Mr. CoRTis's Obfervaiions on Aphidcu 83 



and uniformly; whereas its appearance is extremely irregular, not 

 alike on any two leaves of the fame tree or plant, fome having none 

 of it, and others being covered with it but partially. 



But the'phaenomena of the honey -dew, with all their variations, are 

 cafily accounted for by confidcring the Aphida as the authors of it. 

 That they are capable of producing an appearance exactly fimilar 

 to that of the honey-dew, has already been fhown. As far as my 

 obfervation has extended, thete never exifts any honey-dew but 

 where there are Aphides ; fuch, however, often pafs unnoticed, 

 being hid on the under fide of the leaf. Wherever honey-dew is 

 obfervable about a leaf, Aphides will be found on the under fide of the 

 kaf or leaves immediately above it, and under no other circumflances 

 whatever. If by accident any thing (hould intervene between the 

 Aphidei and the leaf next beneath them, there will be no honey- 

 dew on that leaf. Thus then we flatter ourfclves to have incontro- 

 vertibly proved that Aphides are the true and only fourcc of the 

 honey- dew. 



We have found that where the faccharine fubftancc has dropped 

 from Aphides for. a length of time, as from the Aphis falicis in 

 particular, it gives to the furface of the bark, foliage, or whatever 

 it has dropped on, that footy kind of appearance which arifes from 

 the explofion of gun-powder, which greatly disfigures the foliage, &:c. 

 of plants. It looks like, and is fometimes miflakcn for, a kind of 

 black mildew. We have fome grounds for believing that a iatchdr 

 line lubflancc, IjinijUr to thpt of the Aphis^ drops from the Coccus 

 alfo, and is finally convcrttxl into the fame kind of p^owdcr. 



In moll fca.'ons the natural enemies of the Aphiaes are fufficient 

 to keep them in check, and to prevent them from lioing any cSlen-. 

 tial injury to plants in the open air. liut feafons fometioics occur, 

 very irregularly indeed, on an average, perhaps, once in four or fix 

 years, in vvhicii they arc multiplied to fuch an cxcefs; that the 



M 2 ufual 



