6
to no less an authority than Williston, a concentration of the wing veins to-
wards the anterior margin, and a shortening of the most anterior veins, are evi-
dence of specialisation, and once these changes have taken place they are irre-
versible; a comparison of the venation of Musca and Glossina indicates clearly
that the latter fly, according to this criterion, is the more recent of the two
forms. Secondly, we find that a tendency towards the pupiparous habit, un-
doubtedly a recent development in the Diptera, and one which indicates a
high degree of specialisation, appears along with the blood-sucking habit,
though it occurs somewhat irregularly in the group. Philœmatomyia insignis
is oviparous, but the eggs hatch in a remarkably short time and are larger when
laid than one would expect for the size of the fly. Stomoxys and Lyperosia are
normal in their reproduction; Glossina approaches very closely to the Pupipara.
As the Pupipara are typically blood-sucking and parasitic, there is evidently
some relation between the method of obtaining nourishment and the method of
reproductien. What this relation may be is not decided, but the modification
of the reproductive organs and the divergence from the type found in other
Diptera are so extreme that the descent of non-blood-sucking oviparous forms
from blood-sucking pupiparous ones is not conceivable. The case is not com-
parable with the larviparous habit among the Tachinidœ, for the larvæ of
many Muscid biting flies live in dung or other situations side by side with those
of other oviparous Muscids, whereas those of Tachinids are usually deposited in
the bodies of other larvæ and a shortening of the immature stages is almost
essential to the continuation of the race.
Without going into any further detail in this matter it will suffice to repeat
that the arrangement of the Muscid biting flies in the above order, as more
recent and more specialised than those Muscids which have mouth parts adapted
only for the absorption of fluid from moist surfaces, is in agreement with
the accepted systems of classification, which are for the most part founded on
other anatomical or biological features. The theory that the flagellates of
insects are descended from hæmoflagellates finds little support from entomological
facts; how they originally reached the intestinal tract ,of the insect it is not
the purpose of this paper to discuss.*
*NOTE.—If I may be permitted to make a suggestion here, the argument of the above paragraphs may be
carried further. At that remote geological period when the waters covered the earth, the great majority of
insects, and arthropods which were destined to become insects, must have been aquatic or semi-aquatic in their
habits; during this phase of their evolution free living flagellates, presumably present in abundance then
as now in water rich in organic matter, must have passed into the intestinal tract of the insect either as food
or accidentally with the fool. In thc latter case, presuming that they were able to resist the action of the
digestion by encystment iu response to an unfavourable environment or otherwise, the conditions for the estab-
lishment of parasitism would be present, and the dependence of the flagellate on its insect host for the
completion of its life cycle but a short step.