312 ANIMAL AGGREGATIONS 



compared with that of related snails grown under uncrowded con- 

 ditions. Whitefield says that the shell proportions were so changed 

 that experienced conchologists recognized the dwarfed forms as 

 sufficiently distinct to be placed in a separate species. 



Differential morphological effects associated with partial starva- 

 tion are well known. Child (191 5) has shown that starving adult 

 planarian worms will cause them to return to a juvenile condition. 

 The culture water in which Planaria have been crowded definitely 

 depresses normal head formation in regenerating planarians (Child, 

 1911). In analyzing this effect, Child reports (1911a): ''The pres- 

 ence of metabolic products of Planaria in the water undoubtedly 

 decreases the rate of metabolism, and the effect on regulatory 

 morphogenesis is similar to that of starvation or low temperature, 

 though it may be greater and in extreme cases it approaches that 

 obtained with anesthetics." 



Vernon (1903) has reported differential inhibition in echinoderra 

 larvae due to crowding. He attributes this to food deficiency. 

 Warren (1900) reports modifications of the spine of Daphnia brought 

 about by crowding. Other such cases could be cited, but it will be 

 more profitable to examine in some detail the evidence that crowd- 

 ing affects {a) the appearance of winged forms in aphids, {h) colora- 

 tion and changes in bodily proportions of certain grasshoppers, and 

 (c) the suppression of certain bristles and eye facets in some races of 

 Drosophila. 



WING PRODUCTION IN APHIDS 



Aphids show two changes in the course of their normal seasonal 

 cycle, which may or may not be associated. Their annual history, 

 briefly, runs as follows : In . the autumn sexual forms appear, and 

 an overwintering fertilized egg is formed which hatches out the 

 following spring into a wingless female capable of producing young 

 parthenogenetically. These, too, are usually wingless females capa- 

 ble of parthenogenetic reproduction. After a time winged forms 

 appear which migrate to new host plants where they give rise to 

 wingless females with parthenogenetic powers. The production of 

 winged forms is thus a process distinct from the production of the 

 sexual forms. In the case of the latter in most, but not all, aphids 



