Dragonfly is a predator, they can eat small insects

According to this argument, we can find that oxygen is an important constraint on prehistoric insect evolution, limiting the flying insects the size limit, but in the birds appeared after flying insects began to appear natural enemies, so the bird has become a restricting factor of insect body constraint. Paleontologist Clapham think body large, bloated flying insects in agility, its maneuverability than smaller insects, in other words, flying insect body larger become easy prey target early birds, which is one of the late flight effects of insect body shrink factor. In addition, a speculation that the early birds may eat only large flying insects, not prey on small size of insects, the theory also can to some extent explain why until the disappearance of large flying insects population decline. Dragonfly is a predator, they can eat small insects, but during the Jurassic period, the dragonfly's wingspan is big, we now see the dragonfly several times, so these giant Dragonfly easily become the target of early birds of prey. Clapham was added: in general, the size of the prehistoric insects should consider oxygen levels, but in such as pterosaurs such flight class biological appears, the oxygen content is not the main inducement. According to the latest research of Clapham, he found the pterosaurs and not agile flight mobility as now the bird. Another study found that, if not for the emergence of modern birds, insects posture may be greater