576 THE FERTILISATION OF FLOWERS. [PART Iy. 
specially attractive to Psychode, which are attracted by other sub- 
stances of similar smell; but since the grating of hairs in Arwm 
excludes larger insects we cannot be certain that the smell alone 
would repel them. Delpino supposed that strong perfumes such 
as those of Auta and Anethum are very attractive to flies and are 
avoided by other insects, especially by bees. But according to my 
observations the same flies which visit Anethum and Ruta also 
visit a great variety of other plants, some sweet-scented, others 
scentless, in the same abundance and with the same assiduity. 
Moreover I have observed the flowers of Anethum visited by six 
species of bees and twenty-five other Hymenoptera as well as by 
fifteen species of flies, and Ruta visited by three species of bees 
and eleven other Hymenoptera in addition to nineteen species of 
flies. Delpino’s statement may be more accurate in the case of 
Sambucus, but the flowers of that plant are so little visited that 
we must not yet conclude that they are not sometimes visited by 
pollen-collecting bees. Bees of the genus Prosopis, which them- 
selves emit a strong odour, are chiefly found on strong-scented 
flowers (Reseda, Lepidium, Ruta, Anethum, Achillea, Matricaria), 
and here, as in other cases, the concentration of one class of 
visitors may tend to restrict the visits of others. 
Sweet, aromatic perfumes, pleasant to our own senses, such as 
the scents of Asclepias Cornutr, Rosa Centifolia, Thymus, Lavendula, 
are, as Delpino has shown, specially attractive to bees, but in a 
less degree to other insects also; and here again we find that 
deep situation of the honey is the chief means of restricting - 
insect-visits. 
B.—Lxclusion of Certain Classes of Insect-Visitors by Paucity or 
‘ Concealment of Food Material. 
Absence of honey and inaccessibility of the honey or pollen 
restrict insect-visits much more effectually than peculiarities in 
colour or odour. Honeyless flowers with exserted anthers are 
scarcely visited except by pollen-feeding flies (especially Syrphide) 
and pollen-collecting bees. Lepidoptera and purely suctorial flies, 
(Empide, Conopide, Bombylide) linger at most for a short time on 
honeyless flowers to bore succulent tissues for included sap, but 
usually fly away after a few seconds to other flowers. Beetles and 
sand-wasps devour the pollen of honeyless flowers, but resort far 
more abundantly to flowers which furnish them with both pollen 
and honey; a few voracious Lamellicorns (Cetonia T'richius) 
ae — 
