een to be minute transparent, colorless, snake-like
organisms inclosed in a very delicate sack or sheath. They are but a
little more than one-hundredth of an inch long and about as big around
as a red blood-corpuscle. These are the larval forms of the parasite and
have been called by Le Dantec the micro-filaria.

If blood of the patient drawn from the skin, is examined during the day
few if any of these parasites are found, but if it is examined between
five or six o'clock in the evening and eight or nine o'clock the next
morning they may be found in numbers. During the daytime they have
retired from the peripheral circulation to the larger arteries and to
the lungs, where they may be found in great numbers.

This night-swarming to the peripheral circulation has been found to be a
remarkable adaptation in the life-history of the parasite, for it has
been demonstrated that in order to go on with its development these
larval forms must be taken into the alimentary canal of the mosquito.
Most of the mosquitoes in which the development takes place are
night-feeders, so that the parasites are sucked up with the blood of the
victim. Once inside the stomach they soon free themselves from the
inclosing sheath and make their way through the walls of the stomach and
enter the muscular tissue, particularly the thoracic muscles. Here they
undergo a metamorphosis and increase enormously in size, some attaining
one-sixteenth of an inch in length.

After sixteen to twenty days they leave these muscles and make their way
to other parts of the body. A few may be found in different parts of the
abdomen, but most of them make their way forward into the head of the
mosquito and coil themselves up close to the base of the proboscis,
finally finding their way down into the proboscis inside the labium.
Here they lie until an opportunity offers for them to escape to the warm
blood of a vertebrate. They probably pass through the thin membrane
connecting the labella with the proboscis and there find their way into
the wound made by the puncture when the insect bites. Whether these
parasites can gain an entrance into the circulatory system in any other
way is not known. It has been suggested that the mosquitoes dying and
disintegrating on the surface of water may liberate the filariae which
may later find their way into the system of the vertebrate host when the
water is used for drinking, but most of the investigations made so far
seem to indi