62 METAMORPHOSIS 



bound to occur, and, at the same time, the ancestral form of any 

 given larva is liable to be masked by superimposed adaptive or 

 secondary modifications. Embryological development, although 

 recapitulatory in the main, is well known to be subject to abbrevia- 

 tion and to exhibit secondary features which in no way represent 

 ancestral characters. This is exemplified in the suppression of 

 ancestral appendages from the trunk segments in the embryos of 

 Diptera. In so highly specialised an order many primitive 

 features, common to more generalised insects, are no longer 

 reproduced in the ontogeny. Developmental features of this 

 kind, in their turn, modify the morphological characters of the 

 resulting larvae. There are, consequently, potent influences 

 affecting the forms assumed by insect larvae other than those of 

 ancestry. This fact is abundantly evident in the remarkable 

 adaptations to special modes of life exhibited by many insect 

 larvae, which frequently mask the actual ontogenetic phases which 

 such larvae represent. In the past the tendency has been to 

 overstress the influence of adaptation in determining the evolution 

 of larval forms. The perennial difficulty of determining which 

 characters are ancestral and which are purely secondary thus 

 presents itself. Berlese's theory maintains that insect larvae 

 represent, primarily, ontogenetic phases in the history of the 

 species concerned. They are subject to the same laws affecting 

 other organisms, and, consequently, the particular ontogenetic 

 phase exhibited in the species of one group may be largely obscured 

 by secondary features, and yet be plainly evident in another group. 

 In cases of hypermetamorphosis, where there is a definite 

 sequence of larval types in the ontogeny of a single species, 

 further support is afforded to Berlese's theory ; the sequence 

 follows that of the corresponding embryonic phases of normal 

 insect development. This has been shown to be the case in the 

 hypermetamorphoses of certain parasitic Hymenoptera (Fig. 34). 

 Furthermore, in those Coleoptera where hypermetamorphosis 

 prevails, and in the Mantispidae among Neuroptera, the first 

 instar is a typical campodeiform oligopod larva. In the second 

 instar, change of habit has led to its transformation into an inert 

 maggot-like phase with vestigial thoracic limbs and other 



