thorough innervation of the с allata by the nervus allatus as well as the ana- 

 logy with other incretory glands), the experimental finding is given (c. f. 

 B. Scharer) that a onesided interruption of the nervus corporis cardiaci 

 results in the increase and hyperf unction of the с allatum. The results of the 

 recent experiments of Engelmann and Liischer (1955, 1956) in Leucophaea 

 maderae support this hypothesis. Both authors conclude further, that the 

 found nervous inhibitory effect of the brain on с allatum is induced through 

 hormonal action of another incretory gland, so called pericardial cells. Analo- 

 gous conclusion was reached by Joly (1958) on the basis of his experiments 

 in the migratory locust. Another factor important for the onset of the meta- 

 morphosis should be the competence of tissues to undergo metamorphosis 

 according to B. Scharrer (1946 etc.) and others. 



Unlike all these opinions one of the authors attempted to explain the lack 

 of the JH observed in the last larval instar and thus the onset of metamor- 

 phosis by the activity of the corpus allatum itself (Novak, 1951, 1954, 1956, 

 1959). On the basis of this experiments лvith the lygaeid bug Oncopeltus 

 fasciatus and results of biometrical study of the growth of с allata in 18 species 

 of 8 different orders of Heterometabola and Holometabola as well as by 

 revaluating the results of other authors, Novak has reached the conclusion, 

 that no quahtative change occurs in the corpus allatum function during the 

 last larval instar but the JH production continues, till all the difference against 

 the preceding instars, depending on that the minimum effective concentration 

 of the hormone in haemolymph, is not reached before the further gro^\i:-h 

 has been inhibited by the preceding of the next moulting process. As inevitable 

 cause of this relative decline in the JH production is viewed the observed 

 progressive relative decrease of both surface and volume of the с allata against 

 the volume of body in course of the larval development. In this way the number 

 of larval instars and the onset of metamorphosis is determined. 



The mentioned hypothesis being a part of the Novak's so called gradient- 

 factor theory has made possible a unite explanation of different seemingly 

 contradictory experimental results scarcely understood so far. Hoлvever, 

 a reliable experimental proof in favour of one or the other of the mentioned 

 hypotheses has been lacking so far. This led us to undertake the further 

 mentioned systematic investigation of the effects of implantation of the corpus 

 allatum from different periods of the last larval instar of the bug Pyrrhocoris 

 apterus (L.) into the freshly moulted specimens of the same instar. Such an 

 experiment has not been done as yet, so far as we know, with the exception 

 of the classical experiments of Wigglesworth (1936). The probable cause 

 of this omission is, that according to the existing hypothesis a positive result 

 could not be expected. 



In our experiments the Vtli (last) instar nymphs 5 to 6 days old after 

 moulting were used as donnors. Acceptors were Vth instar nymphs between 



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