Phyli'tic Parallelism in Metaniorphic Species. 487 



diminished to one pair retained in a much reduced 

 condition, that the horny casing of the head, the 

 surface of attachment of the muscles of the jaws, 

 should consequently be lost, and that even the 

 segments of the head itself should become more 

 or less shrivelled up as the organs of sense therein 

 located became suppressed. 



The incongruence manifests itself however in 

 yet another manner than by the relatively greater 

 morphological divergence of the larvae : a dif- 

 ferent grouping is possible for the larvae and for 

 the imagines. If we divide the Hymenoptera 

 simply according to the form-relationships of the 

 imagines, the old division into the two sub-orders 

 Terebrantia or Ditroclia and Aculeata or Mono- 

 troclia will be the most correct. The distin- 

 guishing characters of a sting or ovipositor and a 

 one- or two-jointed trochanter are still of the 

 greatest value. But these two sub-orders do not 

 by any means correspond with the two types of 

 larvae since, in the Terebrantia^ there occur 

 families with both caterpillar-formed and maggot- 

 formed larvae. 



The cause is to be found in that a portion ot 

 these families possess larvae which are parasitic in 

 other insects or in galls, their bodily structure 

 having by these means become transformed in a 

 quite different direction. The mode of life of the 

 imagines is, on the other hand, essentially the 

 same. 



