Perhaps in the first stage they may be carried by the blood stream. They seem in their second larval stage to wander freely through the tissues, especially through the muscular tissues, of the body of their host—usually working upwards, and not infrequently reaching the neighbourhood of the vertebral column before taking up—still in the second larval stage—their final position, where their presence gives rise to the ‘warbles,’ or subcutaneous cysts or tumours, in which the third and fourth larval stages are passed.
It seems odd that an insect pest, which so seriously affects our supply of leather, of meat, and of milk, should have been studied for over a century and yet conceal its chief secret from man. But the problem is much more difficult than the layman thinks.
Whatever be the route the maggot travels through the body of the calf or cow, by the spring the fourth larval stage—when it is about an inch long, and perhaps half as much in breadth—is reached in the ‘warble’ or cyst, under the skin. Here, nourished by the products of the inflammation it sets up, and breathing by two spiracles at the hinder end of its body, which are directed to the opening of the ‘warble’ which it has pierced through the skin, the larva rests until one fine morning it pushes its way, aided by its stout bristles, through the opening and tumbles into the outer world.
Apparently it does not think much of its new surroundings, for it loses no time in hiding under some clod of earth or stone or crevice in the soil, and straightway turns into a dark brown pupa or chrysalis. This stage lasts three to four weeks, and then the perfect fly emerges, and will soon be ready to lay her eggs on some new victim.
Fig. 12.—Cow being chased by fly. Note terrified look of eyes. (From Hadwen.)
As a rule it is the yearlings who suffer most, and then the two-year-olds; the older cattle being comparatively immune. The inexplicable terror which the warble-fly induces in its victims is testified to on all hands, but has never been adequately explained. Hypoderma does not bite, neither does it sting. Many other blood-sucking insects, whose puncture must involve some pain, are tolerated by cattle with a flick of the tail, or are frightened off by a gesture of the head; but the presence of the warble-fly induces a mysterious fear which rapidly spreads through a herd, and results in a general stampede—often referred to by cattle-breeders as the ‘gad.’ This terror communicates itself even to the ‘stalled ox,’ and cattle confined within cowsheds show symptoms of extraordinary unrest when the fly is abroad amongst their kin in the pastures. The resulting evils are, of course, far graver in the unlimited prairies of the West—the great cattle-breeding districts of the United States and Canada—than in our carefully hedged or fenced meadows. A great many ‘dips,’ ointments, and chemical solutions have been recommended for the prevention of the grubs in cattle, but none have proved entirely satisfactory. The tedious method of removing the grub from the tumour is the only safe one. This can be done by the mere pressure of the fingers when the grub is nearly mature and ready to leave its host, or by the use of small forceps should the grub be young and recalcitrant. Once removed the grub should be immediately destroyed, and some such antiseptic as coal-tar applied to the lips of the vacated tumour.
CHAPTER IV
THE MOSQUITO (Anopheles maculipennis)
Part I
Where the water is stopped in a stagnant pond,