In ordinary language the name is used for any species of Siphonaptera (otherwise known as Aphaniptera), which, though formerly regarded as a suborder of Diptera, are now considered to be a separate order of insects.
But the characteristic nature of the avifauna is more clearly brought out when we learn that of the 2000 species just mentioned only about 1070 belong to the higher suborder of Oscines, that means to say, nearly one-half belong to the lower suborder Clamatores.
The Pupipara are also termed Eproboscidea (although they actually possess a well-developed and functional proboscis), and by some dipterists the Eproboscidea are regarded as a suborder .and contrasted as such with the rest of the Diptera, which are styled the suborder Proboscidea.
With many writers it is customary to treat the fleas as a suborder of Diptera, under the title Aphaniptera or Siphonaptera.
It is one of the more typical Opisthobranchs, that is to say, it belongs to the section Tectibranchia, but other members of the suborder, namely, Bulla and Actaeon (figs.