250 POPULAR SCIENCE MONTHLY 



Parallel Evolution. — A similar need among the several related 

 families of the order Orthoptera to which the cockroaches belong has 

 been met by the development of identical structures. Thus both the 

 plications and the fold of the hind wings originated independently in 

 the cockroach and some other families of the order. 



Mechanical Principle. — Both the plications and the longitudinal 

 fold of the Orthoptera wing seem to have been developed in response 

 to the mechanical need of some means of caring for the largely ex- 

 panded inner area of these wings. 



Specialization by Reduction. — The reduction of the long ovipositor 

 of early cockroaches to the short specialized ovipositor of modern forms 

 is apparently an illustration of the law of specialization by reduction. 



Loss of Organs Through Disuse. — The reduction of the wings of 

 several modern species to such an extent that, although presenting 

 normal adult features of venation- and articulation to the body, they 

 are so far vestigial as to be practically functionless, is doubtless to 

 be attributed largely to lack of use, and results from the ground habits 

 of these insects in which they use the wings comparatively little. Not 

 infrequently functional wings are retained by the males, even when 

 lost by the probably less active females of the same species. 



Arrestation of Development.- — The wings of not a few modern 

 forms remain as nothing more than wing-pads similar to the wing-pads 

 of the larval stages. This further reduction is probably an instance 

 of what has been called arrestation. Although other organs of the body 

 have reached maturity, the wings, checked in their development, have 

 not passed beyond the larval stages. Lack of use of the wings by the 

 ancestors of the species, and consequent insufficient blood and food 

 supply to this part of the body, is probably in this case an indirect cause 

 of arrestation. 



