396 THE BIOLOGY OF INSECTS 



insectivorous plants, the Sundews (Drosera) are perhaps the 

 best known among our British Flora, their leaves furnished 

 with stout knobbed hairs, which, when an insect alights, 

 bend over and hold it, while a *' gastric " secretion is poured 

 out over the victim. Many kinds of moorland insects are 

 caught in the Sundew's deadly trap, and a grim illustration 

 of the conflict involved in feeding is afforded by the small 

 dragon-flies (Agrionidae) often to be seen on Drosera leaves ; 

 these creatures of prey, having captured and devoured many 

 weaker insects, for the most part plant- eaters, are them- 

 selves caught and, so far as may be, assimilated by the 

 Sundew. Even in the waters of pools and streams aquatic 

 insects may be caught in the little pouches that are modified 

 from portions of the divided leaves of the bladderworts 

 (Utricularia) and serve to swell the food-supply of those 

 submerged plants. Among tropical insectivorous plants 

 the " Venus Fly-trap " and the various *' pitcher-plants " 

 are familiar to all students of this fascinating branch of our 

 subject. 



Not only these highly organised flowering-plants, but 

 many lowly Cryptogams live in various ways at the expense 

 of insects. The digestive tracts of bees and of many other 

 insects harbour a great variety of bacteria which find 

 nourishment in the partly digested food-products, and in 

 some cases help incidentally to complete the digestive 

 processes. Some of the bacteria harboured by insects are 

 definitely harmful by inducing diseases in their hosts, such 

 as those which cause the various types of '' foul-brood " in 

 hive-bees. The fungus Empusa miiscae is often deadly to 

 autumnal House-flies, as C. G. Hewitt (19 14) and others 

 have pointed out. The internal organs of the fly are com- 

 pletely destroyed by this fungus, and the dead insect is 

 surrounded by a mass of white spores. A fly becomes 

 infected when, from a spore settled among its hairs, a fungal 

 filament (hypha) is extruded, pierces by solvent action 

 through the chitinous coat and then, branching and pene- 

 trating throughout the internal tissues and organs, fills the 

 fly's body- cavity with meshes of mycelium, whence there 



