176 BRANCH ARTHROPODA 



anglos to the body in boring, and deposits her eggs, one in each hole. 

 When the larvae hatch they do much injury by burrowing into the heart- 

 wood, where they feed, grow, and finally form cocoons of silk and fine bits 

 of wood. The winged adult gnaws its way out through the bark. The 

 ichneumon fly Thalessa is parasitic upon Tremex. 



The gall-flies (Cynip'tilfr) live in closed galls during the larval state, and 

 the full-grown larva either makes a hole and emerges and pupates in the 

 ground, or it pupates in the gall and the adult makes a hole through which 

 it emerges. The adult female pierces a hole in the tissue of the leaf with 

 her sharp-pointeil ovipositor which is composed of " several needle-like 

 or awl-like pieces." In the incision thus made she deposits one or more 

 eggs. \\ hen the larva hatches an abnormal growth of tissue begins to 

 form about it, caused, perhaps, from some irritating excretions, or from 

 the physical irritation caused by the pressure of the irritating body. The 

 tiny, footless, white, maggot-like larva feeds, probably through the skin, 

 on the sap of the growing gall. When the gall dies, which is usually about 

 the time the larva is grown, it dries and hardens and forms a protecting case 

 in which the larva (or larvae) pupates, and from which it emerges as a tiny 

 gall-fly in the first or second spring following. 



But one of the strange things about these gall-flies is that, in some cases, 

 the successive generations of the .same species are not of the same form. 

 The adult flies of one generation, which consists exclusively of females, lay 

 their eggs upon a certain host-plant, but the resulting indi\-iduals are not 

 at all like their mothers. This generation includes individuals of both 

 sexes which have developed from " unfertilized eggs," or parthenogenetic- 

 ally. The females of this generation lay their eggs upon a different host- 

 plant, develop very differently shaped galls from those in which they 

 grew up, but, like those of their grandparents, and the resulting individuals 

 are like their own grandparents. Not all gall-flies show this alternation of 

 generation, some species appear always in the same form, but, strange to 

 say, they are usually represented only by females. Although there are 

 two hundred species of gall-flies, each species infests a special part, leaf, 

 branch, or root of one or more particular species of plants. The gall pro- 

 duced by each species of insect is of a definite form. This is a remarkable 

 manifestation of instinct. " It is impossible that intelligence or memory 

 can be of any use in guiding the Cynipidw; no C>Tiips e\er sees its young, 

 none ever pricks buds a second season, or lives to know the results that fol- 

 low the act. Natural selection alone has preserved an impulse which is 

 released by seasonally recurring feelings, sights, or smells and by the simul- 

 taneous ripening of the eggs within the fly. These set the whole physiologic 

 apparatus in motion and secure the insertion of eggs at the right time and in 

 the right place."' 



The Guest Gall-flies (Jn'quilines). — There are many gall-flies which do 

 not themselves form galls, but which lay their eggs in the galls formed by 

 others. The larva; feed and develop here, but ilo not materially disturb 

 the rightful owners. 



Parasitic hymenoptera (Irhneunian'ida') are of great economic interest 

 (Fig. 146). Most of them live wilhin tlie bodies of their victims during the 

 larval stage, (he egg being laid either within or upon the body of the host. 

 In the latter case the larva bores its way into the body and feeds upon the 

 blood, so that the host is not- killed until the lar^•a is grown. Each species 



■Slralton. 



