30 THE AMERICAN BOTANIST 



function so well from the mechanical side tluit thev interfere 

 with pollination by trapping the pollinating insects. The pock- 

 ets in which the common milkweed conceals its pollinia often 

 catch and hold visiting insects !)}• the legs or tongue and thus 

 cause their death. Se\eral other plants, in the same family 

 are known as "cruel plants" from their hal)it of catching in- 

 sects in similar ways. One plant in particular lias an arrange- 

 ment, somewhat like the old-fashioned boot-jack, at the base 

 of the pistil which allows insects to thrust their tongues through 

 in search of nectar but which catches them on the return. The 

 harder the insect struggles to release itself, the more firmly 

 does its tongue become fixed in the trap. Traps of this nature 

 are not confined to the milkweeds, however. They are found in 

 the closely related dogbanes. A good illustration is found in 

 the case of the common dogbane {Apocynuiu androsaeini- 

 folium) which, Raymond C. Osburn reports in Ohio Journal 

 of Science, murders the little syrphid fly, Mesograinnia nnir- 

 ginafa. by wliolesale. As man}- as four flics have been found 

 caught by a single small flower. Other flowers examined gave 

 indications that the insects hatl been caught but had finally es- 

 caped l)y pulling off part of their proboscis or, not infrequent- 

 ly, bv pulling their own heads off. A count of the first him- 

 dred flowers at hand showed that 81 had captured insects, the 

 total number of captures being 140. Although the pinch-trap 

 of these flowers is so much like that of their larger relatives, 

 it appears that they do not hold e\en small insects unless their 

 pr(»bosces are gummed up with the adhesive nectar. The flies 

 were seen to enter one flower after another, only to be caught 

 at last when their tongues were smeared witli nctar. 



.\ \'akii;tn' oi" \'ariktii-;s. — In systematic l)otan\-, an\- 

 form that did n(»t appear distinct enough to be a species was 

 <»nce called a variety, but it has recently been pointed out that 

 varieties mav spring from different causes and thus be \ery 



