57 2 
Insects and Flowers 
which stand close together at about the same height near the mouth of the 
corolla. Both cross- and self-pollination might be brought about in this 
way, but, as Darwin has shown, the foreign pollen would probably possess 
Fig. 763.—Hawk-moth posed before a jimson-weed, Datura stramonium. (After 
Stevens; one-half natural size.) 
the greater potency, and cross-fertilization would be apt to result. Fig. 763 
is a photograph of a sphinx moth and Datura-flower, posed to show the rela¬ 
tive lengths of the moth’s proboscis and the corolla tube.” 
Another kind of specialization in flower structure which tends to pre¬ 
serve the nectar for certain spe¬ 
cific insect visitors is well illus¬ 
trated by the salvias, the snap¬ 
dragon, and other similarly 
irregularly tubular flowers (La- 
biatse, Leguminosae, Scrophu- 
lariaceas, etc.). Probably all 
such flowers are pollinated by 
insects (a few species by hum¬ 
ming-birds) . The irregularity 
in corolla is accompanied by a 
specific disposition of the stamens 
and pistil, so that the insect 
visitors are compelled to visit 
the nectary in one particular 
manner, a manner devised to 
insure their touching, or being 
touched by, the anthers or stigma 
or both. In the snapdragon (Fig. 760) the opening of the flower-cup is 
normally closed, but when a bee alights on the broad keel or platform (com¬ 
posed of two petals grown together) its weight so depresses this platform as 
to open the way into the flower-cup, which closes at once when the bee goes 
in and drinks the nectar. Scrambling and twisting about in the narrow 
chamber it thus thoroughly dusts itself with pollen, or thoroughly dusts the 
Fig. 764. —Salvia-flower. A, showing position 
of pistil and stamens; B, anthers of stamens 
in normal position; C, anthers of stamens 
tipped down; D, bee entering flower; E, flower, 
natural condition. (After Lubbock; natural 
size.) 
