SOME WINGLESS INSECTS 167 



section of the Diptera that section in which, as we have 

 seen, the transformation is, as compared with that of all other 

 insects, the most profound. It is the more remarkable, there- 

 fore, to see how along with a highly specialized form of para- 

 sitic life, there has come about, not only a loss of wings, but the 

 total disappearance of metamorphosis, and even of the succes- 

 sion of moults almost universal in the life-history of all 

 Arthropoda. 



These and many similar facts supply ample justification 

 for the oft-used expression that such wingless, parasitic 

 insects as we have been considering, have " lost their wings ", 

 and the same result is seen to accompany other abnormal 

 life-conditions. Such absence of wings is a secondary, not a 

 primitive character. It is instructive in this connexion to 

 refer to some famous recent researches on the facts of heredity 1 

 as shown by small two-winged flies (Drosophila) whose larvae 

 feed in fruit. In the course of long series of generations of 

 these quickly-breeding insects, kept under careful observation, 

 it has been found that reduction or abortion of the wings may 

 occur in certain individuals and that these suddenly-arising 

 modifications are inherited according to definite rules. It is 

 noteworthy that the extreme condition of degeneration when 

 the wings are reduced to the merest vestiges, is not reached 

 through a long series of ancestors showing gradual retrogressive 

 change, but may appear suddenly, as what is termed a 

 mutation, in the offspring of fully-winged parents. There is 

 no doubt that such changes are due to changes in that sub- 

 stance of the nuclei of the reproductive cells the germ-plasm, 

 as it is called which contains the factors or determiners of 

 inborn characters. The cause of such changes in the germ- 

 plasm remains, however, for the present beyond the bounds 

 of our knowledge. 



But, besides such wingless insects as those we have con- 

 sidered, there are others, briefly referred to in the introductory 

 chapter (p. 3), which have no apparent near relationship 

 to winged groups. These the spring-tails and bristle-tails, 

 with their more obscure allies are believed to be primarily 

 wingless, not only because they have no close affinity with 



1 T. H. Morgan and C. B. Bridges : " Sex-linked Inheritance in Droso- 

 phala ". Carnegie Publication No. 237 . Washington, 1916. 



