FLY FLOWERS 



133 



The white flowers, ornamented at their entrance with two yellow sacs {a) 

 covered with yellow hairs, chiefly attract medium-sized flies, which creep in bodily 

 till their heads project into the hollow spur {c). This is devoid of nectar, but on the 

 inner surface of its lower side there are small unicellular capitate hairs, and the con- 

 tents of the juicy heads of these appear to serve as food for the dipterous visitors. 

 The stiff hairs directed obliquely backwards, that occur at the entrance to the spur 

 (at b), permit the flies to thrust in their heads with ease, and further serve them as 

 a convenient resting-place, but also prevent the head from being rapidly withdrawn. 

 This can only be gradually eff'ected by the fly pressing its body upwards as far as 

 possible from among the blockading hairs that grasp it. In doing this it touches the 

 anthers with its back, folding forwards and upwards the stigmatic lobe that covers 

 them. The flowers being protogynous, the fly regularly effects cross-pollination, 



Fig. 48. Pinguicula alpina^ Z-., a Fly Pinch-trap Flower (after Herm. Miiller). X. Flower seen from 

 the side. XI. The same in longitudinal section (x 3J). XII. Sexual parts of the same (x 7). XIII. Upper 

 half of flower with anthers still closed (x i^J). XIV. Sexual parts of the same (x 7). XV. Sexual parts 

 of a flower, of which the anthers have dehisced. The lower stigmatic lobe is folded upwards from behind, 

 so that its under-surface is visible. XVI, Lower half of the flower. XVII. Two of the capitate hairs, with 

 which the inner wall of the spur is clothed {x Ko). 



applying the pollen of the older flowers to the stigmas of the younger ones. Such 

 flies as are large enough to be gripped, but too feeble or clumsy to withdraw in the 

 proper way, remain sticking and perish by starvation. Pinguicula alpina is therefore, 

 as Hermann Miiller (op. cit.) states, a plant that seizes and kills insects in two 

 different ways. On the one hand, it grips flies with its flowers, and if these are clever 

 enough to free themselves from the clasping bristles they serve as agents of cross- 

 pollination, but otherwise fall sacrifices to their unskilfulness. On the other hand, 

 the plant captures all kinds of small insects by the glandular secretion of its foliage 

 leaves, and aftenvards digests them. 



