DEVELOPMENT 



139 



metabolous insects, it is customary to use the term nymph during the 

 interval between egg and imago. 



As a rare abnormality, a holometabolous larva may possess two 

 pairs of true external wing-pads. This condition has been reported in 



PIG. 209. Cicada tibicen. A, imago emerging from nymphal skin; B, the cast skin; C, 



imago. Natural size. 



several specimens of the meal worm, Tenebrio molitor (by Heymons), six 

 larvae of the museum beetle, Anthrenus verbasci (A. Busck) and one 

 pyrochroid larva, Dendroides canadensis (P. B. Powell). In these larvae 



FIG. 210. Eggs of various insects. A, butterfly, Polygonia inter rogationis; B, house 

 fly, Musca domestica; C, chalcid, Bruchophagus funebris; D, butterfly, Papilio troilus; E, 

 midge, Dasyneura trifolii; F, hemipteron, Triphleps insidiosus; G, hemipteron, Podisus 

 maculiventris; H, fly, Drosophila ampelophila. Greatly magnified. 



all coleopterous it is comparatively an easy step from the internal 

 wing-rudiment to an external wing-pad, as Dr. W. A. Riley has pointed 

 out. He regards the phenomenon not as an instance of atavism a 

 harking-back to a period when the larva bore wings but as an example 



