The bottom two lines scratched in agar traced the outlines of a wax moth larva (about 2 cm long) that was taped on the underside
of the plastic petri dish. Several hundred nematodes were released on the agar surface and after several hours of dispersal
the petri dish was photographed (notice the small white "wiggly worms" scattered about). It is possible to see that more nematodes were swimming over the wax moth (due to metabolic heat conducting through the plastic and agar) than were
swimming in slightly cooler areas. In fact, the agar above the wax moth (that was taped under the plastic petri dish) was found to be
only 0.3° above ambient with an electronic thermometer.
In nature, the nematode seeks out insect grubs and larvae in the soil. This environment is usually moist and
there is a watery film on soil particles (if not free water) so the nematodes can wander in search of insect larvae.
Two main hypotheses on how these insect-killing nematodes find their prey have been put forth: (1) that chemicals
from the insect are attractive or cause arrestment, and (2) that heat is attractive or causes arrestment. Both theories are probably correct, at longer range (a centimeter?) chemicals
play the only role, but at closer range (a few mm) a thermal gradient may elicit a klinokinesis
(more rapid turning at higher intensity of stimulus) near the thermal source. It is well known that metabolic processes of insects (Koidsumi, in Bursell, 1974) in a
well inslulated soil environment can build up heat (even in the so-called cold blooded insects). Normally, the nematode in the third stage ("dauer") leaves the dead insect
body and searches for a new living host. The dauer form can survive many months in the soil. Once an insect is located, the nematode can enter via
the spiracles (air breathing holes along thorax-abdomen sides), mouth or anus. After reaching the body cavity, a symbiotic bacterium, Xenorhabdus nematophilus, is released
and causes death of the insect in 1 to 2 days (Poinar & Thomas, 1966; Poinar, 1979; Thomas & Poinar, 1979).
In this photo of the inner side of a petri dish with agar, there are about 15 nematodes (tiny, faint white worms at left of center) that have wriggled up on the agar film that
thins out on the edges of the plate. Immediately behind the nematodes, on the other side of the curved plastic side was an infrared LED (removed
for the photograph) that had been heating the area during the experiment to about 0.3° C above ambient.
Chemical Ecology
Byers, J.A., & Poinar, G.O., Jr. 1982. Location of insect
hosts by the nematode, Neoaplectana carpocapsae, in
response to temperature. Behaviour 79:1-10.