Insects and Flowers 
575 
held in the stigmatic chamber of the flower, which the bee has just 
deserted.” 
Hive-bees, although common visitors to Asclepias, are really hardly 
strong enough to insure pulling loose from the flowers, and many of them, 
besides numerous flies and small butterflies, get caught and die on the flower- 
heads. Robertson has noted nine species of insects thus killed by A. cor - 
nuti. Bumblebees and large wasps and large butterflies are the most cer¬ 
tain milkweed pollinators. 
Still another markedly different kind of specialization to effect cross¬ 
pollination by insects is that shown by many Araceae and Aristolochiaceae. 
The flower (Fig. 767) in these plants consists of a long tubular perianth 
(spathe) with a constriction near the base, the 
narrow opening into the cavity below being 
nearly closed by stiff downward-pointing hairs, 
so as to make a sort of floral eel-trap. It really 
is an insect-trap: small flies crawl down the 
long tube and through the narrow opening in 
search of nectar; but when ready to return find 
themselves imprisoned by the downward-point¬ 
ing hairs. After a while the stigmas which 
mature before the anthers and have likely 
been pollinated (with pollen brought from 
other flowers) by the entering insects, wither, 
a drop of nectar is secreted for the benefit of 
the captured insects, and the anthers mature, 
exposing their ripe pollen-grains. The hairs 
in the throat of the flower gradually shrivel up 
and release the insects, which are now well 
showered with pollen falling on them from the 
anthers above. Visiting another Arum-flower, 
they hardly fail to rub off some of this pollen 
on the mature stigmas. Sometimes more than 
a hundred small flies will be found imprisoned in a single Arum. 
Classic examples of apparently the wildest vagaries in flower structure 
are those presented by the orchids. But Darwin’s fine work revealed the 
method in all this floral madness. Orchids are pollinated almost exclu¬ 
sively by insects, and the extravagant shapes and color-patterns are all means 
for accomplishing cross-pollination. Any one interested at all in the inter¬ 
relation between flowers and insects should read Darwin’s account of the 
orchids and their insect visitors, in his book “On the Fertilization of Orchids 
by Insects.” As this book is generally accessible, I will here only call atten¬ 
tion to one new and peculiar feature generally characteristic of the speciali- 
Fig. 767.—Flower of Aristolo- 
chia clematitis in longitudinal 
section: A, before fertiliza¬ 
tion by little fly; B, after fer¬ 
tilization. p, pollen-masses; 
s, stigma; b, bristly hairs; 
wb, without bristly hairs. 
(After H. Muller.) 
