120 
Psyche 
[March 
and Doyle, 1977, p. 92). Conversely, angiosperms have been 
instrumental to the evolutionary success of the Apoidea. On the 
basis of the evolutionary dependence of the two groups, can 
anything be said about their relationship in geological time? Two 
possibilities present themselves: 1) the angiosperms evolved first and 
were initially wind pollinated 9 or pollinated by arthropods other 
than Hymenoptera (e.g., Coleoptera, Diptera, Thysanoptera, pos- 
sibly spiders); and 2) the first bees evolved from sphecid wasps prior 
to the origin of the angiosperms by adapting themselves to feeding 
on pteridosperm pollen or reproductive organs. 
A closer look at these possibilities is warranted. Coleoptera and 
Diptera are found in the fossil record at least by the Triassic. This 
supports the argument that they could have served as vectors for 
dispersal of angiosperm pollen. The question arises, if these insects 
were capable of performing essential roles as pollinators, why didn’t 
angiosperms arise earlier in the Mesozoic than the Cretaceous? 
Regal (1977) suggests that the limiting factor to angiosperm dis- 
persal was the presence of seed-carrying birds and mammals. He 
argues that this method of seed dispersal, acting in conjunction with 
insect pollination, provided the selective advantages behind the 
subsequent primary radiation of the angiosperms. This is a sound 
argument, but says little about the insects which may have been 
pollinating these early plants. It would seem that successful dispersal 
of flowering plants is dependent on efficiency at two levels — 
pollination and seed dispersal. The explosive radiation of the 
angiosperms during the Cretaceous indicates that the more special- 
ized insect pollinators, the bees, may have been present in order to 
explain this success. 
This might support the possibility that pollen collecting bees had 
already evolved by the time the first angiosperms appeared. Accord- 
ing to Wilson (1971, p. 75), the “Apoidea can be loosely character- 
ized as sphecoid wasps that have specialized in collecting pollen 
instead of insect prey as larval food.” The possibility, however 
speculative, exists that bees evolved in response to the food source 
presented by the pteridosperms but subsequently abandoned this 
resource when the angiosperms appeared. Certainly one way of 
accounting for the explosive radiation of the angiosperms would be 
9 Stebbins (1970, p. 323) suggests that the earliest angiosperms were not wind 
pollinated. 
