THE ANIMAL FILES

Itch mite

Sarcoptes scabiei

Count the legs at different ages: six becomes eight.

The itch mite is a tiny parasitic arachnid, not an insect. Its rounded body and short appendages suit a life closely associated with mammalian skin. The human-associated form is one variety within a species also associated with other mammals.[1]

Follow the evidence ↓
Itch mite
Image: Kalumet · Commons · CC BY-SA 3.0
Adult legs
Four pairs[1]
Larval legs
Three pairs[1]
Eyes
Absent[1]

A compact body

Adult mites are oval, flattened beneath and rounded above. Short bristles and spines cover the folded surface. There is no obvious narrow division between the front and rear of the body like the waist of a familiar spider.[1]

The number changes during development

The lifecycle proceeds through egg, larva, nymph and adult. Larvae have six legs, whereas nymphs and adults have eight. Leg number must therefore be interpreted together with life stage rather than used as an isolated label.[1]

A tunnel at the outer surface

In the human-associated variety, females extend tunnels in the outermost skin layer while depositing eggs. Hatched larvae move to the surface and use smaller spaces during subsequent moults. These are different locations and activities within one developmental cycle.[1]

A COMMON MIX-UP

Does a six-legged young mite have to be an insect?

No. The six-legged larva is a developmental stage of an arachnid whose later stages have eight legs.[1]

TRY A LITTLE REASONING

What should accompany a leg count in a tiny animal?

Reveal the explanation

Its life stage. Comparing a larva with an adult can otherwise make one species appear to belong to different groups.[1]

Sources & editorial notes

Adapted and translated from the cited Wikipedia revisions, with source-linked reasoning. Image credit and licence are listed separately above.

Adapted and translated from Wikipedia contributors; exact revisions are linked below. CC BY-SA 4.0

  1. Wikipedia contributors — Itch mite (fixed revision; adapted and translated)
  2. NCBI Taxonomy — Sarcoptes scabiei (identity and classification)

Last fact check:

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