阅读理解　　　 What unusual or unique biological train led to the remarkable diversification and 　　　 unchallenged success of the ants for ever 50 million years? The answer appears to be 　　　 that they were the first group of predatory ensocial insects that both lived and foraged 　　　 primarily in the soil and in rotting vegetation on the ground. Eusocial refers to a form (5) of insect society characterized by specialization of tasks and cooperative care of the 　　　 young; it is rare among insects. Richly organized colonies of the land made possible 　　　 by eusociality enjoy several key advantages over solitary individuals. 　　　 Under most circumstances groups of workers arc better able to forage for food and 　　　 defend the nest, because they can switch from individual to group response and back (10) again swiftly and according to need. When a food object or nest intruder is too large for 　　　 one individual to handle, nestmates can be quickly assembled by alarm or recruitment 　　　 signals. Equally important is the fact that the execution of multiple-step tasks is 　　　 accomplished in a series-parallel sequence. That is, individual ants can specialize in 　　　 particular steps, moving from one object (such as a larva to be fed) to another (a second (15) larva to be fed). They do not need to carry each task to completion from start to finish. 　　　 for example, to check the larva first, then collect the food, then feed the larva. Hence, if 　　　 each link in the chain has many workers in attendance, a senes directed at any particular 　　　 object is less likely to fail. Moreover, ants specializing in particular labor categories 　　　 typically constitute a caste specialized by age or body form or both. There has bees some (20) documentation of the superiority in performance and net energetic yield of various castes 　　　 for their modal tasks, although careful experimental studies are still relatively few. 　　　 What makes ants unusual in the company of eusocial insects is the fact that they are 　　　 the only eusocial predators (predators are animals that capture and feed on other animals) 　　　 occupying the soil and ground litter. The eusocial termites live in the same places as ants 　　　 and also have wingless workers, but they feed almost exclusively on dead vegetation.