32 BRITISH APHIDES. 



Chermesmce, Schhoneibrin^e, Pemfhigince, &c. Though 

 now much restricted by its subdivision into other 

 groups, the genus Aphis still continues to be by 

 much the richest as regards tlie number of its included 

 sjDecies. 



As might have been expected, the eHmination of 

 the more typical forms has left a residuum in which 

 the characters hitherto relied on for classification 

 present themselves in a less marked and pronounced 

 degree, and consequently it happens that this original 

 Linnasan genus, which now gives the name to the whole 

 family, no longer furnishes its most typical forms. 

 These special family characters must now rather be 

 looked for in the foregoing genera of Siplionopliora and 

 Drepanosiplmm. 



This absence of sharp external characters renders a 

 natural and scientific grouping difficult, and the want 

 will be the more felt should we find the essential attri- 

 bute of form, as vv^ell as the less important one of 

 colour, liable to variation ; and also suspect phases of 

 inconstancy from the action of external surroundings. 



Mr. J. Monell, of the Botanical Gardens, St. Louis, 

 Missouri, who is doing some good work in this depart- 

 ment of Entomology, writes to me : " The great difficulty 

 with the American Aphides is to distinguish them from 

 allied European species." Also Professor J. W. Trail, 

 of Aberdeen, tells me that at his higher latitude the 

 measurement of several apterous forms of identical 

 species even exceed those I found on the Surrey Hills. 



Mr. "Wallace has more than once interestingly dis- 

 cussed the question of dependence of colour and size 

 upon physical climatic conditions. He has shown that 

 many insular Papilios like HeUconius and Mechanitis 

 partiall}^ change their liveries when differently bred 

 on the slopes of either the northern or the southern 

 Andes. Again, that some insects are more richly 

 banded and coloured when bred on continental rather 

 than on insular tracts. And what is true of insects 

 also seems in this particular to obtain in some 



