362 



EVOLUTION AND ANIMAL LIFE 



FIG. 220. The pigeon horn-tail, Tremex 

 columba, with strong bearing ovipositor. 



birds, leaving their young to be hatched and reared by their 

 unwilling hosts. This is, however, not bodily parasitism, such 



as is seen among lower 

 forms. 



We may also note that 

 parasitism and consequent 

 structural degeneration are 

 not at all confined to ani- 

 mals. Many plants are 

 parasites and show marked 

 degenerative characteristics. 

 The dodder is a familiar 

 example, clinging to living 

 green plants and thrusting 

 its haustoria or rootlike 

 suckers into their tissue to 

 draw from them already 



elaborated nutritive sap. Many fungi like the rusts of cereals, 

 the mildew of roses, etc., are parasitic. Numerous plants, too, 

 are parasites, not on other plants, but on animals. Among 

 these are the hosts 

 of bacteria (sim- 

 plest of the one- 

 celled plants) that 

 swarm in the tis- 

 sues of all animals, 

 some of which are 

 causal agents of 

 some of the worst 

 of human and ani- 

 mal diseases (as 

 typhoid fever, diph- 

 theria, and cholera 

 in man, anthrax in 

 cattle). There are 

 also many more 

 highly organized 



fungi like the whole family of Entomophthorse, and the genus 

 Sporotrichum that live in and on the bodies of insects, often 

 killing them by myriads. One of the great checks to the 

 ravages of the corn and wheat-infesting chinch bug (Blissus 



FIG. 221. Thalessa lunator boring. (After Comstock.) 



