OVIPOSITOR

OF THE BOT-FLY.

THE pictures, with which we are all more or less familiar, of cattle-gentle-eyed cows and bold bulls-standing in a shallow glittering stream overhung with willows and silver-stemmed birches, produce a pleasing sense of rest and coolness in the heat of summer. When gazing at them we almost wish that the conventionalities of life did not exist; and that when the heat is oppressive we too might cool our burning limbs in the purling waters of some sheltered babbling brook.

The oxen, however, do not seek the rippling stream merely for the sake of refreshment. Often it happens that they have been driven there by a few little flies.

HORSE FLY (male).

HORSE FLY (female).

What! flies drive great fiery bulls and gentler cows from the verdant meadows on which they are industriously feeding or peacefully chewing the cud? Yes, the distant hum of half a dozen ox gad-flies is sufficient to send the herds careering madly through the fields, with their tails stretched stiffly out and their noses in the air. On a nearer approach of the insects they rush precipitately into the neighbouring stream, should there chance to be one, whither their tormentors never follow them.

Let us leave them enjoying their relief and refresh

EGGS OF GAD-FLY DEPOSITED ON THE HAIRS OF A HORSE.

ment, and turn to the examination of the insect which has so disturbed their peace.

It is but a little creature, after all; not an inch in

length, and not altogether unlike a blue-bottle fly in shape, though larger, with its abdomen whitish and

HERD OF CATTLE ATTACKED BY BOT-FLIES.

striped with black. It has not, however, the proboscis common to most two-winged insects.

As its mission is not to obtain food for itself the

lanceted tube usually provided for suction is not needed. Its object in life is to place its eggs in such a situation that, when the grubs hatch out, they may be enveloped in the food necessary for their existence, and which their legless condition would prevent them from seeking for themselves. The body of the perfect female fly is, therefore, provided with a wonderful apparatus for this end.

The ox gad-fly's ovipositor (the instrument for puncturing sub

stances and depositing eggs) is

formed like a telescope: that is, the four horny tubes of which it consists lie within each other like the

parts of a closed telescope. When the fly wishes to deposit her eggs, she alights on a cow's back, thrusts out her ovipositor to its full length, drives it right through the thick hide, and, before withdrawing it, lets an egg drop down the tube into the hole thus made for its reception. She is not particular about choosing a spot where the least possible pain may be caused to the animal whose living sensitive flesh is to nourish her young, but sometimes sends her sharp piercer into a nerve, causing the poor ox excruciating agony, driving it, for the time being, quite mad and dangerous.

It is supposed that when the eggs are once inserted (and the process lasts at most a few seconds), the animal has little more to dread either from the mother or her offspring.

This is true so far as the parent is concerned, for she dies as

soon as she has brought

BOT-FLIES.

forth and provided for the sustenance of her young. But it is very improbable that the oxen live on unconscious of, and uninconvenienced by, the appetites