ECOLOGY OF PLOWEES 



359 



accessible to insects, but they 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 undoubtedly served by dense flower-clusters, 

 especially by heads like those of the clovers and by the 

 peculiar form of head found in so-called composite flowers, 

 like the sunflower, the bachelor's button, and the yarrow 

 (Fig. 133). In many such clusters the flowers are special- 

 ized, some carrying a showy strap- 

 shaped' corolla, to serve as an 

 advertisement of the nectar and 

 pollen contained in the inconspicu- 

 ous tubular flowers (see Plate XI). 

 Irregular flowers probably always 

 are more or. less adapted to par- 

 ticular insect (or other) visitors. 

 The adaptations are extremely nu- 

 merous ; — 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 gamopetalous 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. 251. In Fig. 252 one bumblebee 

 stands with his 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 honey-bee entering the violet clings to the 

 beautifully bearded portion of the two lat'eral petals, while 

 jt sucks the nectar from the spur beneath. 



Fio. 251. —A Beetle on the 



Flower of the Twayblade. 



(Enlarged three times.) 



