CONSIDERATIONS FROM STUDY OF INSECTS 257 



nest be put into another colony, it will be set upon and either driven out 

 or killed. Ants never really lose their community odor ; those absent for 

 a long time, on returning, will be easily distinguished by their odor, and 

 eagerly welcomed by the members of the nest. The talking of ants (when 

 they stop each other, when away from the nest, to communicate) is evi- 

 dently a process of smelling, for they caress each other with the antennae, 

 the organs with which odors are perceived. 



Symbiosis. — We have already seen that plants and animals 

 frequently live in a state of partnership or relation of mutual help. 

 Such a state is known as a symbiotic relation. The keeping of 

 the aphids by the ants which use them as " cows" is an example of 

 this relation among two species of insects. The ants provide pro- 

 tection and sometimes food; the aphids give up the honeydew of 

 which the ants are so fond. 



But a wider symbiotic relation exists directly between the flower- 

 ing plants and the insects. We all know the very great service 

 done the plants by the pollination of the flower by the insects, and 

 we know that the return is the supply of pollen and nectar as food 

 for the insects. If it were not for 

 the bees, wasps, and butterflies, 

 it is safe to predict that many 

 of our fruit crops would be 

 almost entire failures. Do you 

 know why ? 



Parasitism. — One of the near 

 relatives of the bee called the 

 ichneumon fly does man indi- 

 rectly considerable good because 

 of its habit of laying its eggs 

 and rearing the young in the 

 bodies of caterpillars which are 

 harmful to vegetation. Some 

 of the ichneumons even bore 

 into trees in order to deposit 

 their eggs in the larvse of wood- 

 boring insects. It is safe to say 



that by the above means the ichneumons save millions of dollars 

 yearly to this country. 



HUNT. E8. BIO. 17 



Thalessa boring in an ash tree to deposit 

 its eggs in the burrow of a horntail 

 larva, a wood borer. From photo- 

 graph, natural size, by Davison- 



