38 



ZOOLOGY 



colony is very perfectly organized. The duties of the workers are 

 divided among different gangs. Thus, when a tree has been selected, 

 one gang ascends the trunk and cuts the leaves in pieces of a definite 

 shape. The pieces flutter to the ground, are picked up by another 

 gang and carried to the entrance of the ant-hill, where they are de- 

 posited to be carried into the nest by a third relay. The bits of 

 leaves are used to line certain of the passageways, and a fungus is 

 grown upon them which serves the ants as food. The leaves are 

 probably stored to provide this fungus food. 



The parasitic Hymenoptera have the habit either of lay- 

 ing eggs in the body of another insect, one of the plant- 

 lice, a caterpillar, or other species, or else they lay their 

 eggs in the nest of some species of insect so that the larva 

 can make its own way into the host (Figs. 36-38). The 

 long, tail-like ovipositors of the female ichneumon are, in 



some cases, used to drijl 

 holes into trees occupied 

 by insect burrows so that 

 her eggs can be laid 

 therein. These parasitic 

 species are invaluable to 

 agriculture in keeping 

 down injurious insects. 



The gall-wasps, popu- 

 larly not distinguished 

 from the strict gall-flies, 

 are familiar to us from 

 their works. They lay 

 eggs in various kinds of 

 plants, especially in oaks and members of the rose family. 

 An excessive growth of the plant tissue, called a gall, is 

 caused either by a poison dropped into the plant with the 



FIG. 37. Cocoons of Microgaster, a para- 

 sitic hymenopter, on a sphinx larva. 

 Photo, from the living object by V. H. L. 



