ox IXDIAX rAItASlTlC FLIES. 
715 
not uncommon in Bengal. The nasal fly, bot or head-maggot of 
sheep must have attracted the notice of shepherds in all ages. The 
fly is now found in all four continents and in Australia, the parasite 
having, thanks to man, travelled round the world with its host, 
but whether introduced or indigenous to India is uncertain. 
The adult is rather a small fly, but larger than a house-fly, and of 
a yellowish- grey mottled and hairy apjiearance. Like other CLstrids, 
it avoids the shade and loves the hot bright sunshine. During incle- 
ment weather it hides and emerges in the warmth, to fly with much 
liveliness at a great height and to bask on stones heated by the sun. 
According to some observers the approach of the flies alarms the sheep, 
who huddle together with heads down and bury their noses in the dust. 
After being touched about the nostrils by the female fly, they snort 
and rub their noses on the ground According to other observers, 
the fly has been seen to settle on the nostrils of the sheep, and to 
deposit as many as fifteen eggs in succession, without causing any 
immediate annoyance to the future host. 
There has been divergi.nce of opinion as to whether eggs or larvae 
are eleposited, but there is little doubt that in Europe both habits 
obtain. It has been suggesteel that this depends on the weather.* 
The pregnant female, if the weathen’ is fine, deposits eggs round the 
sheep’s nostril. But if low tempeiature makes her inactive, the ova 
develop within the body of the parent during the period of delay. 
Larvae are then laid in the sheep’s nostrils. The larvae enter the nasal 
cavities of the sheep, creep into the frontal and maxillary sinuses, 
even into those of the horn-cores, and there develop. They pass 
through the usual three stages of an CEstrid larva. In the first stage 
they are white and transparent, with a length of 2 millimetres. When 
they have attained maturity they develop dark transverse streaks, 
and sometimes attain a length of 30 millimetres. The parasites 
remain about ten months in these cavities of the head, feeding on the 
mucus. It is not unusual to find three or four together, and at the 
suitable season they may be found together in all stages of develop- 
ment. Yet they rarely occasion any morbid disturbance in the host 
unless they are far advanced in development and exceedingly numer- 
ous. During the months of larval growth they attach themselves by 
their mouth-hooks to the membranous lining of the cavity. On 
attaining maturity, the larva lets go and creeps about until it passes 
into the nose, from which it is expelled on to the ground by the vio- 
lent snorting of the host. There pupation takes place within twenty- 
four hours. The pupal envelope, at first red, becomes brown and 
ultimately black. In about a month the perfect fly emerges and 
another life-cycle begins. So far as is known, the life-history of the 
other members of this group is somewhat similar. Other species of 
• “ Note on the Deposition of the Eggs and Larvae of CEstnis ovw, Linn.”, by 
Walter E. Collinge, Jour. Econ. Biology, (1906) vol. I, p. 72. 
