METAZOA INSECTA. 477 



Spathcgaster baccarum Linn. (PI. 1217, figs, i, 2), and 

 leading up to the complicated structure of Cynips kollari 

 Hartig (No. 1218, complex gall of Cynips; No. 1219, 

 young and adult Cynips quercus spongifica O. S.). In 

 Spathegaster the simple gall (PI. 12 17, on oak leaf and 

 the peduncle of the flowering catkin of the oak) consists 

 of nutritive tissue enclosed in thin-walled parenchyma, 

 while in Cynips the gall has an inner gall enclosed in 

 thick-walled parenchyma, surrounded by spongy tissue and 

 covered by a differentiated epidermis. The gall is an 

 abnormal growth of the plant caused by animal agency 

 working from within (Adler). 



The female punctures the oak leaf and deposits her 

 egg, but the gall does not begin to develop until the larva 

 is hatched. It is therefore through the agency of the 

 young gall-fly that the plant is excited into active gall- 

 growth. "The moment the larva has for the first time 

 wounded the surrounding cells with its delicate mandibles, 

 a rapid cell growth begins." 1 



The larva spends its whole life within the narrow con- 

 fines of the gall. Having little need of antennae and legs, 

 these organs exist as vestiges. The pupal stage is also 

 passed in the home of the larva, and the adult fly (No. 

 1219) makes a hole in the outer wall in order to escape. 



These insects illustrate alternation of generations, since 

 the brood of the first generation (the spring gall flies, 

 Cynips quercus spotigifica O. S.) is made up of males and 

 females, while that of the second generation (the fall gall 

 flies, Cynips quercus aciculata O. S.) is composed wholly 

 of agamous females. 



The parasitic Hymenoptera seem to be more nearly 

 related to the Terebrantia already described than to the 

 next division of Hymenoptera, the Aculeata, and for this 

 reason they are now considered. All the species of the 

 immense family of Ichneumonidae are parasites, destroy- 



1 Adler, loc. cit ., p. 101. 



