FLOWERS 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. In all cases it must be understood 

 that these so-called " adaptations " have not been produced 

 to ward off insects, but that having appeared from one 

 cause or another they have proved to be useful in this 

 particular. 



