ON INDIAN PARASITIC FLIES. 373 
Section Pupipara. .. Hippoboscide a 
Streblide meh As 
Nycteribiidee ig > Imayo parasitic. 
(Braulide) .. i 
Various considerations, interesting to the student of parasites, 
will occur to the reader who thinks over the facts disclosed by this 
table. In the first place it will be noted that parasitism is far more 
prevalent in the Cyclorrhapha than in the more primitive Orthorrha- 
pha. Indeed in the Nematogerous division, which includes the most 
primitive flies known, there are no parasitic species at all; and in 
the Brachycerous division only two families, neither of which are 
very large ones. This absence of parasites from the primitive groups 
of Diptera is confirmation of Metchinkoff’s dictum: that among 
parasites we are to look for the latest products of evolution. 
In the secon4 place, it will be noted that parasitic larvae and para- 
sitic wnagines are sharply contrasted and separated. Only in the 
Pupipara do we find adult flies parasitic on mammals and birds. Larval 
parasitism is far more widely spread. The nourishment of the indi- 
vidual and the reproduction of the species is the aim and end of all 
animals. In insects the larval stage is the period of feeding and of 
growth; the imaginal stage is the period of love and procreation. 
Now parasitism is ultimately a matter of securing nourishment. The 
parasite solves the problem of nutrition, but is often confronted with 
difficulties when the problem of reproduction has to be faced. This 
explains why in Diptera, to deal only with the one order under review, 
the feeding-stage, rather than the reproductive, should more com- 
monly be the parasitic stage. It is an advantage to a larval fly to 
be well supplied with food and, as an incident, to be a parasite. It 
isequally an advantage toa mature fly to be free living, and to 
move about seeking the other sex or depositing eggs. The 
breeding habits of the Pupipara, which do not lay eggs and nourish 
the larva within the maternal abdomen, enable the mature fly to enjoy 
the advantages of a parasitic life. There is no free larval stage to be 
considered ; but against this, nourishment for the adult fly is essential, 
for without it the young could not be reared. As a result the adult 
fly is parasitic. 
There are other reasons why the larval stage should be parasitic. 
The habits of Diptera easily pass over into parasitism; but this 
involves some changes in the way of life which probably come 
more easily at an immature stage, whilst development and growth 
are in progress. The greater number of dipterous larvae are vege- 
table feeders, but many live on decomposing animal matter. From 
this it is but a step to live and feed in and on the living bodies of 
other insects or snails, reptiles, birds and mammals. For obvious 
reasons no dipterous larva is known to be parasitic in a fish. The 
