The shells or exuviae of these arachnids are common in beach drift in certain areas of the world.
From the en.wikipedia.org
The shells of sea urchins are called tests, and the moulted shells of crabs and lobsters are called exuviae.
From the en.wikipedia.org
The authors predicted that the areas littered with exuviae decoys would be seen as poor hunting grounds by the wasps who would move on to other patches.
From the sciencedaily.com
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Cast-off skins or coverings of various organisms during ecdysis
Exuviae is a term used in biology to describe the remains of an exoskeleton and related structures that are left after ecdysozoans (including insect, crustacean or arachnid) have moulted. ...
The coverings of an animal that have been shed or cast off, particularly the molted exoskeletons of arthropods; Roman military term for weaponry and equipment stripped from the person of a foe; booty
Cast skin of an arthropod, e.g. mite, spider, insect.