370 INSECTA. 



discussing first the development of the external form of the body, 

 and then the rise of the internal organs of the imago. 



A. Development of the external form of the Body. 



The external form of the imaginal body is already complete in 

 rudiment in the pupa, so that the passage of the pupa into the 

 imago takes place merely by an unfolding and hardening of parts 

 already present. The form of the body of the imago must therefore 

 be prepared in the last larval stages, and attains complete develop- 

 ment at the pupal moult (the transition to the pupal stage). 



In most cases the transition from the larval shape to that of the 

 imago consists principally of a modification of parts already present, 

 new rudiments participating in it only to a limited extent. In the 

 Lepidopteran caterpillar, for example, the head, together with the 

 antennae and mouth-parts, and, further, the thoracic limbs (though 

 in an essentially modified form) pass over direct from the larva to 

 the pupa. The compound eyes and the wing-rudiments arise as new 

 rudiments. The latter appear on the meso- and meta-thorax of the 

 larva in the form of imaginal discs (icing-discs). The same is the 

 case in very many other Holometabola, in which the transition from 

 the larva to the pupa rests essentially upon a transformation of parts 

 already present. We ought here further to mention modifications 

 which occur in the abdomen, and which consist partly of the growth 

 of the abdominal rudiments (extremities'?) into external genital organs 

 (ovipositors, poisonous stings, pp. 299 and 300), and partly of an 

 apparent diminution in the number of segments. The latter may be 

 brought about by a fusion of distinct segments, or by a union of the 

 first abdominal segment with the meta-thorax (Hymenoptera), or else 

 may be traced to a transformation of the most posterior segments 

 into a telescopic genital appendage (ovipositor, penis). 



In those cases in which the larva is limbless (Diptera, many 

 Hymenoptera, and Coleoptera), the limbs of the imago also arise 

 as new formations in the form of imaginal discs (limb-discs). 



The metamorphosis of Corethra (Weismann, No. 130) may serve 

 as an example of the simpler type of metamorphosis. The larva 

 belongs to the eucephalic type of Dipteran larvae, and consequently 

 the head of the adult is present as a rudiment in this stage; this 

 larval head, through certain modifications of its parts, passes directly 

 over into the pupa. Even the compound eye is already found in the 

 larva, a rare and exceptional occurrence among the Holometabola. 

 The thoracic limbs, the wings and the halteres, on the contrary, are 



