ADAPTATIONS IN ANIMALS AND PLANTS 39 



in the stigmatic chamber while the corpusculum and its broken 

 retinacula are carried off, still firmly attached to the insect's 

 foot. The insect continues its visits and the retinacula at- 

 tached to its feet now get fixed in corpuscula as the claws did 

 before, and the insect's foot, after repeated visits, may some- 

 times be seen bearing corpuscula with their retinacula often 

 dichotomously arranged. "' 



While the poUinia of the common milkweed is frequently 

 attached to the claws of insects and rarely to the hairs and 

 mouth parts, in the allied species of milkweed, A. verticUlata, 

 the pollinia are usually attached to the hairs of insect visitors. 

 The peculiar structure of the milkweed flowers is such that 

 small, weak insects are often entrapped and lose their hves by 

 getting their feet entangled in the pollen masses or caught in 

 the slits of the crown, from which they are unable to release 

 them. I have found numbers of dead house flies on some of 

 the flowers of the common milkweed, which had been too weak 

 to extricate their feet from the slits. Similarly, on other occa- 

 sions, I have found honey bees and Syrphus flies which had been 

 held captive in the same way. Among unbidden guests of the 

 common milkweed, ants are frequently seen, especially the 

 larger, dark-colored species. At certain times I have seen them 

 swarming over the stems and flowers. The species of swamp 

 milkweed, A. incarnata, shown in the illustration, is easily 

 distinguished from the common milkweed species. The dull 

 crimson pink blossoms appear from July to September. It 

 is fertilized by bees and bee-like flies and will be found full 

 of interest when studied from the standpoint of its insect 

 visitors acting as fertilizers of the flowers. It is also found 

 at the border of ponds and in wet meadows. The common 

 milkweed, on the other hand, grows in great luxuriance on 

 dry, sandy ground. 



1 "Fertilization of Flowers," p. 398. 



