5
into the causes that produce this variation, but I believe that sunshine, tem-
perature and rainfall contribute towards such variations. In certain places
this species was found to breed in water which had some foul smell about it.
When the larvae of A. stephensi are disturbed, they all disappear from the
surface. It is generally believed that they go to the bottom of the cistern or
well. I have observed that the larvae go to the bottom in most cases in which
the water is not over about two feet deep. But in deeper waters, most of
them do not reach the bottom. They go into the water and then stick to the
sides of the cistern or tub, with their tail end upwards; and considerable dis-
turbing was required to dislodge them from their resting places. I then sus-
pected that there was some gripping organ in their tail. An examination of
the tail-end showed no such structures, but the tips of the hairs in the supra-
anal tuft on the ninth segment of the abdomen were curved and hook-like.
It is with these hairs that the larva clings to the sides of the cisterns; and in
doing so, its dorsal surface is in contact with the wall of the cistern. If this
were not the case, the larvae when disturbed would be compelled to go to the
bottom and thus run the risk of being killed by the enormous pressure of
water. Hence perhaps this adaptation of the tail-hairs. These
hooked hairs are not peculiar, however, to A. stephensi alone.
A. fuliginosus and sinensis also have them. Whenever the water
in a tank or pond is disturbed, the larvae rarely go to the
bottom, as thereby they run the risk of being caught by their ever-
wary enemies, the fish. But they get in between the roots of the water-plants
and hang therefrom by their tail-hairs. Very often they also make use of their
mandibles for this purpose. When holding on in this way, head upwards,
from the submerged parts of the water-plants or algae, it is with great diffi-
culty that they can be dislodged.
This fact is significant with regard to Stegomyia fasciata which breeds
in almost all domestic situations in which A. stephensi breeds, but which
rarely breeds with it in the wells. If Stegomyia were to breed in wells, when
the water is disturbed, the larvae would have to go to the bottom of the well
because they lack these tail-hairs of A. stephensi with which to hold on to
the sides. It is likely that mosquito larvae being air-breathing organisms
cannot ordinarily stand much pressure at the depths of a well. Stegomyia
is a bottom and side-feeder; therefore it has to go to the bottom of its breed-
ing place, unlike A. stephensi, which feeds on the surface. These facts
explain why Stegomyia has rarely been found in waters which are over three
feet deep. In the cisterns on the tops of houses, in which A. stephensi breeds,
few of them breed Stegomyia also. Out of ten cisterns breeding A. stephensi,
one perhaps may have Stegomyia also.
In several garden-tubs, Culex concolor (a species whose larva resembles
Stegomyia in all outward appearances) was found to breed along with
A. stephensi. This species is destructive to all forms of mosquito larvae; it
is a very active and voracious creature, often attacking its own kind when no
other food is available. In some tubs, at first, both A. stephensi and Culex
concolor were found to be breeding together, and some days later it was
found that the number of A. stephensi was rapidly diminishing. Some days
thereafter, there was nothing other than Culex concolor in the tub. A
similar observation was made by Dr. Bentley in Bombay. Unfortunately,
this enemy of A. stephensi does not breed in cisterns on house-tops. This is
probably because Culex concolor is a low flier; it attacks man doggedly while
walking or standing on the ground, but it rarely gets even to the first floor,
even though the locality had plenty of it. This species is essentially'a wild
mosquito and occurs in jungly tracts.
Desvoidea obturbans is another predaceous species occurring in the
suburbs of Calcutta. It breeds in foul water in company with Leucomyia
gelida and Anopheles rossi, on both of which larvae it feeds. It is also very
cannibalistic; we can never keep several of them together without missing
many of them. I have also observed them to breed in stagnant water inside
a bad latrine in Tengra.