May, 1920] The Syrphid Fly and Apocynum 265 



The old explanation was that such flowers penalize those 

 visiting insects found guilty of being too weak to function 

 satisfactorily as pollinators, by condemning them to death and 

 carrying their execution into immediate effect. Even if such a 

 teleological explanation appealed to one, he might with perfect 

 propriety inquire what good it would do the flower to penalize 

 itself with sterility at the same time, since the captured flies 

 may block up the entrance to other insects. Moreover, if the 

 insects learned anything by the death or capture of their fellows 

 one could see the logic of such an explanation, but apparently 

 they do not. Instead they keep on going to their death in spite 

 of the "horrible examples" right under their noses, just as they 

 have done, no doubt, for ages past, and the flowers, similarly, 

 keep on interfering with their own pollination by holding the 

 flies in captivity. Certainly, any flower that habitually clogs up 

 its own system with insects, after devising special structures to 

 prevent their being useful, is open to criticism by the etiologist. 



Ohio State University, Columbus, Ohio. 



EXPLANATION OF PLATE L 



Fig. 1. Flower of Apocynum androsaemifolium with three Mesogramma marginata 

 entrapped. 



Fig. 2. Flower partly cut away, to show stigmatic surface of pistil (s), ring of 

 attachment of anthers (r), nectaries (n), and opening of pollen sacs (p). 



Fig. 3. Looking into a flower-cup, showing two heads of flies with proboscis 

 caught between anthers, and part of a proboscis stuck on the outer side 

 of an anther. 



Fig. 4. Characteristic position of entrapped fly. The proboscis is held between 

 the anthers close to their attachment to the stigma. 



Drawings by Mrs. Walter V. Balduf. 



