18S3.J 111 



judgment in suspension for a "while, but in guarding myself througli 

 the expressions M. Lichtensteiu quotes, I by no means hardily assert 

 the unreliableness of the present published observations on migration. 

 It is from undoubted evidence alone that theory passes into re- 

 cognised fact. 



Peculiar difficulties attach to experiments connected with life- 

 habits, and it is granted to be no easy task to eliminate all sources of 

 error in conducting them. Here we must assume that all possible 

 precautions were taken that the "clean garden earth" contained no 

 underground Aphides or their ova ; and that the roots of the maize 

 plants were previously as free from such. 



The comparatively slight differences of character to be remarked 

 between the larvae of the ItJdzohiidcB and other underground forms 

 which are now known to be rather numerous, and the consequent 

 difficulty in making a good diagnosis, render a confusion of species 

 not unlikely. 



But let us assume that the larva; of Tetraneura ulmi leaviug their 

 galls have been successfully transferred to the roots of the maize 

 plant {Zed), and that there they have undergone pupation, and that 

 the images have, by their wing venation, &c., proved themselves to be 

 normal forms, identical with those simultaneously producing the per- 

 fect sexes on the bark of the elm. Then are we to assume that the 

 maize-root is necessary to the economy of this insect ? I think we 

 must answer this question in the negative. 



In England and in Belgium Tetraneura ulmi is often common on 

 the elm-trees. In the former country the maize is exotic, and one 

 may say it is almost exclusively cultivated for ornament. Certain it 

 is that in parts of Kent the insect is common, where the Indian corn 

 is not be found for miles round. 



[ In June, 1877, I noticed that the elms of the neighbouring dis- 

 tricts of Spa, in Belgium, were covered by the galls of Tetraneura, yet 

 I did not mark any cultivation of Zea in the fields around. 



It may be urged that Graminece, other than the maize, are resorted 

 to, but if the elm-bark be selected for the nidus of the ova, the under- 

 ground habit would seem to have nothing to do with winter quarters 

 and oviposition. 



I would invite the attention of some competent observer, in whose 

 quarters Tetraneura ulmi is common, to search the couch-grass, 

 Triticum repens, in autumn, and, if possible, to settle this point of 

 habitat. 



A similar difficulty suggests itself in the case of BrijoVius croati- 



