Scute

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Scutes on an alligator foot

A scute (

mesonotum in insects as well as some arachnids (e.g., the family Ixodidae
, the scale ticks).

Properties

Scute of the carapace of a Texas tortoise
Children's Museum of Indianapolis
.

Scutes are similar to

mammals
, and are believed to be the primitive form of dermal armour in reptiles.

The term is also used to describe the heavy armour of the

snakes or certain fishes, such as sturgeons
, shad, herring, and menhaden.

Mammals

Prehistoric ancestors of mammals, the

mammals. The horny scales of pangolins are only rarely called scutes, but "scute" is used to describe the heavy armour of the armadillo
.

Turtles

The turtle's shell is covered by scutes formed mostly of keratin. They are built similarly to horn, beak, or nail in other species.

Fish

pineconefish

Some fish, such as

pineconefish, are completely or partially covered in scutes. River herrings and threadfins have an abdominal row of scutes, which are scales with raised, sharp points that are used for protection. Some jacks have a row of scutes following the lateral line on either side. Sturgeon
have five rows of scutes instead of scales.

Birds

The tarsometatarsus and toes of most birds are covered in two types of scales. Large scutes run along the dorsal side of the tarsometatarsus and toes, whereas smaller scutellae run along the sides. Both structures share histochemical homology with reptilian scales, however work on their evolutionary development has revealed that the scales in bird feet have secondarily evolved via suppression of the feather-building genetic program.[1][2][3] Unblocking the feather suppression program results in feathers growing in place of scales along the tarsometatarsus and toes.[1][2][3] Dinosaur species very close to the origin of birds have been shown to have had "hind wings" made of feathers growing from these areas, suggesting that the acquisition of feathers in dinosaurs was a whole-body event.[3] The bottoms of bird feet are covered in small, keeled scale-like structures known as reticulae. Evolutionary developmental studies on these scale-like structures have revealed that they are composed entirely of alpha keratin (true epidermal scales are composed of a mix of alpha and beta keratin).[3] These data have led some researchers to suggest that reticulae are in fact highly truncated feathers.[3][4]

Insects and other arthropods

The term "scutum" is also used in

metanotum
, though rarely applied in that context).

Conspicuous scutum on a typical female hard tick before she has fed. Note the pale eye-spots near the edges of the scutum, roughly between the 2nd and 3rd legs
The same scutum is relatively less conspicuous after the tick has fed, because it has not changed in size, whereas the tick has swollen as it engorged
In the typical male hard tick, the conscutum covers practically the whole back

In the

hard ticks, the Ixodidae, the scutum is a rigid, sclerotised plate on the anterior dorsal surface, just posterior to the head. In species with eyes, the eyes are on the surface of the scutum. The flexible exoskeleton posterior to the rigid scutum of the female tick, is called the alloscutum, the region that stretches to accommodate the blood with which the mature female tick becomes engorged. Males do not engorge nearly as drastically as females, so they do not need a flexible alloscutum; instead the rigid scutum covers practically the entire dorsal surface posterior to the head, and may be referred to specifically as the conscutum.[5]

In some species of Opiliones, fused abdominal segments are referred to as a scutum.[6]

See also

References

  1. ^ a b Sawyer, R.H., Knapp, L.W. 2003. Avian Skin Development and the Evolutionary Origin of Feathers. J. Exp. Zool (Mol Dev Evol) 298B:57–72.
  2. ^
    S2CID 44477271
    .
  3. ^ .
  4. .
  5. .
  6. .
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