[Illustration: FIG. 42. Alpine Lousewort (_Pedicularis asplenifolia_). _A_, flower seen from the left side, enlarged three times; the arrows show the path by which the humble-bee enters. _B_, the same flower, seen from the left, after removal of the calyx, the lower lip and the left half of the upper lip. _C_, ovary (_ov_), nectary (_n_), and base of style. _D_, tip of style, bearing the stigma. _E_, two anthers turned towards one another. _o_, upper lip. _u_, lower lip. _gr_, style. _st_, anthers. _kr_, corolla-tube. _k_, calyx.] There are not a few of such flowers adapted to a very restricted circle of visitors, and in all of them we find contrivances which close the entrance to all except what we may call the welcome insects; sometimes there are cushions of bristles which prevent little insects from creeping up from below, or it is the oblique position of the flower which prevents their getting in from the stem; sometimes it is the length and narrowness of the corolla-tube, or the deep and hidden situation of the nectar, which only allows intelligent insects to find the treasure. Very remarkable are those flowers which are adapted to the visits of flies, for they correspond in several respects to the peculiarities of these insects. In the first place, flies are fond of decaying substances and the odours given off by these, and so the flowers which depend for their cross-fertilization on flies have taken on the dull and ugly colours of decay, and give out a disagreeable smell. But flies are also shy and restless, turning now hither, now thither, and cannot be reckoned among the 'constant' insect visitors, that is to say, they do not persistently visit the same species; it is, therefore, evident that they might easily carry away the pollen without any useful result ensuing. Moreover, their intelligence is of a low order, and they do not seek nectar with the perseverance shown by bees and humble-bees. It is not surprising, therefore, to find that many of the flowers adapted for the visits of flies are so constructed that they detain their visitors until they have done their duty, that is to say, until they have effected, or at least begun, the process of cross-pollination. [Illustration: FIG. 43. Flower of Birthwort (_Aristolochia clematitis_) cut in half. _A_, before pollination by small flies. _b_, the bristles. _B_, after pollination. _P_, pollen mass. _N_, stigma, _b_, the bristles. _b´_, their remains. After H. Müller.] [Illustration: FIG. 44. Alpine Butterwort (_Pinguicula alpina_). _A_, section through the flower. _K_, calyx. _bh_, bristly prominences. _sp_, spur. _st_, stamen. _n_, stigma. _B_, stigma and stamen more magnified. After H. Müller.]