MORPHOLOGICAL EFFECTS OF CROWDING 



315 



similar, except that there was practically no crowding. Normally, 

 as we have stated, the alate aphids give rise to wingless forms. 

 Reinhard found that crowding these alate individuals caused an 

 increase in the percentage of winged offspring in the next generation. 

 In the light of these experiments, Reinhard concluded that ''crowd- 

 ing is a potent, if not the dominant, factor in controlling wing de- 

 velopment in Aphis gossypii.^' While this conclusion appears entire- 



TABLE XXXVI 



Data on the Effect of Crowding on Wing Development in Aphids. 



The Check Progeny Were Sisters to Those Crowded, and 



of the Same Generations 



(From Reinhard) 



Generation 



13 



18 



19 



29 



47 

 48 



55 

 59 



Parentage 



Apterous 

 Apterous 



.\pterous 

 Apterous 

 Apterous 



Apterous 

 Apterous 

 Apterous 

 Apterous 

 Apterous 



Date 

 Isolated 



1925 

 Dec. 9 

 Dec. 20 



1926 

 Jan. 9 

 Feb. 10 

 Feb. 16 



April 17 

 July 25 

 July 28 

 Sept. 15 

 Oct. 3 



Num- 

 ber 

 Iso- 

 lated 



9 

 17 



19 

 13 



First 



Young 



Observed 



De\'eloping 



Wings 



1926 

 Jan. 9 

 Jan. 15 



Jan. 20 

 Feb. 16 

 Feb. 24 



April 23 

 Aug. 9 

 Aug. 18 

 Sept. 23 

 Oct. 19 



Winged 



Forms after 



Progeny 



Well 

 Crowded 



Many 

 Many 



Few 

 Many 

 Predomi- 

 nate 

 Few 

 Few 

 Many 

 Many 

 Few 



Check Progeny 

 Not Crowded 



Apter- 

 ous 



25 

 19 



32 

 75 

 16 



16 

 17 



14 



Alate 



ly reasonable, it must be remembered that the factors involved in 

 bringing about this result are not fully revealed. Observers agree 

 that starvation is also an effective agent in producing winged aphids, 

 and anyone who has undertaken to rear aphids under crowded con- 

 ditions will appreciate the difficulty of separating these two factors 

 with certainty. One of Reinhard's observations indicates that 

 crowding is the more effective agent of the two. Starvation did 

 not cause winged forms to increase the number of winged progeny 

 in the next generation, while crowding did do so. Other possible 

 factors to be tested include the physical effect of the presence of 



