FLY FLOWERS 



131 



Fig. 45. Asclepias syriaca^ JL., 

 a Pinch-trap Flower. Flower seen 

 from above after removal of sepals 

 and petals (x 3^) : a, nectar reser- 

 voir ; d, conical process of the 

 same ; c, upper membranous part of 

 the stamens ; rf, outer side of the 

 lower part of the stamens enclosing 

 the pollinia ; e, lateral expansion of 

 the stamen which with the lateral ex- 

 pansion of the neighbouring stamen 

 bounds the slit_/J in which the insect's 

 foot and later a poUinium is caught. 



the insects when they feel themselves held fast. The result is that the pollinia 

 fastened to the chps become attached to the visitors, and are thrust into a stigmatic 

 cavity by the latter, of course unknowingly and unintentionally. The pollinia stick fast 

 to the stigma and remain upon it to effect fertilization 

 after the insect has released itself (see Fig. 45). 



Pinch-trap Flowers are not exclusively Fly Flowers, 

 though in species of Vincetoxicum, flies of moderate 

 size, to the proboscis of which the clips attach them- 

 selves, are the agents of pollination. In species of 

 Stapelia also, the clips hold on to the proboscis-bristles 

 of large carrion-flies, which are attracted by the 

 colour of the flowers, and by the well-marked smell 

 of decaying flesh. Similarly, in species of Ceropeja 

 the pollen is transported by small flies, and here too 

 there are little pitfalls in the flower, as in Aristolochia, 

 where visitors are retained as prisoners for a time, 

 so that these plants form a transition stage between 

 Pitfall Flowers and Pinch-trap Flowers. 



In other Pinch-trap Flowers various other insects 

 besides flies appear as cross-pollinating agents. On 

 the flowers of species of Asclepias, for instance, are 

 to be found, in addition to flies, bees, wasps, fossorial 



wasps, and Lepidoptera, to the claws or limb-bristles of which the clips remain 

 hanging (see Fig. 46). In the species of Arauja the clips get attached to the 

 proboscis of large bees, while in the species of Stephanotis they are affixed to the 

 proboscis of hawk-moths, in which this organ is very long (cf. Delpino 'Relazione sull' 

 apparecchio dellafecondazione nelleAsclepidee,' Torino, 1865; and'Ult.oss.,'Atti Soc. 

 ital. sc. nat., Milano, xi, 1868, 

 pp. 224 et seq. ; H. Miiller, 

 Kosmos, ii, 1877, P- 33o)- 



The species of Cypripe- 

 dium (see Fig. 47) are also 

 Pinch-trap Flowers, but here 

 the whole insect is seized, 

 not merely individual parts of 

 the body (proboscis, claws, 

 or bristles), as in the flowers 

 already mentioned. The visi- 

 tors may partly be flies, partly the less specialized and less intelligent bees, but in our 

 native species Cypripedium Calceolus they only consist of the latter. Hermann 

 Miiller (Kosmos, ii, 1877, p. 333), however, holds that the purple spots on the upper 

 side of the third stamen, which is metamorphosed into a shade for keeping out light, 

 are an adaptation originally evolved in relation to flies. 



The labellum, which is deeply excavated like a wooden shoe, bears on its floor 

 a coating of juicy hairs, and at the bases of these there are sometimes a few minute 

 drops of nectar. Small bees of the genus Andrena try to get into this cavity of the 



Fig. 40. 



Foot of a Lepidopterid with ii clips (K) and 8 pollinia 

 of Asclepias curassavica. 



K 2 



