It appears from observation of the embryo that whilst the first prosthomere of Centipedes has its appendages reduced and represented only by eye-patches (as in Arachnida, Crustacea and Hexapoda), the second has a rudimentary antenna, which disappears, whilst the third carries the permanent antennae, which accordingly correspond to the second antennae of Crustacea, and are absent in Hexapoda.
Mosquitoes, termites, bees, ants, centipedes, millipedes, locusts, grasshoppers, butterflies, dragonflies, sandflies and spiders' are found in immense numbers.
Aristotle had included in one class "Entoma" the six-legged arthropods which form the modern zoological class of the Hexapoda or Insecta, besides the Arachnida, the centipedes and the millipedes.
The names Condylopoda and Gnathopoda have been subsequently proposed for the same group. The word refers to the jointing of the chitinized exo-skeleton of the limbs or lateral appendages of the animals included, which are, roughly speaking, the Crustacea, Arachnida, Hexapoda (so-called " true insects "), Centipedes and Millipedes.
White and red ants are very prevalent, as are mosquitos, centipedes, spiders and beetles.
They were until recently classified with the Chilopoda (Centipedes), with which they have no close affinity, but only a superficial resemblance.
Linnaeus in his Systema naturae (1735) grouped under the class Insecta all segmented animals with firm exoskeleton and jointed limbs - that is to say, the insects, centipedes, millipedes, crustaceans, spiders, scorpions and their allies.
His order of wingless insects (Aptera) included Crustacea, spiders, centipedes and other creatures that now form classes of the Arthropoda distinct from the Hexapoda; it also included Hexapoda of parasitic and evidently degraded structure, that are now regarded as allied more or less closely to various winged insects.
The term, however, is somewhat elastic in its current use, and students of centipedes and spiders are often reckoned among the entomologists.
Birds and mammals take the first place; the leading collections devote a good deal of attention to reptiles and batrachians; fishes and aquatic invertebrata are most often to be found only when there are special aquaria, whilst non-aquatic invertebrates are seldom to be seen and at most consist of a few moths and butterflies, spiders, scorpions and centipedes, molluscs and crustaceans.
Insects are numerous, and of about 500 species of beetle some 80% are not known to exist elsewhere; cockroaches and green locusts are pests, as are, also, mosquitoes,' wasps, scorpions, centipedes and white ants, which have all been introduced from elsewhere.