Of the more than 30 million species of animals estimated to exist
(only about 10 million of which have been formally described
and named), phylum Mandibulata claims the largest number.
There are more than 1.2 million species of insects alone not to
mention centipedes, millipedes, and less familiar species in this
phylum. Uniramia (one branch) indicates unbranched appendages.
Both unbranched (uniramous), as in insects, and branched
(biramous) appendages, as in crustaceans, consist of a linear
series of joints, as do all arthropod appendages; but both insects
and crustaceans are considered mandibulates. The mandibulate
body has three distinct parts: head (cephalum), thorax, and
abdomen. As the mandibulate changes size or form, it secretes
a new exoskeleton and sheds the outgrown armor. Although
many mandibulates are dioecious, parthenogenesis is common
in insects. Many species of mandibulates metamorphose: the
fertilized egg hatches a larva that develops into a series of juvenile
forms that differ considerably from the adult.