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Cuticle features (Sclerotization - Pigmentation) - Outgrowths - Bibliography - Web resources

This page overviews the prominent features of the larval integument, as the sclerotization and pigmentation of the cuticle, and the features of the tiny and not fleshy outgrowths, as hairs, setae, spines, etc. The fleshy outgrowths, which appears as appendages, but are cuticular structures provided with muscles, will examined in the next page.

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Cuticle features

Most Diptera larvae live in aquatic or other wet habitats as moist substrates or the body of living organisms, animals or plants. Throughout the order, only a few larvae live in dry habitats, as the dry soil, or directly weathered. As result of these facts, the Diptera larvae usually do not need special adaptations to protect them versus unfavorable conditions, as the abrasive action of the substrate, the dehidration, the predation. In most Diptera larvae, therefore the integument is smooth and more or less hairless, tiny and weak, depigmented or translucent (Teskey, 1981, Courtney et al., 2000).

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Sclerotization

The thickening and chitinization of the integument, often restricted to some areas of the body, is observed only in few groups and is related to singular habits and ambient conditions. The most evident cases concern the larvae of Asilidae, Stratiomyidae, and Xylomyidae. The asilid larvae live in dry soils and are predacious. The mobility of these larvae and the ambient conditions expose them to dehidration and abrasion by the soil particles, thus they have a resistant and leathery cuticle (Teskey, 1981). The larvae of Xylomyidae and some subfamilies of Stratiomyidae are terrestrial and live in decaying matter, as leaf litters, bark debris, otherwise the surficial layers of the soil. For this reasono, probably, they have a thick and robust cuticle and rough due the presence of concretions of calcium carbonate, that is a prominent feature of these families (Teskey, 1981; Courtney et al., 2000; Tremblay, 2005).

In other groups, the sclerotization of the integument is not extended to the entire body, but is restricted to certain body regions. For example, the larvae of Xylophagidae have the integument slightly chitinized, with sclerotized plates in the dorsal and lateral areas of the thorax and in the dorsal side of the last segment (Teskey, 1981; Courtney et al., 2000). The Psychodidae larvae of subfamily Psychodinae have usually two sclerotized dorsal plates on each thoracic segments and the first addominal and three plates on each successive segments, from 2 to 7 (Teskey, 1981, Quate & Vockeroth, 1981 e Wagner, 1997). The larvae of many Cecidomyiidae have in the ventral side of the prothoracic segment a small sclerotized appendage, variously shaped, called sternal spatula (Teskey, 1981; Gagné, 1981; Tremblay, 1991; Skuhravá, 1997). Sclerotizations mor or less accentuated can be observed in various families of Nematocera in the last segment related to the anus or the posterior spiracles (Teskey, 1981). Instead among the larvae of Cyclorrhapha, the sclerotization of the integument is infrequent and appears in few families (Courtney et al., 2000).

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Pigmentation

Throughout the Diptera, the pigmentation of the larva integument is a morphological character of little importance, being the cuticle depigmented, with variable colors from white to yellowish, especially in forms that do not live outdoors (Teskey, 1981).

True pigmentations are presumably due to granules of pigment within cromatocytes of the fat body or the epidermis and are know only in few nematoceran families (Teskey, 1981; Courtney et al., 2000). In other cases, the pigmentation is due to the hemolymph containing carotenoids, hemocyanin or hemoglobin. Cause of these hematic pigments, the color may change, depending on the groups, from grey to brown, green, and rarely to red (Teskey, 1981; Courtney et al., 2000).

Any indirect pigmentations are instead given by the pubescence of the cuticle and may have various patterns, sometimes useful to taxonomic diagnosis (Teskey, 1981; Courtney et al., 2000).

