110
Oesophagostomum radiatum
The migration of infective larvæ
The larvæ having reached the infective stage show a tendency to leave
the fæces in which they develop by virtue of a migratory instinct they possess.
When the fæces are surrounded by water, as in a petri-dish culture, they crawl
up the edge of the inner dish and swim vigorously on reaching the water. They
also likewise move towards the soil, if the dung happens to be laid on it.
Migration is hindered if the fæces is not spread uniformly. If in the inner;
petri-dish, the dung is placed in the middle so as not to touch the sides, the
larvæ experience difficulty in crossing the surrounding space to reach the edge
of the dish and get entangled in the fluff of the blotting-paper. Apart from
this instinctive migration their movements are governed by external conditions
as moisture, temperature, light,etc. With an increase in temperature they
attempt to protect themselves from drying by concealment at the bases of
grasses or by burrowing the soil if the latter is moist.
Hydrotropism. —The reaction to the presence of water that these larvæ
show has been described under ' Moisture'.
Thermotropism.—The influence of a moderate or excess of heat on the
larvæ is detailed under ' Temperature '.
Heliotropism.—When a petri-dish containing numerous infective larvæ
was left on the stage of a binocular microscope, and diffused sunlight properly
reflected through it, all the larvæ assembled in the lighted portion in about
thirty minutes. The reaction in this case should be regarded as positive
heliotropism.
The same dish was later removed to the table and covered by a thick
cardboard which had a large hole in its middle. An electric lamp was so
adjusted as to throw its light on the dish through the hole, to discover if the
larvæ will be attracted by it. Even after four hours, the larvæ were found
to be scattered and did not confine their movement to the illuminated area of
the water. Bright light has therefore no influence on the migration of the
larvæ.
Migration, in nature, always seems to be achieved in the early hours of
the morning when both coolness and diffuse light prevail. The twilight of the
evening may also be likewise preferred. A culture of dung made at 10-30 A.M.
on a day did not show emigrated larvæ in the water-jacket in sufficient
numbers till 5 P.M. on the fifth day, when the glare of the sun and the heat
had subsided in intensity, though the infective stage was reached at 10-30
A.M. on the same day. In another culture, a few larvæ did emigrate at
mid-day when a large tray was left over the petri-dish to keep off the light for
some hours.
Thigmotropism.—The movement of the larvæ in water is faster than in
agar plates. Ecdysis of the first-stage larva is not achieved in a fluid medium
which does not offer any resistence on the sides. The larva could liberate
itself from its old skin by piercing solid fæcal masses in a fæcal emulsion.
The infective larvæ tangle themselves in the fungal mycelia which develop
on the fæces, or in the algal filaments growing at the bottom of a bottle
containing tap-water and exposed to bright light.
Geotropism.—Migration in the soil of these infective larvæ is mainly
vertical, either downwards or upwards. Lateral migration occurs to a negli¬