Our birthwort (_Aristolochia clematitis_) and the Cuckoo-pint (_Arum maculatum_) are pit-fall flowers, whose long corolla-tubes have an enlargement at the base, in which both pistil and stamens are contained. In the birthwort (Fig. 43) the narrow entrance-tube is thickly beset with stiff hairs (_A_, _b_), whose points are all directed towards the base. Little flies can creep down quite comfortably into the basal expansion, but once there they are kept imprisoned until the flower, in consequence of the pollination of the stigma, begins to wither, the first parts to go being these very bristles (_B_, _b´_), whose points, like a fish-weir, prevented the flies from creeping out. Other 'fly-flowers,' as for instance the Alpine butterwort (_Pinguicula alpina_) (Fig. 44), securely imprison the plump fly as soon as it has succeeded in forcing itself in far enough to reach, with its short proboscis, the nectar contained in the spur (_sp_) of the corolla. The backward-directed bristles hold it fast for some time, and it is only by hard pressing with the back against the anthers (_st_) lying above it, and against the stigma (_n_), that it ultimately succeeds in getting free, but it never does so without having either loaded itself with pollen, or rubbed off on the stigma the pollen it brought with it from another similar flower. The Alpine butterwort is protogynous, that is to say, the pistil ripens first, the pollen later, so that the possibility of self-fertilization is altogether excluded.