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Spines of botfly
Fig. 1 - Ventral spines of the larva of a botfly, Gasterophilus Leach (Brachycera: Oestridae), arranged in two transverse rows for each segment.
Author: Alan R. Walker
(Home page)
Modified from the original picture at Wikimedia Commons
(License: Creative Commons BY-SA)

Outgrowths

When they are present, the cuticular complements are outgrowths of various kinds, as tubercles, spicules, spines, hairs, setae and, finally, fleshy processes. The appearance of these structures changes depending on the groups and may be an useful element of taxonomic determination. In general they are more frequent in larvae of lower Diptera (Nematocerous, orthorrhaphous Brachycera, and cyclorrhaphous of the Aschiza group), wheread throughout the Schizophora are usually confined to the last abdominal segment (Teskey, 1981).

In the most simple configuration, the cuticular outgrowths are microtrichia or spicules of homogeneus size that cover uniformly the entire body or broad parts of this, as in many families of both Nematocera and Brachycera (Teskey, 1981). In larvae of some families it can be observed trichoid processes with different sizes arranged in patterns with may be peculiar. For example, the Ptychopteridae larvae have an uniform pubescence given by microtrichia and additional longer hairs grouped in transvere series (Teskey, 1981). The larvae of certain Scatopsidae have transverse stripes of bristles in the posterior part of each segment, preceded by shor longitudinal rows (Teskey, 1981; Cook, 1981).

Several zoophagous larvae of Schizophora affecting vertebrates have cuticular outgrowths used to anchor or move within the host tissues: most larvae of Oestridae sensu lato have robust spines or scales grouped along the anterior edges of the segments (Teskey, 1981; Wood, 1987; Tremblay, 1997; Minář, 2000b, 2000c, 2000d). Their appearance and arrangement are useful to diagnostic purposes. Similar arrangements can be observed also in many larvae of Calliphoridae, usually in multiple sets of reclinate spines, arranged along the anterior edge of each segment (Shewell, 1987; Tremblay, 1997; Rognes, 1998).

Larva of forcipomyia
Fig. 2 - Larva of biting midge Forcipomyia Meigen, 1818 (Nematocera: Ceratopogonidae).
On the body there are water droplets borne by the secretive dorsal setae.
Author: Cristophe Quintin
(Home page at Flickr)
Modified from the original picture
(License: Creative Commons BY-NC 2.0)

More evident are the setae, which of distribution, size, and shape are variable features (Teskey, 1981). About their distribution, the setae may be assembled in pattern more or less defined or scattered on entire body, present in all segments or some of them. Usually they appears as well developed hairs but sometimes have singular shapes or other features. For example, the larvae of Lutzomyia vexator (Coquillett) (Nematocera: Psychodidae) have typical clavate setae (Quate & Vockeroth, 1981). The larvae of Culicidae may bear palmate setae, as in Anophelinae, or branched, as in some Culicinae (Stone, 1981; Minář, 2000a).

The larvae of Forcipomyiinae (Nematocera: Ceratopogonidae), subfamily which includes the genera Forcipomyia Meigen and Atrichopogon Kieffer, have a complex and heterogeneous system of setae, present in all thoracic and abdominal segments. In general, the setae are arranged in multiple longitudinal rows and those of the four dorsal series are carried by tubercles more or less prominent (Downes & Wirth, 1981; Boorman, 1997). The setae of the two median series are associated to mechanoreceptors (Urbanek et al., 2015), but in Forcipomyia, the trichogenous cell has acquired a secretory function and secretes a hygroscopic substance, through the spatulate apex of the setula (Urbanek et al., 2011). Due this anatomical and physiological feature, the dorsum of the Forcipomyia larvae often bears several water droplets which give them a unique appearance (Murray, All about Forcipomyiinae, galleria fotografica). The main purposes of these glandular secretions should be to prevent the dehidration and allow the cutaneous respiration: the droplets run down along the setae and expand on the cuticle of the larva, covering it with a liquid veil (Urbanek et al., 2015). Indeed this feature is found in larvae of terrestrial species, that are common under the bark and among the moss (Boorman, 1997).

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Bibliografia

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Web resources

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Last update of this page: 28 May 2019
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