86 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE 
Such flowers open in the evening, and are then most fragrant. 
Conspicuousness is often increased by numerous flowers being 
associated together. 
For the entertainment of their insect-guests flowers may pro- 
vide sweet sap, or produce a great excess of pollen, a highly 
nutritious substance, or secrete nectar. In highly-specialized 
forms at least the last sort of food is generally the most important, 
and is usually produced deep down in the recesses of the flower, 
often in long spur-like tubes, 
where it is only accessible to 
long - tongued insects, such 
as bees and butterflies. 
Flowers not only attract 
guests and provide refresh- 
ment, but their whole struc- 
ture is often modified, it may 
be in a very complex way, 
to secure the benefits of 
cross-pollination, z.e. to en- 
sure that pollen which is 
brought is deposited on the 
stigma by arriving guests, 
and make certain that de- 
parting guests take with 
them a fresh supply of the 
: ae same material. 
Fig. 1079.—The Nottingham Catch-Fly (Srlene nutans) by 
Night, a flower being visited by a moth (Dranthecia albi- In many cases the adap- 
“aoa oe of dead creeping insects are seen adher- {ations existing between 
flower and insect are so 
perfect as to leave no room for doubt that each has influenced 
the evolution of the other. Some of the adjustments that have 
come into existence will best be understood by reference to con- 
crete examples, the first two of which will be taken from the 
remarkable Orchis Family, the members of this being notable for 
the great variety of arrangements which they display in relation to 
insect-pollination. In one large and handsome species (Pha/e- 
nopsis Schilleriana, fig. 1080) the attraction of colour is provided by 
five spreading leaves, equivalent to sepals and petals, of which the 
most remarkable (the ade/dum) is one which hangs down from 
the centre of the flower. It begins in a narrow stalk, but soon 
