He had to stand very still and strain his ears for an instant before he caught it. It was unmistakable, but so faint that, under calmer circumstances, he would have been amazed by the acuteness of Janice's hearing. But now there was no room in his mind for amazement. Only horror. "All right," he said. "It's following us. We might have known that it would. I sensed its rapacity the instant I saw it, and if they kept it from attacking us they must have had a reason." "What reason, David? Don't try to spare me, if you think you know." "I don't know for sure. How could I? But I can make a guess that has some ugly implications. It could be a kind of insectlike--" He hesitated, fighting for control, reproaching himself for his harshness. He was making no attempt to soften the blow and he knew why. He had spoken with a slight edge of angry impatience in his voice because he did not want her to know how profoundly the thought had unnerved him. It was more than chilling, for there was something diabolically calculated, wholly vicious, about the use of such a horror as a weapon. A relentlessly pursued man--an escaped prisoner floundering in desperation through a swamp or trapped in a mountain gully--can experience many dreads. But only one that claws at his mind like a sharp-taloned, utterly merciless harpy. The appeal in Janice's eyes put a quick end to his hesitation. "What could it be, David? Tell me." "A kind of insectlike bloodhound. A scent-tracking animal can demoralize a fleeing man in a very terrible way, and I'm sure they know it. I think they deliberately kept it from attacking to give us time to escape from the building. Then they let it pick up our scent. They may be planning to prolong the pursuit until we abandon all hope and just wait helplessly for the worst to happen." He gave her no time to reply. Tightening his grip on her hand he pulled her forward, his voice urgent, rising a little as the distance beneath them dwindled and they could see the shining expanse of metal at the base of the stairway. "I didn't tell you that to frighten you. But we've got to build up our mental defenses fast. You've got to try not to think of it as a bloodhound. Our instinctive fear of insects isn't as great as our fear of savage, wolflike animals. Bloodhounds aren't wolflike, but to a pursued man there are no beasts more sinister. We must keep thinking of it as an insect." "Are you sure that will help, David? An insect so large--" "On Earth all insects are small. It's natural for us to think of them as small. Remember, man's fear of insects is real enough. But it can be overcome, thrown off. All small, crawling things make us recoil in revulsion. But the bloodhound image strikes a deadlier chord of fear. We've got to shut our minds to it." * * * * * "Then why didn't you keep silent about it? If you hadn't told me, if I hadn't known at all--"