S. A. Arendsen Hbin 235 



from egg to beetle, i.e. it is possible to get the beetles at one's dis- 

 posal at any time of the year. This gives the great advantage, that 

 the work, which often accumulates suddenly in experiments with cross- 

 breeding, can be divided over a longer period. 



The temperature of the incubators varied from 22|° — 27|°, averaging 

 25'' C. In this temperature, and in favourable feeding-conditions, the 

 duration of the larval period varies from 5 to 10 or 12 months. By far 

 the greater number of the larvae pupated within 11 months. But 

 every culture produces also larvae which pupate much later, and whose 

 larval period lasts 14 — 18 months. Even with the children of one and 

 the same mother the larval period may differ widely. 



How far hereditary factors, too, play a part here, will be treated in 

 another paper. For the present we may as well say that the investi- 

 gation is impeded in a high degree by the great influence of outward 

 circumstances. I have not been able to confirm from my experiments 

 the information here and there to be found in literature (Krizenecky, 

 p. 54) that insufficient feeding at a certain moment would be favourable 

 to pupation or, what is the same, that the larval stage would be 

 shortened ^. 



4. Numher of Moults. 



It is a rather astonishing fact that in the literature^ the statement 

 can always be found that the larva moults four times, and that after 

 the fourth moult the pupa appears. This assertion has evidently never 

 been tested because a simple experiment would immediately have shown 

 its fallacy. In our experiments undertaken for that purpose the number 

 of moults was never less than 10 and never more than 19. In a series 

 from 8 larvae originating from the same mother and born on the same 

 day the number of moults were 13 — 12 — 14 — 11 — 13 — 13 — 14 — 13 

 with a larval period of 187 days in 7 larvae and 209 days in one larva. 



B. The Pupa. 



1. Differences of Sex. 



It was of no small importance for the purpose of the experiments 

 when it was found that the difference of sex in the pupae (once 

 recognized) is so easy to ascertain that any inexperienced assistant can 

 soon be made familiar with it. 



1 I have discussed this topic aloug with others in a paper "Technical experiences 

 in the breeding of Tenehrio molitor,'' published in the proceedings of the li. Ac. Sci. 

 Amsterdam, 1920. 



'■^ Brehm, p. 128. Frisch, Vol. iii. p. 1. Saling, pp. 2—8. Sturm, pp. 21, 22. 



