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317
During the first quarter of the hour, the fish
filled their stomachs with the larvae, in the second
quarter the larvae passed on to the intestine, in the
third quarter the food had moved on to the mid
gut, and at the end of 90 minutes the larvae were
seen in the posterior part of the gut as well.
Similar experiments on the intensity of digestion
in Panchax panchax 1 were carried out by Hora
and Nair [1938]. It is interesting to note that
Colisa lalia exhibits as great an intensity of diges-
tion as P. panchax, as in the experiment, 20
Colisa lalia had, between them, devoured 564
mosquito larvae in 1½ hours, out of which 511
were found in stomach and 53 in intestine, while
20 P. panchax [Hora and Nair, 1938], fed on mixed
diet and dissected two at a time at intervals of
30 minutes, accounted for 280 mosquito larvae
and pupae, 16 Chironomid larvae, 14 water mites,
13 Corixiid bugs and 1 May-fly nymph in five
hours.
(b) In another experiment ten starved Colisa
lalia were fed for an hour on abundance of
mosquito larvae and were then transferred to
another receptacle containing clear water without
any food. The fish were dissected in two lots of
five specimens each after two hours and 3¼| hours
respectively to find out the time taken by the food
to pass from the stomach into the gut. In the
first lot, one fish had four mosquito larvae in the
stomach and the other had 10, while intestines
in all the fish were full of digested remains of
larvae. In the second lot, dissected after 3¼
hours, stomachs were empty, while intestines
were full of digested remains. The food, obviously,
had passed totally into the gut within three hours.
(c) Four starved Colisa lalia were fed for 15
minutes on mosquito larvae and then put in a
jar containing clean water at 8-30 a.m., tempera-
ture of water being 90ºF. In two hours their
excreta came out showing that the food had been
digested. The fish dissected after two hours,
i.e. at 10-30 a.m., had two mosquito larvae in
its stomach and digested food in posterior part of
the gut, while another specimen dissected at
11 a.m., i.e. 2½ hours after feeding, had its stomach
empty and digested food in posterior part of the
gut. In Colisa lalia, therefore, it may be said
with some confidence that food takes two to three
hours to travel from mouth to anus.
B. In field
(i) Feeding capacity. Capacity of the larvi-
vorous fish to feed on mosquito larvae in tanks and
ponds was studied by examination of their gu
contents. The result of the investigations showt
(Table III), that Colisa lalia is as active in the
field as in the laboratory in devouring mosquito
larvae. When introduced into a tank where
mosquito larvae are to be found, these fish have
invariably been found to feed on larvae. It is
only occasionally that some algal filaments or
other vegetation are found in their gut. Out of
112 guts of Colisa studied during the various
months of the year, 32 contained algae or green
vegetation. In August, specimens of Colisa,
studied from one of the tanks at Lyallpur, had
numerous green, round bodies and Diatoms.
The contents of the guts looked like a green mass
mixed with semi-digested remains of numerous
Crustacea (Daphnia and Cyclops), and Rotifers.
Study of these microscopic animals from the tank
from which Colisa were netted showed that these
Crustacea and Rotifers had green bodies in their
guts. Evidently the green vegetation seen in
Colisa was not eaten by it directly, but was the
one consumed by the Crustacea and the Rotifers
on which the fish had fed.
In Ambassis, out of 109 guts examined, 15 only
had any vegetation. The fish, however, feeds
voraciously on Crustacea, especially Cyclops.
Its cyclopscidal activities have recently been
demonstrated by Job [1941, 1].
Barbus (Puntius) sophore is an active larvivorous
fish for demonstration purposes in the laboratory,
but its usefulness to destroy mosquito larvae in
tanks and ponds is rather doubtful. Sen [1937]
found that the food of these fish 'consists almost
entirely of the plankton flora and filamentous
algae, such as Spirogyra and Oscillatoria. Only
a small proportion (less than 10 per cent) of the
fish was found to have traces of animal food which
consisted of minute Crustacea of the groups
Copepoda and Cladocera'. The present investi-
gation corroborates Sen's observations. The
fish mostly feeds on mud, decayed vegetation,
acquatic weeds, and occasionally relishes mosquito
larvae. Out of 102 stomachs, studied during
the year, 77 had green vegetation, algae and weeds
The utility of carp minnows as controlling agent
for mosquitoes is said to lie in their algivorous
activities. These fish feed on algae and thus prove
useful in reducing the food of the larvae [Prashad
and Hora, 1936]. Roy [1938] also refers to the
destruction of acquatic vegetation in the ponds
in Java by Puntius javanicus, and remarks that
'where the fish are in sufficient numbers the vege-
tation in the ponds disappears completely.'
(ii) Number of fish required to stock a tank.
In one of the tanks at Lyallpur, measuring 60
ft. by 50 ft., 30 Ambassis baculis and 30 Barbus
(Puntius) sophore were introduced in March
Three weeks later it was observed that mosqui-
toes were breeding freely, and their population
1 Syn. Aplocheilus panchax (Hamilton).