222 SYSTEMATIC ZOOLOGY 



They have two pairs of transparent, membranous wings, not 

 richly veined, and the posterior pair smaller than the anterior. 

 These two pairs of wings are attached together during flight, by 

 little hooks on their adjacent edges. The mouth parts are 

 adapted to biting and sucking. Generally the first or the first 

 two segments of the abdomen are very narrow, forming a stalk 

 connecting the thorax with the remainder of the abdomen. 

 There is a complete metamorphosis. The male possesses a 

 large, protrusible penis, and in the female the abdomen is pro- 

 vided with an ovipositor or with a sting. 



The larvae of most of the Hymenoptera are grublike, without 

 any appendages, but in many of the wasps they closely resemble 

 caterpillars and feed upon leaves ; they have three pairs of tho- 

 racic legs and from six to eight pairs of abdominal legs; thus 

 they are readily distinguished from the true caterpillars, which 

 never have over five pairs of appendages on the abdomen. The 

 gall wasps ( Fig. 226) deposit their eggs on the leaves of various 

 plants or trees; when the female places the egg, she pierces the 

 leaf and there results a very rapid growth of the plant tissue 

 about the developing larva, thus forming a gall ; in some cases, 

 however, it appears that it is not the sting of the parent, but the 

 action of the young larvae themselves, that leads to gall forma- 

 tion. The galls of some oaks contain a considerable amount of 

 tannic acid and so have a certain industrial value. 



Very interesting Hymenoptera are the ichneumon flies, which 

 constitute a very large group. The female deposits her eggs 

 almost exclusively within or upon the bodies of other insects, 

 usually upon the larvae. The ichneumon larvae then feed upon 

 the larval host, and finally spin cocoons for themselves within 

 or upon them. It is not uncommon to find caterpillars com- 

 pletely covered with the minute cocoons of these insects ; in 

 this way the ichneumon flies are of considerable advantage in 

 destroying these pests. 



The ants possess a relatively high degree of psychical devel- 

 opment. They live in communities with a marked division of 

 labor; the sexually functional males and females are typical 

 winged Hymenoptera, but the majority of the members of a 

 colony are wingless, and are nonfunctional females, which are 

 the workers (Fig. 227). They are sometimes divided into two 



