n amber, as proved by the deposits on the
shores of the Baltic, the proverbial "fly" is more numerous than any
other creatures, and with very few exceptions representatives of all the
existing families have been found. The famous Tertiary beds at
Florissant, Colorado, have yielded a considerable number of remarkably
well-preserved _Tipulidae_ (in which family are included the most
primitive of existing Diptera), as also species belonging to other
families, such as _Mycetophilidae_ and even _Oestridae_.

Diptera as an order are probably more widely distributed over the
earth's surface than are the representatives of any similar division of
the animal kingdom. Flies seem capable of adapting themselves to
extremes of cold equally as well as to those of heat, and species
belonging to the order are almost invariably included in the collections
brought back by members of Arctic expeditions. Others are met with in
the most isolated localities; thus the Rev. A. E. Eaton discovered on
the desolate shores of Kerguelen's Island apterous and semi-apterous
Diptera (_Tipulidae_ and _Ephydridae_) of a degraded type adapted to the
climatic peculiarities of the locality. Many bird parasites belonging to
the _Hippoboscidae_ have naturally been carried about the world by their
hosts, while other species, such as the house-fly, blow-fly and
drone-fly, have in like manner been disseminated by human agency. Most
families and a large proportion of genera are represented throughout the
world, but in some cases (e.g. _Glossina_--see TSETSE-FLY) the
distribution of a genus is limited to a continent. As a rule the general
_facies_ as well as dimensions are remarkably uniform throughout a
family, so that tropical species often differ little in appearance from
those inhabiting temperate regions. Many instances of exaggerated and
apparently unnatural structure nevertheless occur, as in the case of the
genera _Pangonia_, _Nemestrina_, _Achias_, _Diopsis_ and the family
_Celyphidae_, and, as might be expected, it is chiefly in tropical
species that these peculiarities are found. To a geographical
distribution of the widest extent, Diptera add a range of habits of the
most diversified nature; they are both animal and vegetable feeders, an
enormous number of species acting, especially in the larval state, as
scavengers in consuming putrescent or decomposing matter of both kinds.
The phytophagous species are attached to various parts of plants, dead
or a