The duration of the increase, peak and decline in abundance of the immature stages of sycamore and cereal aphids each year is ephemeral. These temporary resources are exploited by a sequence of aphidophagous insect predators. The temporal sequence in the appearance of the immature stages of coccinellids and syrphids in the sycamore and cereal aphid systems is defined. In spring in the sycamore aphid system and early summer in the cereal aphid system the immature stages of syrphids consistently appeared before those of coccinellids. In the case of the sycamore aphid the autumnal peak in abundance was on average larger than the spring peak, and although attacked by more syrphids, it was not exploited by coccinellids. These temporal patterns in the attack sequence are associated with a difference in the lower developmental thresholds (LDT) of these two groups of predators. The LDT of syrphids (4°C) enables them to be active at lower temperatures and to develop faster between 10° and 27°C than coccinellids, whose LDT is 10°C. As a consequence, early in the year, when temperatures are low but increasing, syrphids appear before and complete their development more quickly than coccinellids, and in the latter half of the year, when temperatures are generally lower and decreasing, only syrphids are likely to be able to complete their development before the aphids disappear. Thus, the niche shift between syrphids and coccinellids is possibly more a consequence of a phylogenetic constraint than a response to competition and or intraguild predation. The relevance of these findings for the ecology of intraguild predation is discussed.