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.) 



