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INSECTS AND THEIR RELATION 
that fed upon diseased meat, contain the anthrax microbe 
in their bodies and may infect through their excreta. 
But our subject lies more with those Diptera which possess 
biting mouth-parts and actually suck blood. Such insects 
must be more dangerous than non-biting flies, since they can 
deposit the disease-germs beneath the skin, and actually into 
the blood-circulation, through the agency of the bite ; whilst 
the others merely deposit them upon the surface. 
We have all heard of the accidents which may occur to 
man or animal through the use of a hypodermic syringe that 
is not clean and sterile. The mouth-parts of a biting fly may 
be likened to this contaminated syringe-needle. By a previous 
bite it has acquired an infection transmissible at a second or 
subsequent feed. If it be at an immediate second feed, which 
takes place within a short time, and if the germ involved does 
not require to undergo any developmental processes in the fly 
body, the infection is said to be mechanical. For such 
mechanical transmission it is theoretically conceivable for 
any biting fly, whose proboscis is like the syringe- needle, to 
acquire and to give any infection of a microbe present in the 
blood or on the skin of the bitten patient. Tsetse, mosquito, 
sand- fly, horse-fly, and so on, may be culpable; but, fortunately, 
a limitation to this transmission is enforced by Nature, which 
has endowed these several species of flies with certain habits 
of life. Some will not enter a stable ; others a room. A few 
bite only at night ; others only by day. One must live near 
the water in which it breeds ; a second requires almost desert 
dryness. Unless we play into Nature’s hands, and expose 
our animals or our men to the ideal conditions of transmission, 
these mechanical carriers are easily evaded ; for in them the 
germs do not usually live long, and a second bite at several 
days’ interval from the infecting bite will be harmless. 
In opposition to the mechanical transference of disease is 
that known as vital or cyclical. In this, the germ, having 
entered a suitable biting fly with the blood it has ingested, 
commences a specific development, and converts the host 
fly into a breeding-place and reservoir for the disease. After 
the lapse of time necessary for the germ to undergo its full 
development, the fly can infect not only at a succeeding feed, 
