THE INSECTS 



137 



animal, which finally becomes an adult. In some types, these changes are 

 fairly gradual ('hemimetabolous' insects with an incomplete metamor- 

 phosis), in others there are first a series of larval stages in which little 

 alteration occurs except increase in size, but these are followed by a rather 

 sudden radical reorganisation by which the adult is produced ('holometa- 

 bolous' inseas with a complete metamorphosis). The period during which 

 the metamorphosis occurs is known as pupation, and the pupal form 

 usually differs considerably both from the larval and the imaginal phases 

 of the Hfe-history. 



GMH 



Figure 8.11 



Diagram of the hormonal control of moulting and metamorphosis in insects. 

 The letters above refer to the relevant larval organs: br. the brain; tisc, the 

 neurosecretory cells; R.G., the ring-gland (in Diptera) consisting of cc, the 

 corpus cardiacum, ca, the corpus allatum, and Ic, the lateral cells, which 

 probably function as the prothoracic gland, which in other forms lies some 

 distance from the corpus allatum, and is indicated as ptg; ib is an imaginal 

 bud. The letters below refer to the active principles: N, the nervous con- 

 nection to the corpus allatum; A, the activator passing from the neurose- 

 cretory cells to the prothoracic gland ; GMH, the growth and metamorpho- 

 sis hormone given out by this gland; JH, the juvenile hormone produced by 

 the corpus allatum. 



Both the moulting of the larva and its metamorphosis to the adult are 

 controlled by hormones (Reviews: Seidel 1952^, Wigglesworth 1954, 

 Bodenstein 1954). There are at least two main hormones involved, and 

 probably more. The anatomical structures in which the hormones are 

 produced are not always easy to homologise from one group of insects 

 to another, so that the details of the story are complex; only a general 

 summary can be given here. 



