COMPOSITION OF THE HEAD IN HYMENOPTERA 55 



a 



The composition of the head in the Hymenoptera. --Ratzeburg stated 

 in 1832 that the head in the adult Hymenoptera (Cynips, Hemiteles, 

 and Formica) does not correspond to that of the larva, but is derived 

 from the head and the first thoracic segment of the larva. West- 

 wood and also G-oureau made less complete but similar observations, 

 though Westwood afterwards changed his opinion, and the same 

 view Avas maintained by Eeinhard. Our OAvn observations (as seen 

 in Fig. .38) led us to suppose that this Avas a mistaken vieAv ; that 

 the larval head, being too small to contain that of the semipupa, was 

 simply pushed forward, as in caterpillars. Bugnion, however, re- 

 affirms it in such a detailed Avay tint Ave reproduce his account. He 

 maintains that the 

 A r ie\vs of Ratzeburg 

 are exact and easy to 

 verify in the chalcid 

 genus Encyrtus, ex- 

 cept, hoAvever, that 

 Avliich concerns the 

 ventral part and the 

 posterior border of 

 the prothoracic seg- 

 ment. 



As the time of 

 transformation ap- 

 proaches, the head of 

 the larva, he says, is 

 depressed and soon 

 concealed under the 

 edge of the protho- 

 racic segment; the latter elongates, becomes thicker and more con- 

 vex, and Avithin can be seen the t\vo oculocephalic imaginal buds. 

 The head of the perfect insect is derived not only from the head of 

 the larva, but also from the portion of the prothoracic segment which 

 is occupied by the buds, i.e. almost its entire dorsolateral face. But 

 the hinder and ventral part of this segment (which contains the 

 imaginal buds of the first pair of legs) takes no part in the forma- 

 tion of the head ; these parts, according to Bugnion, towards the end 

 of the larval period detaching themselves so as to become fused Avith 

 the thorax and constitute the pronotum and the prosternuui. 



FIG. 38. Larva (a) of a chalcid, about to pupate, with the 

 head, including- the eyes and three ocelli, in the prothoracic seg- 

 ment : I/, c, pupa. 



This mode of formation of the head may be observed still more easily in 

 Rhodites, Heraiteles, and Microgaster, from the fact that their oculocephalic 



