March 1S96.I 



PSYCH E. 



369 



speciinens flj to other hills either in the same 

 or neighboring fields, where the ants are 

 waiting to receive them and proceed to es- 

 tablisli colonies. This second generation 

 bring fortli viviparous young (mostly wing- 

 less) ; and generations of viviparous females 

 continue to develop on corn roots through- 

 out the summer. In autumn the true sexes 

 are produced (both being apterous), and the 

 eggs are deposited by the oviparous females 

 in the mines of the ant colonies. These eggs 

 are cared for by the ants through the winter, 

 and the voung lice that hatch from them in 

 spring are provided for as already described.* 



While the above observations are 

 sufficient perhaps to indicate that the 

 great majority of aphides spend the 

 winter in the egg state, it is by no 

 means trne that they all do so. There 

 are many species in which so far as we 

 can judge no sexed individuals or eggs 

 are ever developed. One of these, 

 which is often extremely abtmdant on 

 the branches of alders in New Eng- 

 land is the flocculent aphid {P<t)iphi- 

 gus tesseliata) . This insect abotnuls 

 throughout tiie sinnmer months in the 

 condition of parthenogenetic females ; 

 and in autumn enormous numbers of 

 little aphides are produced. These 

 migrate dowia the branches and tnnik 

 to the bases of the slirubs, where on the 

 larger roots or amojig the leaves and 

 rubbish thev settle down for the long 

 and drearv New England winter. No 

 doubt millions of them perish or are 

 washed a^vav. but in spring those that 

 are left crawl up the alder stems, and 

 finding satisfactorv positions insert their 



* Bulletin Illinois State Laboratory of Natural History, 

 V. in, art V 



beaks through the bark and begin to 

 feed and grow. In a short time they 

 mature and give birth to yr)ung. They 

 secrete a large amount of flocculent 

 material, causing aflected branches to 

 appear as if covered with a cottony 

 vegetable growth. They are not usu- 

 ally attetided by ants, and the large 

 amount of ''honey-dew" the\' excrete 

 encoin'ages the growth of a black 

 ftmgus. 



It has already been explained that 

 the divers methods of hibernation 

 adopted by the aphides may be ex- 

 plained bv the principles of natinal 

 selection. With these insects we have 

 all the essentials for the working of the ' 

 method of elimination which permits 

 onlv the fittest to survive. Individuals 

 are produced in such enormous numbers 

 that a large proportion of them may 

 well be sacrificed witlmut injiuv to the 

 species. 



The habit of migrating In smnmer 

 from trees to herbs may also be ex- 

 plained ill a similar manner. By so 

 migrating the aphides obtain at least 

 three important advantages, viz. : 

 (i) escape from enemies; (2) more 

 succident food ; (3) lessening the injury 

 to, or even saving from destruction 

 their host-plant. Bv returning to the 

 trees when the herbs begin to die, they 

 find a comparatively safe place for the 

 deposition of their eggs. In both tlie 

 spring and autmnn migrations the laws 

 of natural selection would find oppor- 

 tunitv to operate. 



To bring out moie clearly the bearing 

 of the laws of natiual selection upon 



