FACILITIES FOR INSECT VISITS 



425 



401. Facilities for insect visits. Eegular flowers with radial 

 symmetry usually have no special adaptations to make them 

 singly accessible to insects, but lie open to all comers. They 

 do, however, make themselves much more attractive and afford 

 especial inducements in the matter of saving time to flower- 

 frequenting insects by being grouped. This purpose is undoubt- 

 edly served by dense flower clusters, such as those of the lilac, 

 the phlox, and the elder, and especially by heads like those of 

 the button bush (Oephalanthus) and by the peculiar form of 

 head found in so-called composite flowers, like the sunflower, 

 the bachelor's button, and the yarrow 

 (Fig. 144). In many such clusters the 

 flowers are specialized, some carrying 

 a showy strap-shaped corolla, to serve 

 as an advertisement of the nectar and 

 pollen contained in the inconspicuous 

 tubular flowers. Flowers with bilateral 

 symmetry probably always are more 

 or less adapted to particular insect (or 

 other) visitors. The adaptations are 

 extremely numerous ; here only a very 

 few of the simpler ones will be pointed 

 out. Where^ there is a drooping lower 

 petal or, in the case of a sympetalous corolla, a lower lip, 

 this serves as a perch upon which flying insects may alight and 

 stand while they explore the flower, as the beetle is doing in 

 Fig. 324. In Fig. 325 one bumblebee stands with her legs 

 partially encircling the lower lip of the dead-nettle flower, while 

 another perches on the sort of grating made by the stamens 

 of the horse-chestnut flower. The honeybee entering the violet 

 clings to the beautifully bearded portion of the two lateral 

 petals, while she sucks the nectar from the spur beneath. All 

 bilaterally symmetrical flowers seem to be specially adapted to 

 compel visiting insects to enter them in the best way to 

 secure transference of pollen. 



FIG. 324. A beetle on the 

 flower of the frvvayblade 



Slightly enlarged. After 

 Behrens 



