FLOWEKS AND INSECTS 137 
hard. In the flower clusters of many latex-secreting 
plants the epidermis of the stem is very smooth and deli- 
cate, and easily pierced by the claws of ants and other 
creeping insects who seek to maintain footing on the 
smooth surface. Wherever the epidermis is pierced the 
latex gushes out, and by its stiffening and hardening glues 
the insect fast. 
(5) Protective forms. In some cases the structure of 
the flower prevents the access of small creeping insects to 
the pollen or to the nectar. In the common snapdragon 
the two lips are firmly closed (see Fig. 74), and they can be 
forced apart only by some heavy insect, as the bumble-bee, 
alighting upon the projecting lower lip, all lighter insects 
being excluded. In many species of Pentstemon, one of 
the stamens does not develop pollen sacs, but lies like a bar 
across the mouth of the pit in which the nectar is secreted. 
Through the crevices left by this bar the thin proboscis of 
a moth or butterfly can pass, but not the whole body of a 
creeping insect. Very numerous adaptations of this kind 
may be observed in different flowers. 
(6) Protective closure. Certain flowers are closed at 
certain hours of the day, when there is the chief danger 
from creeping insects. For instance, the evening prim- 
roses open at dusk, after the deposit of dew, when ants are 
not abroad ; and at the same time they secure the visits of 
moths, which are night-fliers. 
Numerous other adaptations to hinder the visits of 
unsuitable insects may be observed, but those given will 
serve as illustrations. 
