the secondary sexual characters and the Gonads. 341 



to be excluded, Gynandromorphs are occasionally met 

 with in which the head or head and thorax show fully- 

 developed male characters and the rest of the insect shows 

 female characters. In others exactly the opposite con- 

 dition is met with. 



When the whole of the external structure of the head 

 or thorax shows male characters it is almost certain that 

 the structures within are male in constitution also. 



If this be admitted one can exclude any part of the 

 head, thorax or abdomen as the possible site of a gland 

 producing an internal secretion, which causes the develop- 

 ment of the secondary sexual characters in insects. 



Thus we are reduced either to accept the view that in 

 insects the tissues are sexually differentiated from the 

 first, and do not need the stimulus of any internal secre- 

 tion in order to attain the fullest development ; whereas, 

 in vertebrates, although the tissues are sexually differen- 

 tiated from the beginning, the differentiation cannot be 

 completed without the influence of the internal secretion 

 of the gonads and other glands. 



Or we must accept an alternative explanation such as 

 that of Geoffrey Smith, that there is in all animals a sexual 

 formative substance, which in some is capable alone of 

 perfecting the sexual characters, but which in others 

 requires the co-operation of the internal secretion of the 

 gonads. It is difficult to accept the former hypothesis 

 even in the case of insects, because the parasite Stylops 

 in bees and subjection to abnormally high or low tem- 

 peratures in butterflies and moths can in some instances 

 modify the secondary sexual characters at a late stage of 

 development. 



But on the latter hypothesis, unless the evidence derived 

 from a study of gynandromorphism be untrustworthy, the 

 sexual formative substance must be produced in various 

 parts of the body if not by all the tissues. 



BlBLIOGKAPHY. 



Cockayne, E. A. Trans. Ent. Soc. 1916, p. 322. 



. Journ. of Genetics, 1915, V, 2, p. 75. 



Dannenberg. Zeitschr. f. Wissen. Insektenbiol. 1913, ix, 



pp. 239 and 291. 

 Harrison, J. W. H. Entomologist, 1916, xlix, p. 53. 



