484 Studies in the Theory of Descent. 



embryo of the grub possesses a complete head, 

 consisting of four segments and provided with the 

 three typical pairs of jaws. These head segments 

 do not subsequently become formed into a true 

 horny head, but shrivel up ; whilst the jaws dis- 

 appear with the exception of the first pair, which 

 are retained in the form of soft processes with 

 small horny points. We know also that from the 

 three foremost segments of the embryo the three 

 typical pairs of legs are developed in the form of 

 round buds, just as they first appear in all insects. 2 

 These rudimentary limbs undergo complete de- 

 generation before the birth of the larva, as also do 

 those of the whole 3 of the remaining segments, 

 which, even in this primitive condition, show a 

 small difference to the three foremost rudimentary 

 legs. ' 



The grub-like larvae of the Hymenoptera have 

 therefore descended from forms which possessed a 

 horny head with antennae and three pairs of 

 gnathites and a ^-segmented body, of which the 

 three foremost segments were provided with legs 



2 [See for instance Lubbock's " Origin and Metamorphoses 

 of Insects," chap. iii. ; and F. M. Balfour's " Comparative 

 Embryology," vol. i., 1880, pp. 327356. This last work 

 contains an admirable resume of our knowledge of the embry- 

 onic development of insects up to the date of publication. 

 R.M.] 



* Are not the 4th, nth, and i2th segments destitute of the 

 rudiments of legs as in the larvse of all existing saw-flies ? I 

 might almost infer this from Biitschli's figures (see 'for instance 

 PI. XXV., Fig i7,\). 





