156 DISPERSION OF POLLEN BY ANIMALS. 



lay their eggs, and in so doing dust the stigmas with their freight. To sum up, the 

 flowers of the Nottingham Catchfly and of other species of Caryophyllaee« above 

 referred to are adapted to the small NoetuaB of the genera Dianthoecia and Manies- 

 tra, and are visited exclusively, or, at any rate, principally, by those insects. The 

 Nocture obtain honey from them, and the females find in them homes suitable for 

 their eggs. The return made by the moths to the plants consists in the conveyance 

 of pollen from flower to flower and the consequent conversion of ovules into seeds 

 which would not be effected spontaneously. 



The relations just described occur also among several other groups of plants and 

 Lepidoptera. A number of species of the small blue butterflies belonging to the 

 genus Polyommatus stand in the same relation to Leguminosas and Rosaceas. The 

 beautiful Polyommatus Hylas visits the flowers of Lady's-fingers (Änthyllis Vul- 

 neraria) and in doing so transfers the pollen from one plant to another. The female 

 lays her eggs in the ovaries of the flowers she visits, and from the eggs issue cater- 

 pillars which feed on the young seeds. When mature the caterpillars forsake the 

 ovaries and retire underground to pass through the chrysalis stage. The same 

 relation exists between Polyommatus Bceticus of Southern Europe and the Bladder- 

 Senna {Colutea arborescens), between Polyommatus Areas and the Great Burnet 

 (Sanguisorba officinalis) and in many other cases; only, besides the butterflies 

 named, others alight with a freight of pollen on the flowers of these plants, but do 

 not lay eggs in the ovaries, and only receive honey in return for their conveyance 

 of the pollen, so that these cases are really only partially of the same category. 



On the other hand, the life-history of one of the moths living on the capsule- 

 bearing species of the genus Yucca, and named Pronuba yuccasella, has been made 

 out, and must here be dealt with in some detail, as it affords one of the most 

 wonderful examples of the dispersal of pollen by means of egg-laying insects. The 

 flowers in all species of Yucca are arranged in large panicles (vol. i. fig. 154, p. 659), 

 and each is bell-shaped and suspended at the end of a smooth, green stalk. The 

 perianth-leaves, six in number, are yellowish -white and are consequently visible 

 from a considerable distance in the dusk and on moonlight and starry nights. 

 After the flower-buds open, which happens regularly in the evening, the perianth 

 forms a widely-open bell (cf. fig. 240 ^ ). The dehiscence of the small anthers, which 

 are supported on thick and velvety filaments, takes place simultaneously with the 

 divergence of the petals, and a golden-yellow adhesive pollen is to be seen in the 

 spiral slits of the anthers. Each flower is wide open for one night only; by the 

 next day the free extremities of the six perianth-leaves bend towards one another 

 causing the flower to assume the form of a balloon or bladder with six narrow 

 lateral apertures (fig. 240 ^ ). In the twilight and by night, numerous small 

 yellowish- white moths (Pronuba yuccasella; see fig. 240*) which have a metallic 

 glitter in the moonlight flutter about the flowers of the Yucca plants. The females 

 penetrate into the interior of the wide-open bells and there endeavour to possess 

 themselves of the pollen, not with a view to devouring it, but that they may carry 

 it away. For this purpose they are furnished with a special implement. The first 



