362 THE BIOLOGY OF INSECTS 



or artificially. In the case of the Small Tortoiseshell 

 Butterfly {Aglais urticae), however, it has been claimed 

 that among a small number of insects reared at normal 

 temperatures, the darkening that had been artificially 

 induced in the parents reappeared to some extent in the 

 offspring. This has been claimed as a definite case of '' use- 

 inheritance," but it may be objected that the strain of 

 insects used in such experiments possessed an innate 

 tendency towards darkening of wing-pattern, a suggestion 

 evidently hard to prove or disprove. It is also said by 

 upholders of Weismann's theory that if such effects do 

 reappear in a second generation, the result must be attributed 

 to the direct action of the surroundings on the germ-plasm ; 

 but the germ- cells are during the time of action of the 

 environmental influence within the body of the parent, and 

 any effect on them must be in some way due to transmission 

 of stimulation, apparently by some soluble substance, 

 through the body- tissues. 



In connection with tliis question the researches of 

 W. L. Tower (1906, 1918) on colour modifications in 

 American leaf-beetles of the genus Leptinotarsa are con- 

 sidered by many to be important though his methods have 

 been criticised by opponents of his views. These insects are 

 mostly yellow or brown with black markings on the pro- 

 thorax and dark stripes on the wing-cases ; the patterns 

 of the adult beetles can be modified by changed conditions 

 of temperature and humidity during the larval and pupal 

 stages. Tower claims that such modifications were not 

 transmitted by inheritance if the abnormal conditions were 

 restricted to the earlier stages of the insects' development, 

 but that the modifications became hereditary when the 

 abnormal conditions were continued through the maturation 

 period ; then it is stated that the offspring of the modified 

 beetles resembled their parents, though not subject to the 

 same abnormal surroundings ; hence a direct action of 

 the environment on the germ-cells may be inferred. A 

 different interpretation is offered by B. Diirken (1923) 

 from the result of his researches on the influence of 



