Synopsis of a hundred common non-anopheline mosquitoes of the ...

8
contain-ng a gorged female, is closed with a plug of absorbent cotton which has been
moistened in sugar water; the tube is then paraffined and placed upside down in an
empty tin. The tin should be protected from ants by placing it on a dish with water.
The tube must be put down with the moist plug undermost to prevent the sugar water
from running along the side of the tube, causing the mosquito to stick to the glass wall.
Mortality is usually large and it is very important to have a large number of mosquitoes.
The purpose is to obtain mature, very mobile Maria larvae in the head and proboscis
of the mosquitoes. If successful the mosquito may be considered a good carrier. The time
it takes for the filaria larva to reach this stage in the tropics is from 8 to 14 days and it is
for this length of time that our mosquitoes must stay alive. If IO per cent live longer than
IO days, the results may be considered satisfactory, provided one starts with a large
enough number of specimens. The mosquitoes which die prematurely are not entirely
without value, if they are not desiccated. They may contain living worms and may be
used for dissection. If, however, they have been dead for more than one day they are
useless. It is absolutely necessary to control test-tubes and cages once or twice a day
for dead specimens which may then be identified and dissected. Experimental infections
in our region were positive in the following Culicines (for W. ba~~~~fti) : Culex vishmi,
C. fuscocephalus, C, whitmovei, C. amulirostris and Aedes scutellaris.
What to do with the mosquitoes to be dissected?
(I) Identify the specimen, (z) Take it by a wing with pincers or with fingers and cut
off the legs. (3) Cut off the wings and the abdomen with fine scissors. Place the head and
thorax in a drop of 0.4 per cent NaCl on a glass slide. Use a dissecting microscope or, if
not available, a monocular one. In the latter case use a very low power. A magnification
of 25 to 30 times is sufficient. Larger magnification makes dissecting more difficult,
especially with a monocular microscope.
Sometimes, before dissection has started, larvae appear from the insertion of the
legs or from the proboscis. Dissecting is done with two needles, which should not be too
coarse. It is a good idea to keep two different sizes of needles: thicker ones and fine ones.
If no holders for the needles are available, they may be improvised by cutting off the
heads of mosquito pins, heating them and inserting the hot end in a small wooden stick,
just long enough to lie comfortably in the hand. Bending the needle 45 degrees at a
point about 8 cm before the tip will make it a good dissecting instrument. Pull the
proboscis off the head and the head off the thorax; open the head, but be careful not to
damage the eyes otherwise the pigment will stream over the mount. Now open the
thorax a,ld pull the muscles apart; be careful not to damage any of the tissues. If mature
larvae are present they have had three chances to show themselves: (I) when the pro-
boscis ar.d (2) the head were pulled off, and (3) when opening the thorax, These larvae
are very mobile and move about in the saline drop; they are I-I& mm long and even
with a magnification of 25 are very clearly visible; it is impossible to overlook them.
From an epidemiological point of view it is important to find these large, mobile, mature
larvae; incompletely developed larvae may be found in mosquitoes which are never
carriers because in these specimens the larvae die before they are mature. Mature larvae
are only found in head and thorax; the development of the larvae takes place in the
thorax between the bundles of muscles; the details of this process may be found in a
handbook on the subject. It is important, when searching for immature stages, that the