574 THE FERTILISATION OF FLOWERS. [PART LY. 
tissues of the flower are devoured by some insects, especially 
beetles. But this cannot be considered of advantage to the plant. 
In many Orchids fleshy processes of the labellum are devoured 
by insects ; in Polystachya (Malaxidee) the labellum becomes filled 
with loose cells on which insects feed ; but we have no accurate 
observations as to the value of these attractions. 
The shelter which many flowers afford from rain and-wind is 
in many cases a powerful attraction. For instance, the males of 
many bees pass the night within flowers of Campanula or take 
shelter there from rain; and many small Diptera resort for shelter 
to the flowers of Arwm ‘and Aristolochia, 
b.—THE EXCLUSION OF CERTAIN INSECTS, AND CONSEQUENT 
INCREASED ATTRACTION OF OTHERS. 
While, on the one hand, it is of advantage for flowers to be 
visited by the utmost possible variety of insects, since the like- 
lihood of cross-fertilisation increases with the number of visitors, 
yet, on the other hand, the attraction of all kinds of insects has 
several disadvantages. For many visitors are positively hurtful, 
as, for instance, voracious beetles, which may devour the repro- 
ductive elements of the flower; and each class of insects will be 
the less attracted the more the store of food is removed by others ; 
easily accessible honey is therefore the least likely to attract the 
most diligent, long-tongued insects. Besides, easily accessible 
honey and pollen are also the most liable to be spoilt by rain. So 
exclusion of certain insects may be advantageous to the plant, 
if thereby injurious insects are excluded, if certain other insects 
are stimulated to more diligent visits, or, lastly, if the honey and 
pollen are sheltered from rain. The great majority of flowers 
possess contrivances for more or less restricting insect-visits. 
a.—Limitation of Insect-Visits by Colour and Odour. 
The plants that we have discussed furnish only one class of 
instances of colour directly limiting insect-visits. 
All dull yellow (dirty-yellow, brownish-yellow, yellowish-white) 
flowers (Buplewrum, Anethum, Pastinaca, Rhus Cotinus, Galiwm 
Mollugo, Ruta, Neottia, Euonymus, Euphorbia, Adoxa, Alchemilla) 
are entirely or almost entirely avoided by beetles; closely allied 
white flowers (Umbelliferw, Rubus, Rosa, Galiwm verwm) are visited 
by beetles, more or less to their injury; and brightly coloured 
