Biomedical Engineering Reference
In-Depth Information
Chelicerata (Chapter 29)
Arachnida
Acari
Sarcoptiformes
Sarcoptidae
*Sarcoptes
Trombidiformes
Demodicidae
*Demodex
Cheyletidae
*Cheyletiella
Hexapoda (Chapter 30)
Crustacea (Chapter 31)
Platyzoa
Platyhelminthes (Chapter 26)
Acanthocephala (Chapter 28)
Fungi (Chapters 33
37)
The subclasses of Class Arthropoda that harbor human infectious organisms
are Class Chelicerata, Class Hexapoda (Chapter 30), and Class Crustacea
(Chapter 31). Class Chelicerata contains a variety of organisms that would
appear, at first glance, to be unrelated: spiders, mites, ticks, scorpions, and
horseshoe crabs. The common names do not help much. Horseshoe crabs are
not true crabs; true crabs belong to Class Crustacea (Chapter 31).
All the members of Class Chelicerata have chelicerae, embryonic appen-
dages that form prior to, and in the vicinity of, the mouth. In most species
within Class Chelicerata, the chelicerae are feeding pincers. In spiders, the
chelicerae are fangs. Another feature of the chelicerates, which helps to
distinguish them from insects, is the absence of antennae.
The enormous difference in size between a horseshoe crab (60 cm or
less) and a Demodex mite (less than 1 millimeter) is yet another reminder
that biological classes are not determined by similarities (such as size), but
by phylogenetic relationships (such as chelicerae). Aside from their class-
specific chelicerae, chelicerates inherit the body plan of their superclass,
Class Arthropoda. This means that
they have a heart
that pumps blood
though a major body cavity called a hemocele.
Most members of Class Chelicerata are non-infectious in humans. Only
three genera of Class Chelicerata live in, or on, humans, and both genera
belong to the subclass of arachnids named Class Acari, which includes mites
and ticks. Readers should not confuse mites and ticks with insects. Insects
are members of Class Hexapoda (Chapter 30).
Ticks are vectors for a variety of infectious pathogens. Tick species
Ixodes scapularis transmits Babesia microti (babesiosis), Borrelia burgdor-
feri (Lyme disease), and Anaplasma phagocytophilum (human granulocytic
