The boundaries of this pest are well defined; and we have heard the Dutch colonists speak of “De Kant van de Vleigen,” or the “edge of the flies,” with as much precision as a municipal officer in defining the boundary of his parish. Frequently one side of a waggon road will be spoken of as infested, while the other is safe; and sometimes the hunters speak of riding up to the “edge of the fly,” and going in on foot to shoot game. Whether the rather more than half-reasoning elephants know that the fly kills horses or not, we cannot undertake to say; but they certainly connect the idea of comparative security with the “fly country,” just as an Australian horse knows that the stockyard is the only place in which he is never flogged, and retreats to it whenever he hears the cracking of the dreaded stock whip.
That the fly is local there can be no doubt; and the only chance, so far as we have heard, of this pest being found out of its proper district, is that a herd of buffaloes or other game may be chased to some distance, and carry the fly with them; but if they remain long the insect will leave them and return to its proper range.
The fly, a little more than half an inch long and more slender in proportion than a common house fly (the illustrations given below, Figs. 1 and 2, give the true size and an enlarged view of the insect), hovers steadily over the devoted cattle with a peculiarly rapid motion of the wings. To speak either of its bite or its sting would convey an erroneous idea; it seems to pierce the skin, and dilute the blood it wishes to drink by the injection of a fluid, just as the mosquito does; the surplus liquid mingles fatally with the blood. We believe neither goats nor sheep are injured by the virus. The piercing apparatus must be of considerable length, as the insect will penetrate a pilot coat and full suit of underclothing; the puncture leaves no subsequent pain like that of a mosquito, but is startling enough at the moment. The description of the fly may be thus summed up: The abdomen is marked with transverse stripes of yellow and dark chesnut, fading towards the back, and imparting the appearance of a longitudinal stripe of yellow down the centre of the back. The belly is a livid white; dusky glassy-brown wings folding over each other; eyes, brownish purple. It has six long legs. Its proboscis is about one-sixth of an inch long. It has tufts of hair on the body, which are dingiest about the mouth, on the back, and near the tail. It is keen of smell, quick of sight, and its flight is rapid and straight.
It is said that a peculiar breed of dogs, known as the “Moscoba,” or Bàylyè dogs, remain exempt from injury, from having from time immemorial been reared in the fly district, and escaped a cow-milk diet as the natives say. It has no injurious effect upon game whatsoever.
The Cape Colony, the Free State, Kafirland, Natal, and most of the Transvaal Republic, as well as Namaqua and Damara land—the Kalihari Desert and the desert between the Bō-tlét-lē and Zambesi rivers—may be regarded (speaking generally) as clear of the fly, but the hunters on the various tributaries of the Limpopo suffered very heavy losses; and Mr. Coqui and party, who travelled from Origstadt to Delagoa Bay, lost all their cattle, we believe, from the tsetse, not far from the last-named place. When travellers first began to find their way to Lake Ngami many, for want of local knowledge, lost sometimes half their cattle. At Tette, and the other Portuguese possessions on the Zambesi, very few cattle and no horses are kept, but Senhor Pascoal possessed a few donkeys. The natives in the vicinity have no cattle. We do not remember that we saw the tsetse there, but possibly this may have originally prevented their introduction, and the fly may have died out in places where the wild animals have been destroyed. When we travelled from Walwisch Bay, we fell in with no fly all the way to Lake Ngami, but turning thence to the north-west we feared to push too far to the northward, as the banks of the Teoughe, and probably the woods some distance from it, were known to be infested.
From the lake eastward we travelled in comparative safety along the Bō-tlét-lē River, and turned north over the elevated riverless plain towards the Zambesi. In the valley of that great river system we first felt ourselves in actual proximity to the fly. At Dāká, the cattle grazed in safety; but a servant, who was sent to outspan, ten or twenty miles to the west, had to return because he had got into an infested locality; and when we started with one waggon only to visit the Falls, we found that patches of mimosa and other forests on the banks of the Matietsie River were also frequented by these little pests.
We tried to save the oxen by rushing them past whenever the edge of the bush approached too closely to the river; but an accidental delay exposed the cattle to the fatal influence of the tsetse. At the Anyati, or Buffalo River, we had to leave the waggon and cattle as the long bed or sand hill covered with mopani and other trees, between it and the Falls, was known to be infested.
Mr. Baldwin also on his way to the Victoria Falls, from Natal, left his waggons in Moselekatses country, on account of the fly in the intervening districts, and made his way on foot. A fellow traveller of ours had a safe camp for several months with cattle at Boana; and at Logier Hill on the Zambesi (lat. 18° 4′ 58″ S.; long. approx. 26° 38′ E.) we do not remember to have seen any, though we resided there from September to the following February, in 1862–63.
In the parts about Chobè the fly is found near rivers only, in or near rich soils, and marshy spots—generally in mimosa or mopani forests. It sometimes shifts its position, and has been known to leave a spot which has been greatly hunted with guns—probably because the game had diminished or left.