A most remarkable feature in the bite of the tsetse is its perfect harmlessness in man and wild animals, and even calves, so long as they continue to suck the cows.  We never experienced the slightest injury from them ourselves, personally, although we lived two months in their habitat, which was in this case as sharply defined as in many others, for the south bank of the Chobe was infested by them, and the northern bank, where our cattle were placed, only fifty yards distant, contained not a single specimen.  This was the more remarkable, as we often saw natives carrying over raw meat to the opposite bank with many tsetse settled upon it.

The poison does not seem to be injected by a sting, or by ova placed beneath the skin; for, when one is allowed to feed freely on the hand, it is seen to insert the middle prong of three portions, into which the proboscis divides, somewhat deeply into the true skin; it then draws it out a little way, and it assumes a crimson color as the mandibles come into brisk operation.  The previously shrunken belly swells out, and, if left undisturbed, the fly quietly departs when it is full.  A slight itching irritation follows, but not more than in the bite of a mosquito.  In the ox this same bite produces no more immediate effects than in man.  It does not startle him as the gad-fly does; but a few days afterward the following symptoms supervene:  the eye and nose begin to run, the coat stares as if the animal were cold, a swelling appears under the jaw, and sometimes at the navel; and, though the animal continues to graze, emaciation commences, accompanied with a peculiar flaccidity of the muscles, and this proceeds unchecked until, perhaps months afterward, purging comes on, and the animal, no longer able to graze, perishes in a state of extreme exhaustion.  Those which are in good condition often perish soon after the bite is inflicted with staggering and blindness, as if the brain were affected by it.  Sudden changes of temperature produced by falls of rain seem to hasten the progress of the complaint; but, in general, the emaciation goes on uninterruptedly for months, and, do what we will, the poor animals perish miserably.

When opened, the cellular tissue on the surface of the body beneath the skin is seen to be injected with air, as if a quantity of soap-bubbles were scattered over it, or a dishonest, awkward butcher had been trying to make it look fat.  The fat is of a greenish-yellow color and of an oily consistence.  All the muscles are flabby, and the heart often so soft that the fingers may be made to meet through it.  The lungs and liver partake of the disease.  The stomach and bowels are pale and empty, and the gall-bladder is distended with bile.

These symptoms seem to indicate what is probably the case, a poison in the blood, the germ of which enters when the proboscis is inserted to draw blood.  The poison-germ, contained in a bulb at the root of the proboscis, seems capable, although very minute in quantity, of reproducing itself, for the blood after death by tsetse is very small in quantity, and scarcely stains the hands in dissection.  I shall have by-and-by to mention another insect, which by the same operation produces in the human subject both vomiting and purging.