202 INDIRECT BENEFITS DERIVED FROM INSECTS. 



dron, Kalmia, Rohinia, Silene, Lythrum, Populus baJsamifera, &c.^ yVnd 

 under the third class will arrange those which ensnare by their leaves, 

 whether from some irritability in them, as in Dioncea, Drosera, &£C., or 

 merely from their forming hollow vessels containing water into which the 

 flies are enticed either by their carrion-like odor, or the sweet fluid which 

 many of them secrete near the faux ; as in Sarracenia, Nepenthes, Aqua- 

 rium, Ccphalotus, he, the tubular leaves of which are usually found stored 

 with putrefying insects. In this last class may be placed the common 

 Dipsacus of this country, the connate leaves of which form a kind of basin 

 round the stem that retains rain-water, in which many insects are drowned. 

 To these a fourth class might be added, consisting of those plants whose 

 flowers smelling like carrion (^Stapelia hirsuta, &c.) entice flies to lay their 

 eggs upon them, which thus perish. 



The number of insects thus destroyed is prodigious. It is scarcely pos- 

 sible to find a flower of the Muscicapa asclepiadea that has not entrapped 

 its victim, and some of them in the United States closely cover hundreds 

 of acres together. 



What may be the precise use of this faculty is not so apparent. Dr. 

 Barton doubts whether the flowers that catch insects, being only temporary 

 organs, can derive any nutriment from them ; and he does not think it 

 probable that the leaves of Dionaa, &c. which are usually found in rich 

 boggy soil, can have any need of additional stimulous. As nothing, how- 

 ever, is made in vain, there can be little doubt that these ensnared insects 

 are subservient to some important purpose in the economy of the plants 

 which are endowed with the faculty of taking them, though we may be 

 ignorant what that purpose is ; and an experiment of Mr. Knight's, nursery- 

 man in King's Road, London, seems to prove that, in the case o( DioncBa 

 at least, the very end in view, contrary to Dr. Barton's supposition, is the 

 supplying the leaves with animal manure ; for he found that a plant upon 

 whose leaves he laid fine filaments of raw beef was much more luxuriant 

 in its growth than others not so treated.^ Possibly the air evolved from 

 the putrefying insects with which Sarracenia purpurea is sometimes so 

 filled as to scent the atmosphere round it, may be in a similar manner 

 favorable to its vegetation. 



Most of the insects which are found in the tubular leaves of this and similar 

 plants enter into them voluntarily ; but Sir James Smith mentions a curious 

 fact, from which it appears that in some cases they are deposited by other 

 species. One of the gardeners of the Liverpool Botanic Garden observed an 

 insect, from the description of one of the Crabronidce, which dragged several 

 large flies to the Sarracenia adunca, and having with some difficulty forced 

 them under the lid or cover of its leaf, deposited them in its tubular part, 

 which was half filled with water ; and on examination all the leaves were 

 found crowded with dead or drowning flies. "^ What was the object of this 

 singular manoeuvre does not seem very obvious. At the first glance one 

 might suppose that, having deposited an egg in the fly, it intended to avail 



' Small flies are sometimes found sticking lo the glutinous stigma of some of the Orchi- 

 deae lil^e birds on a limed twig (Sprengel, Entdecktes Geheimniss, 21.) ; and ants are not un- 

 frequently detained in the milky juice which the touch of even their light feet causes to 

 exude from the calyxes of the common garden lettuce. — Ann. of Bot. ii. 590. 



'^ Elements of the Science of Botany, 62. 



^ Smith's Introduction to Botany, 195. 



