493 
Annual Report of New York 
evening of August 1st they were seen in prodigious numbers near 
Ilighgate, and soon covered the plants in the gardens, causing con¬ 
siderable mischief to the dahlia buds. They died, however, in two 
or three days, after some very heavy showers occurred. He further 
states that they had done great injury to the turnip crops in York¬ 
shire. A letter from Mr. Lister, with specimens, informed him that 
hundreds of acres of late sown turnips were almost destroyed by these 
flies. Mr. L. had five acres which were quite destroyed, and several 
more that were much injured. 
Hitherto it has been the current opinion of naturalists that every 
plant had its own parasites — one or more species of plant-lice living 
upon it and unable to subsist upon any other member of the vegetable 
kingdom. Linnaeus hence supposed it was unnecessary to give any 
descriptions of these insects, deeming that the several species were 
amply distinguished by simply naming the plants to which they 
respectively pertained. But the more accurate researches of the 
present day have greatly modified this view, and have shown that the 
same species of aphis is liable to occur upon different plants which are 
related to eacli other, and which pertain to the same botanical genus, or 
even to different genera of the same Natural Order. And some 
species occur which are still more ubiquitous, sustaining tliemsqjves 
upon plants which are quite dissimilar, and which pertain to different 
Orders. Thus the species of these parasites are now known to he 
much less numerous than has hitherto been supposed, many of them 
having received several names in consequence of their occurring upon 
different plants. We have a notable instance of this in the aphis now 
under consideration. Establishing itself and subsisting as it does 
upon quite a number of plants and trees which are more or less dis¬ 
similar in their nature, it has confidently been regarded as a distinct 
species on each of these plants and trees, and has thus received a cor¬ 
responding number of names. But when we come to fully study the 
characters and marks which this aphis presents in each of the stages 
of its development, the exact form and size and colors of the different 
parts of its body and of each of its several members, the position of < 
the veins in its wings, and especially the peculiar row of flocculent 
snow-white spots along each side of the back of its pupa, the size and 
shape and situation of each of these spots, we are furnished with such 
an assemblage of diagnostics as enables us to clearly recognize this spe¬ 
cies on whatever plant we meet with it. And, finding them to be 
identical in such a multitude of particulars, we shall not be able to 
persuade ourselves they are different species, however dissimilar the 
