STRUCTURAL EFFECTS 141 



gestion that a high concentration of excretory 

 products has the same effect also has value and, 

 in fact, further researches may show that any 

 condition that tends to reduce the rate of metab- 

 olism of the females at critical periods will in- 

 crease the percentage of males produced. As 

 we know from work reported in the last chapter, 

 crowding these animals does have such an effect. 

 A somewhat less important change in body form 

 is illustrated by the effect of crowding upon wing 

 production in the plant lice commonly called 

 aphids. These aphids are small, usually greenish 

 bugs which push their beaks into juicy plant tissues 

 and obtain their food by sucking the juices of 

 their host plants. Their usual life cycle runs 

 something as follows: In the autumn a sexual 

 form appears and an over-wintering fertilized egg 

 is formed which hatches out the following spring 

 as a wingless female capable of producing young 

 without fertilization, that is, like the cladoceran 

 females, she is parthenogenetic. Her immediate 

 offspring all tend strongly to be parthenogenetic 

 females like herself. These reproduce very rap- 

 idly; the young settle near their mother and soon 

 produce a crowded condition on the host plant 

 which may lead to the withering of the plant. 

 About this time winged, parthenogenetic females 

 appear which can and do migrate to other neigh- 



