158 



THE AMERICAN NATURALIST [Vol.LV 



galls. A third (Mayr,''05), including some genera re- 

 lated to the remarkable parasitic Perilampus, which we 

 mentioned a few moments ago, produce conspicuous 

 galls on certain plants. 



The production of galls by the phytophagous Chalcids 

 is quite suggestive, since many forms related to Isosoma 

 (Harmolita) are parasites of gall-making Cynipids. 

 Megastigmus also belongs to a group including many 

 parasites of Cynipids. Since we do not know exactly 

 how galls are formed, however, the matter can not be 

 profitably discussed at the present time. 



Although they may not aid us greatly in formulating 

 any general causes leading to divergence in habits among 

 related forms, I should like to append a few observations 

 made by various entomologists which suggest a variety 

 of factors. 



The effect upon the parasite of almost complete elimi- 

 nation of a host through excessive parasitism has often 

 been commented upon by entomologists. An especially 

 clear case has been described by Aldrich ( '12) where an 

 invasion of the western pine-butterfly was suddenly 

 checked by Theronia fulvescens. The parasite then found 

 it necessary to eke out an existence from scattering and 

 less suitable forest insects and under such stress, selec- 

 tion must be very keen (cf. p. 147). Complete parasitism 

 of 100 per cent, of the related cabbage butterfly by Apan- 

 teles glomeratus has also been reported by Chittenden 

 ('05). 



Errors or aberrations of instinct have also been occa- 

 sionally observed. Thus Marchal ('07) saw a Chalcidid 

 parasite of coccdnellids [Lygellus) repeatedly oviposit in 

 the pupal exuvia when living material was not available. 

 Still more incongruous is the behavior of TricTio gramma 

 observed by Holloway ( '12) who found this insect actually 

 ovipositing in small globules of partly solidified plant 

 juice on the foliage of okra plants. One of the common 

 hosts of this egg-parasite, the cotton boll-worm, fre- 

 quently oviposits on the leaves of okra and the globules 



