ASCLEPIADEAE 



91 



inevitably held fast in the clip. The visitor will then draw back its proboscis 

 with a jerk, pulling away the clip with the two connected poUinia, and carrying 

 them off. When first extracted from their loculi the poUinia are wide apart, but 

 the bands connecting them with the clip twist inwards as they dry, bringing the 

 poUinia so close together that they can easily be introduced into a slit. Should 

 the insect now visit another flower the poUinia are readily pushed into one of 

 the slits, guided by which they will slip into the stigmatic chamber, there to 

 effect crossing, for when the insect draws back its proboscis they are torn away 

 from the bands connecting them with the clip. At the same time a new clip 



Fig. 254. VinceioxicuTn officinale, Moenchi^h^T H&rm.MuW^r). A. Flower after removal of the 

 five petals (X 7). B. Do. with calyx also removed, seen directly from above (x 14). C The dilated 

 end of the style, surrounded by the anthers. The poUinia lying hidden in the anthers, and the bands 

 connecting them with the clips, are indicated by dotted lines. D. Inner side of a clip with attached 

 pollinia ( X 80). E. Outer side of do. a, capitate end of the style ; b, membranous process of the 

 connective, lying upon a; c, outer side of loculas; d^ wing-like lateral margin of anther, which, with 

 the corresponding wing of the adjacent anther, bounds the slit x that widens below ; e, nectar pit ; f, clip, 

 to which two pollinia (Ji) are attached by bands ig) ; z, nectar-secreting coronal appendages (cucuUi). 



with its pollinia will be attached to the proboscis. It is only to the proboscis 

 bristles of Muscids that the clips regularly get fixed. Other visitors (Empids, 

 Syrphids, wasps, &c.) lack these bristles, and it is exceptional for an insect of 

 the kind to carry off a clip on the end of its proboscis. 



F. Heim (Bull. soc. linn., Paris, ii, 1893) says that this species is pollinated by 

 large Diptera, which usually do not succeed in getting nectar. Small flies are 

 also found in the flowers, and these remain sticking in the clips, thus preventing 

 the pollinia from being used for crossing. This is a case of insect-visits being 

 not merely useless, but harmful. Heim believes that spiders protect the flowers 

 against such unwelcome guests, and speaks of this as a case of ' arachnophily. 

 The sam.e remark applies to Apocynum venetum L. 



Visitors. — Vide supra. The following were recorded by the observers and for 

 the localities stated. — 



Sprengel, flies with clips attached to their proboscis. Herm. MuUer (Alps), 

 12 Muscids, mostly with pollinia attached to their proboscis bristles: also, as 



