Naturally, m a group of animals as diverse as the snakes, and with so many varied enemies. there are numerous defensive reactions and devices. There is. however, one general pattern of behavior. In the presence of suspected enemy the first reaction is to try to escape observation; if this fails, the next resort is the flight co some inaccessible retreat, but if this is not possible, or is circumvented. various kinds of intimidatory gestures and warning devices are brought into play; in the last resort the snake attacks. This pattern vanes with the circumstances; some stages may be omitted or combined unpredictably whilst all some notoriously irascible species may dispense with all the preliminaries and attack almost at once. though seldom or never without some provocation. Amongst the factors that increase aggressiveness are hunger, the mating season and surprise, with the last mentioned the commonest; when hunting for food or for mate. activity and the aggressive instinct are both at their peak. Owing to their poor sense of hearing snakes are very liable to be, quite literally, caught napping and a similar situation arises during their periods of temporary blindness just before sloughing (蜕皮）begins. By far the greatest number of snake-bit accidents result from the unwitting disturbance of resting snakes. and this hazard is much increased with species that are well disguised and whose natural instinct is to trust to this concealment as their principal defense. As well as differences in aggressiveness between individuals of the same species according to the circumstances and conditions, there are also notable differences between species, even closely allied species; and the reports of those who have been attacked may understandably be lacking in objectivity. So it is impossible to forecast. even in outline, how any encounter will develop. The Hamadryad, for example, is usually credited with being amongst the most aggressive of snakes. and there are many accounts of unprovoked attacks, yet on one occasion fourteen men and seven dogs passed and returned within two yards from a nest and no snake was seen although the female. which guards the nest, could not have been far away. 测试题 When a snake meets a potential enemy, its primary device is______________.