It moves, in fact, by rolling itself rapidly along, rotating like a wheel on the extremities of its spokes, or like the clown in a pantomime, hurling himself forward on hands and feet alternately. Its celerity is so great that Colonel Montague, who was one of the first to describe it minutely[1], says its speed exceeds that of any known insect, and as its joints are so flexible as to yield in every direction (like what mechanics call a "ball and socket"), its motions are exceedingly grotesque as it tumbles through the fur of the bat. [Footnote 1: Celeripes vespertilionis, _Mont. Lin. Trans._ xi. p.11.] [Illustration: NYCTERBIA.] To enable it to attain its marvellous velocity, each foot is armed with two sharp hooks, with elastic opposable pads, so that the hair can not only be rapidly seized and firmly held, but as quickly disengaged, as the creature whirls away in its headlong career. The insects to which it bears the nearest affinity, are the _Hippoboscidæ_, or "spider flies," that infest birds and horses; but, unlike them, the Nycteribia is unable to fly. Its strangest peculiarity, and that which gave rise to the belief that it was headless, is its faculty when at rest of throwing back its head and pressing it close between its shoulders till the under side becomes uppermost, not a vestige of head being discernible where we would naturally look for it, and the whole seeming but a casual inequality on its back. On closer examination this, apparent tubercle is found to have a leathery attachment like a flexible neck, and by a sudden jerk the little creature is enabled to project it forward into its normal position, when it is discovered to be furnished with a mouth, antennæ, and four eyes, two on each side. The organisation of such an insect is a marvellous adaptation of physical form to special circumstances. As the nycteribia has to make its way through fur and hairs, its feet are furnished with prehensile hooks that almost convert them into hands; and being obliged to conform to the sudden flights of its patron, and accommodate itself to inverted positions, all attitudes are rendered alike to it by the arrangement of its limbs, which enables it, after every possible gyration, to find itself always on its feet.