370 
ON INDIAN PARASITIC FLIES. 
BY 
HaARro_tp RUSSELL, F.L.S., F.Z.9. 
iD 
This paper is concerned with parasitism among Diptera and parti- 
cularly among Indian flies. Of the dozen or thirteen families of two- 
winged flies, in which parasitic habits have been developed, all but 
one are represented in the insect fauna of India. In some cases it is 
the larva, in others the perfect or winged insect, which is a parasite. 
There are also cases of wingless flies, which pass the whole of their 
existence on the bodies of the hosts from which they draw their 
nourishment. By no means all blood-sucking flies are parasites ; for 
parasitism, in the strict sense, implies that a hostis essential to the 
well-being and development of the parasite. On the other hand all 
Diptera which are parasitic in the imaginal, or final, stage of their 
lives, are blood-suckers. When the respective life-histories of an 
(Hstrid anda Hippoboscid fly are contrasted, it will be seen how 
entirely larval parasitism differs from imaginal parasitism and what 
adaptations have been developed to secure the well-being of the 
larva and the perfect insect, respectively. 
By way of preface some generalities on two-winged flies are neces- 
sary to render intelligible what follows on the parasitic species. The 
Diptera are physiologically among the highest insects; none get 
through their life-history more rapidly; and no insects go through 
such a complete metamorphosis as flies. In the Muscid flies we have 
the most highly specialised imago and the most degraded larva known 
in any group of insects. The larvee are usually called grubs or maggots. 
Thoracic legs are always absent ; but many larve have pseudopods by 
which they move about with great facility. 
Diptera may at once be recognised by the presence of only one pair 
of wings. The second or hind pair are represented by poisers or 
balancers called halteres, which are small organs consisting of a stem 
andaknob. The halteres are, obviously, homologous with wings ; and 
in the parasitic wingless forms they usually disappear along with the 
wings. They vibrate during flight, and besides acting as balancers, 
they also serve as sense organs. The basal part is supposed to contain a 
structure which allows sound to be perceived. The wing veins, 
which are of great taxonomic importance, are comparatively few 
and for the most part run longitudinally. If the halteres represent 
wings, it is remarkable that no organs intermediate between the two 
should be found in any Dipteron. The thorax is welded into a single 
mass instead of being composed of three clearly divided segments. 
The legs are characterised by five-jointed tarsi and are often long and 
slender. In the purely parasitic Hippoboscide they tend to become 
shorter but exceedingly stout. The foot of a fly is composed of a 
