Metamorphosis 



641 



stimulating the prothoracic glands to secrete 

 their hormone. The brain hormone corre- 

 sponds to the factor variously designated as 

 "molting hormone," "metamorphosis hor- 

 mone," "pupation hormone," and "growth 

 and differentiation hormone." 



Corpus Allatum Hormone. An agent pro- 

 duced by the corpus allatum. When acting 

 with prothoracic hormone it modifies the 

 tissue response in such a way as to lead to 

 a larval molt. This factor was formerly desig- 



three aspects of insect development: (1) the 

 molting process as it occurs in the larva, (2) 

 pupation or the metamorphic molt, and (3) 

 the differentiation of adult structures from 

 imaginal discs or other larval sources. Re- 

 sults on the Diptera will be considered 

 separately after the general discussion, since 

 these results at first did not seem to fit in 

 with results in the other insects. For reviews 

 of various aspects of the problem published 

 during the last ten years see Needham ('42), 



RMODNIUS 



PLATY5AMIA 



DIPTtRA 



Larval 

 Molts 



Dram 

 Pro+horacic Glanol 



tissues 

 I 

 Larval Molt 



Pupal 

 Developments 



Bra i n 

 Prothoracic Gland 



^Tissues 



Imoginal Molt 



Brain 

 Prothoracic Glanol 



C.A. 



^^ Tissues 

 Larval Molt 



Brain 



Prothoracic Gland 

 C.A. J 



Tissues 

 Pupal Development 



o/ia/oause 



cold 

 Brain 



Prothoracic Gland 



C A X* 



^■^' Tissues 



Imoqinal Development 



/ Prothoracic Region) 

 C.A. /^i 



^Gojionx Tissues 

 Larval Molt 



rinq Brain 



Pupal & Innao|lnal Dewel 



Fig. 225. Schemata of endocrine relations in insect metamorphosis according to the Wigglesworth- 



Williams concept. 



nated as "inhibitory hormone" and "juvenile 

 hormone." 



Prothoracic Gland Hormone. An agent pro- 

 duced by certain glandular cells of the in- 

 sect's thorax (in some extending also into the 

 head region). It seems to act directly upon 

 the tissues, stimulating them to assume the 

 adult condition except when acting in con- 

 jimction with C.A. factor as noted above. 

 The prothoracic factor was and still is often 

 designated as the "growth and differen- 

 tiation hormone," a term appropriate in 

 indicating the wide action of the hormone 

 but objectionable in its extreme vagueness 

 and its previous use for the brain hormone. 

 The prothoracic gland is presumed to corre- 

 spond to the glandular structures of different 

 insects variously called "intersegmental 

 gland," "ventral gland" and "peritracheal 

 gland." 



It will be profitable to consider separately 



Bodenstein ('42), Joly ('45), Wigglesworth 

 ('45), Mendes ('47), Scharrer ('52a), Turner 

 ('48), and Williams ('49). 



Larval Molting. The clearest evidence that 

 blood-borne factors control the molting proc- 

 ess in pre-metamorphic stages comes from 

 implantation experiments. Limbs or pieces of 

 skin of caterpillars of various species trans- 

 planted from one larva to another molt 

 with the host, not at the same time as the 

 donor animal. This is true even if transplant 

 is made from one species to another, or from 

 pupa or imago to larva (Bodenstein, '33; 

 Furukawa, '35; Piepho, '39). Since such 

 results follow when the implant is in the 

 fat body or haemocoele and not in contact 

 with the host hypodermis the molting stimu- 

 lus would appear to be transmitted by the 

 blood. 



Direct attempts by injections to demon- 

 strate the presence of the molting factor in 



