12 EVOLUTION IN COLOR-PATTERN OF THE LADY-BEETLES. 



lady-beetle larvae are feeding. These larvae then disperse in search of 

 another colony of aphids. In this they are rarely successful, for colonies 

 of that species of aphid in the immediate neighborhood are subject to simi- 

 lar attacks and with the larva's limited capacity for wandering the chance 

 of finding another species of aphids is indeed slight. As a consequence, more 

 larvae would be successfully reared if the females laid their eggs singly in 

 good aphid territory, for large batches of suitable aphids are frequently seen 

 without lady-beetle eggs or larvae. Any change in this direction is not 

 possible, however, because there are no favorable variations by which the 

 lady-beetles could locate a sufficient number of colonies to distribute their 

 eggs properly. 



The less of two evils, then, is to risk overcrowding and the consequent 

 failure of some of the batches. We ordinarily think of overcrowding as 

 having no other result than the desirable one of the survival of the fittest, 

 but this other result of group-suicide may and frequently does follow. I 

 have often seen a batch of eggs laid on a plant where I could see that the 

 colony of aphids could not last through the larval life of the coccinellids 

 because of syrphids or attacks on the aphids of disease. The larvae would 

 then disappear among surrounding plants destitute of aphids. In Epi- 

 lachna, on the other hand, the laying of eggs in batches is advantageous, 

 for shortage of food-supply is unlikely. In confinement the larvae, except 

 those of Epilachna, are cannibalistic when the aphids are exhausted. This 

 is probably not an important feature in nature, however, because the larvae 

 are so rapidly dispersed when the food is exhausted. 



Although assortative mating is apparently not operative in these beetles, 

 there is a very remarkable assortative association. At Oakland, California, 

 the typical Hippodamia convergens and its spotless variety are both common, 

 yet a hibernating mass taken by Mr. Nunenmacher and kindly given to me 

 consists almost wholly of the spotted variety, as shown in table 6. If 

 other hibernating masses are found to be similarly segregated we have a 

 noteworthy condition. It is doubtful if this produces much or any passive 

 assortative mating, however, as dispersive flights for food doubtless pre- 

 cede mating in the spring. 



