GROWTH 87 



But this cannot, of course, be a general explanation, applicable to 

 those insects which moult repeatedly though starved. In insects such 

 as locusts, which feed continuously, it seems to be the act of swallow- 

 ing that provides stimuli which go via the frontal ganglion to the 

 neurosecretory cells of the brain and thus maintain a continuous 

 production of the moulting hormone. 



Metamorphosis 



Thus far we have considered only the factors which control growth 

 and moulting. But it is characteristic of insects that moulting may be 

 associated with a more or less striking change in form or 'metamor- 

 phosis'. In some groups of insects the larva does not transform 

 directly into the winged adult but into an intermediate, non-feeding 

 stage, the pupa, which then moults again to produce the perfect 

 insect. This pupal stage, however, is not a necessary feature of insect 

 metamorphosis. The change in form from the larva to the adult may 

 be just as great in such insects as dragon-flies (Odonata), whiteflies 

 (Aleurodidae), or may-flies (Ephemeroptera) which have no pupal 

 stage, as it is in the alder fly (Megaloptera) with a well-defined pupa. 



In the larva of some insects, notably in Diptera and Lepidoptera, 

 many of the organs of the adult are already recognizable as clusters 

 of embryonic cells, the so-called 'imaginal discs' of Weismann. But 

 these likewise are not a necessary feature of metamorphosis. In many 

 insects which show a striking metamorphosis, such as dragon-flies, 

 they do not occur; and they are absent, for example, from the abdo- 

 men of caterpillars which is transformed into the totally different 

 structure of the butterfly. 



The essential feature of insect growth is the existence within the 

 epidermis of a latent capacity to develop into several organisms of 

 widely different form. The potentiality to form the pupa or the adult 

 remains latent within the functioning cells of the larva; metamorpho- 

 sis consists of the realization of these latent genetic capacities for 

 growth and differentiation. 



The control of this process has been studied in most detail in 

 Rhodnius. During the first four larval stages the corpus allatum (a 

 small endocrine gland lying just behind the brain) secretes a so-called 

 'juvenile hormone' or 'neotenin' which ensures that the larval 



