is still recognizable. Only the larvae of Trichomyiinae lack the division into annuli (Quate & Vockeroth, 1981; Wagner, 1997a).

In regard to the Trichoceridae larvae, authors provided conflicting statements. Teskey (1981a) included this family into the Nematocera groups whose larvae have the number of primary segments reduced to 8, but with a pseudosegmentation similar to that of Psychodinae. The same interpretation, in the Manual of Nearctic Diptera, was given by Alexander (1981), who explicity stated that the body of Trichoceridae larvae is composed of 11 postcephalic segments with a supplementary subdivision into two annuli

in the thoracic, the first, and the eighth abdominal segments, three annuli for the remaining (segments 2 to 7). In the Manual of Palaearctic Diptera Dahl & Krzemińska (1997) provided a different interpretation. According to Authors, the larval body of Trichoceridae has 12 postcephalic segments: following this scheme, the thoracic and the last abdominal segments should be undivided, whereas all the abdominal segments preceding the last should be divided into three annuli. Both the interpretations, also in the attached tables, imply a body divided into 28 pseudosegments. The disagreement id probably caused by the difficulty of discerning between the primary and the supplementary segmentations. I think that Teskey (1981a) and Alexander (1981) gave the correct interpretation the correct interpretation. Apart from the fact that the primary segmentation would be the same in three related families (Anisopodidae, Psychodidae, and Trichoceridae, although as incertae sedis), their interpretation is coherent with the position of anterior spiracles: in all larvae, the anterior spiracles lies always in the prothoracic segment; in Trichoceridae larvae, the spiracles lies in the second pseudosegment. According to the interpretation by the american Authors, this pseudosegment is the posterior annulus of the prothoracic segment, whereas is the mesothoracic segment according to the interpretation of the european Authors. However, the latter interpretation should assume an anatomical peculiarity which has not references in literature, since the Trichoceridae larvae are certainly classified as amphipneustic (Alexander, 1981; Dahl & Krzemińska, 1997).

Larva of Trichogramma annulata
Fig. 9 - Larva of Trichocera annulata Meigen (Nematocera: Trichoceridae) from dorsal and ventral views.
The head is at the left.
Author: Caroline Harding, Ministry for Primary Industries, New Zealand.
Modified from the original photos
(License: Creative Commons BY 3.0 AU)

In regard to the Tipuloidea (=Tipulidae sensu lato), which were specifically described only in the Manual of Nearctic Diptera, no references about a supplementary subdivision were given. Teskey (1981a) mentioned the Tipulidae sensu lato (including also Cylindrotomidae, Limoniidae and Pediciidae) among the nematocerous that have the larval body composed of 11 postcephalic segments (3 thoracic and 8 abdominal). In the same book, Alexander & Byers (1981) have not reported about the segmentation of larval body. In the general dissertation about the dipteran larvae of the Manual of Palaearctic Diptera, Courtney et al. (2000) have generically mentioned the Tipulidae among the families that have larvae with a pseudosegmentation. In fact, observing the plates attached to the dissertation by Alexander and Byers, these larvae seem to have rudimentary subdivisions, although not well defined, on the base of cuticular outgrowths, hairs or setae, or apparent annuli, at least in larvae of genus Liogma Osten Sacken (Cylindrotomidae), Prionocera Loew (Tipulidae), Antocha Osten Sacken and Limonia Meigen (Limoniidae).

Other reports about supplementary subdivisions in nematocerous larvae was given for restricted groups within some families, i.e. the Leptoconopinae among the Ceratopogonidae (Downes & Wirth, 1981; Courtney et al., 2000) and some Keroplatidae (Courtney et al., 2000).

In Brachycera a supplementary segmentation is clearly present in larvae of two families of the Asiloidea and in the larvae of the Vermileonidae. Furthermore Courtney et al. (2000) referred about occurrence of the supplementary subdivision in Syrphidae and Sepsidae larvae.

Among the Asiloidea this feature is prominent in two related families, the Therevidae and [taxa|Scenopinidae]], which have similar larvae. As previously reported, these larvae have a serpentine shape, strongly elongated and tapered at the anterior and posterior ends (Fig. 6). The secondary subdivisions are uniform and make it difficult to distinguish the primary segmentation (Teskey, 1981a; Courtney et al., 2000). The body seem divided into 20 postcephalic segments according to Teskey (1981a) and Majer (1997), 19 segments according to other authors (Irwin & Lyneborg, 1981, Krivosheina N.P., 1997c). The discordance is probably caused by a different perception of the subdivisions in the last segment. It is even discordant the interpretation of the primary segmentation on the base of the position of the posterior spiracle, which lies in the 14th abdominal pseudosegment. Teskey (1981a), based on the concept that the posterior spiracles have borne by the segment 8, has inferred that the 13 abdominal pseudosegments originate from the first 7 urites; so, the first 6 abdominal segments and segments 8 and 9 would be divided into two rings, whereas the segment 7 would be undivided. Furthermore, the posterior spiracle would lie in the anterior ring of the segment 8. The author who described these families started by another approach: each of segments 1 to 8 would be divided into two rings (Irwin & Lyneborg, 1981; Kelsey, 1981) and the posterior spiracles would lie in the antepenultimate segment (Irwin & Lyneborg, 1981; Kelsey, 1981), to be precise, on the posterior ring (Krivosheina N.P., 1997c). Following the statement of these Authors, it must be inferred that the posterior spiracles has shifted on the segment 7 and the segments 8 and 9 are undivided. The opinion of Majer (1997) seems undefined: the Author has reported about the appearance of 20 postcephalic pseudosegments and the subdivision into two rings for each of first 6 abdominal segments. These statements are coherent with the interpretation given by Teskey (1981a), but Majer did not provide further details.

Larva of Lampromyia iberica
Fig. 10 - Larva of Lampromyia iberica Stuckenberg (Brachycera: Vermileonidae).
The head is at the left.
Author: Ferran Turmo Gort (Home page at Flickr)
Modified from the original picture
(License: Creative Commons BY-NC-SA 2.0)

The larve of Vermileonidae have various morphological peculiarities (Fig. 10), also including a strong pseudosegmentation, with the supplementary division of some segments, especially the toracic (Teskey, 1981b, Nagatomi, 1997; Courtney et al., 2000). The integument is marked by several dorsal and ventral wrinkles, which divide each segment into a variable number of narrow rings. In general, this supplementary division involves the three thoracic segments and the first abdominal, each uniformly divided into 5-6 rings that make difficult to distiguish the primary segments. The following abdominal segments are usually well defined, because they lack a supplementary division, as in Vermileo Macquart, or this is not uniformly arranged, as in Lampromyia Macquart. In Vermiophis Yang, the supplementary division involves all postcephalic segments except the urite 8. It is intense and uniform up to segment 6 and not uniform in the segment 7.

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Bibliography

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