none The two species Diadromus collaris and D. pulchellus do not show the same adaptations to two different thermal regimes; interspecific competition in population cages under a 25 degree C-15 degree C thermoperiod is more favorable to species D. pulchellus. D. collaris is a eliminated after 3 to 4 generations only. Under the thermoperiod 30 degree C-20 degree C, both species coexist at relatively equal and constant level during several generations. Population fluctuations of the progeny of inseminated and virgin female yielded information on possible mechanisms involved in the coexistence of both Diadromus species in the zone of sympatry.