NATURAL SELECTION 



675 



blind and four eyed fishes surNdved a sea- 

 son under predation pressure from frogs 

 and insects. 



Darlington (1936) described the con- 

 vergent regression of the hind wings in 

 various genera of carabid beetles, in all of 

 which nonfunctional vestiges of the hind 

 wings are present. The reduced hind wing 

 is an example of a vestige of an ancestral 

 character functional only in the adult stage. 

 Controversy still exists over recapitulation 

 of ancestral adult structures (de Beer, 

 1938, p. 58, 1940; see also p. 636). 



Various external parasites such as fleas, 

 sucking lice, and Mallophaga are wingless, 

 and the mesothoracic wings of certain flies, 

 including the sheep tick (Melophagiis 

 ovinus) , have undergone extreme regressive 

 evolution. In other examples, one sex has 

 lost the wings, while the other retains 

 functional wings. The female gypsy moth 

 (Porthetria dispar) has well-developed 

 wings, but is incapable of sustained flight, 

 while the male has functional wings. The 

 female of the white-marked tussock moth 

 (Hemerocompa Jeiicostigma) has small 

 wing rudiments useless for flight, while the 

 male has functional wings. The male of 

 a termitophilous braconid (Termitohracon 

 emersoni) has reduced wings, while the fe- 

 male has normal functional wings (Brues, 

 1923). Some of the summer generations of 

 the woolly apple aphid (Eriosoma laniger- 

 iim) lack wings without even a vestige, 

 while other summer generations of females 

 may be winged, and the fall sexual genera- 

 tion of males and females is wingless (see 

 pp. 123,347,703). 



One of the most extreme cases of regres- 

 sive evolution is found in the adult stage 

 of the crustacean Sacculina, parasitic upon 

 crabs. The parasite is attached to the host 

 by rootlike projections that penetrate 

 throughout the crab and absorb nutriment. 

 The appendages, muscles, nerves, sensory 

 organs, and digestive tract are all degen- 

 erate, and only the reproductive organs of 

 the parasite are unimpaired. If it were not 

 for the free-swimming larva with jointed 

 appendages, it is doubtful whether the 

 crustacean affinities of the parasite could 

 have been recognized. 



Regression of metabolic functions is prob- 

 ably characteristic of the evolution of 

 parasites. One may assume that free-living 

 forms commonly synthesize some essential 



Fig. 247. Transformations of polar coordi- 

 nates centered in the eye of fishes exhibiting re- 

 gressive evolution. A, Normal-eyed river fish 

 {Astyanax mexicanus); B, fully blind derived 

 form (Anoptichthys jordani) from a cave (La 

 Cueva Chica); C, fully bfind derived form 

 from a cave ( Cueva de los Sabinos ) . ( Redrawn 

 from Breder. ) 



