VIII 



ARTHROPODA 



277 



hard external cuticle was pared off by a fine scalpel, and the spot 

 so exposed was covered with fresh paraffin. 



The metamorphosis of the family Chrysomelidae, amongst 

 Coleoptera, which we have studied in Galerucella and Dorypliora, is 

 very simple as compared to the metamorphosis of the family Muscidae 

 in Diptera. The latter may be regarded as exhibiting the most 

 complicated metamorphic process of all Insecta. Amongst the 

 Coleoptera, however, which have a more prolonged period of larval 

 existence than the Chrysomelidae, still simpler conditions are found, 



FIG. 223. Diagrams illustrating the metamorphosis of Musca vomitoria. (After Van Rees. ) 



A, maggot with deeply embedded imaginal discs. B, early stage of pupation. C, later stage of 

 pupation, ofc 1 , first abdominal segment ; ceph, head region largely invaginated, the invaginated part 

 represented by a white line ; cut, cast-oft' cuticle ; I 1 ' 3 , the imaginal discs for the legs, each enclosed 

 in its sac ; halt, imaginal disc for halteres (" balancers ") ; thi~ 3 , the three thoracic segments ; w, 

 imaginal disc for wing. 



so that little difference is to be detected between their life-histories 

 and those of the so-called Hemimetabolous insects, such as Orthoptera, 

 Odonata (Paraneuroptera), and Hemiptera. Thus, according to Tower, 

 in many Coleoptera, and in particular in the families Cerambycidae and 

 Buprestidae, the imaginal disc for the wing consists of an area of 

 ectoderm which shrinks away from the larval cuticle, dipping very 

 slightly below the general level. On this area the wing develops as 

 an external appendage. The imaginal discs only appear towards the 

 close of larval life (Fig. 217, A). 



A slightly more complicated condition is found in the family 

 Scarabaeidae where the imaginal disc, which likewise appears late, 



