There are many large and poisonous spiders and flies; fleas and mosquitoes abound.
Fleas are wingless insects, with a laterally compressed body, small and indistinctly separated head, and short thick antennae situated in cavities somewhat behind and above the simple eyes, which are always minute and sometimes absent.
With many writers it is customary to treat the fleas as a suborder of Diptera, under the title Aphaniptera or Siphonaptera.
The wingless forms in question are always allied to winged forms, and there is every reason to believe that they have been really derived from winged forms. There are also insects (fleas, &c.) in which metamorphosis of a " complete " character exists, though the insects never develop wings.
Lamarck at the same time founded the class Crustacea for the lobsters, crabs and water-fleas, also until then included in the order Aptera of Linnaeus.