232 THE NATURAL HISTORY 



and therefore impregnated with the particles of flowers in summer 

 weather, our senses will inform us ; and that this clammy sweet 

 substance is of the vegetable kind we may learn from bees, to 

 whom it is very grateful : and we may be assured that it falls in 

 the night, because it is always seen first in warm still mornings. 

 On chalky and sandy soils, and in the hot villages about London, 

 the thermometer has been often observed to mount as high as 

 83 or 84 ; but with us, in this hilly and woody district, I have 

 hardly ever seen it exceed 80 ; nor does it often arrive at that 

 pitch. The reason, I conclude, is, that our dense clayey soil, 

 so much shaded by trees, is not so easily heated through as those 

 above-mentioned : and, besides, our mountains cause currents of 

 air and breezes ; and the vast effluvia from our woodlands temper 

 and moderate our heats. 



LETTER LXV. 



TO THE SAME. 



THE summer of the year 1783 was an amazing and portentous 

 one, and full of horrible phaenomena ; for, besides the alarming 



showed that aphides discharge a sugary fluid from the intestine, which collects on 

 leaves in dry summer weather ; he was also aware, like Goedaert before him, that 

 ants are fond of this excretion, and protect and caress the aphides for the sake of it. 

 It was, however, long believed that there is also a true vegetable honey-dew, not 

 excreted by insects at all. This was taught by Treviranus, Boussingault and 

 Hooker, and Darwin vigorously maintained the same doctrine (Cross- and Self- 

 fertilisation, p. 404). It cannot, however, stand the observations and experiments 

 of Biisgen (Jen. Zeits. f. Nat., 1891), who shows that except small quantities 

 excreted by sundry insects and by the parasitic fungus, Claviceps purpurea, all 

 honey-dew is formed by aphides and scale-insects. The fluid is dropped or squirted 

 out, and may fall in a fine shower on distant objects. When fresh drops are ob- 

 served on a leaf, it is instructive to place a piece of paper or a glass slip on the 

 same spot. It will generally become sprinkled with honey-dew before long, and 

 thus will show that the sugary fluid does not exude from the leaf, but is dropped 

 from above. Moreover, the drops do not grow steadily, as we should expect them 

 to do if they exuded from the plant, but increase only by running together. The 

 tongue or proboscis of an aphis is beautifully contrived for penetrating the tissues 

 of a leaf or shoot. It works its way in between the hard and impenetrable structures, 

 often taking a sinuous course till it reaches the soft bast, whose vessels are then 

 tapped. Linnaeus originated the oft-repeated error, still to be found in modern 

 books, that the tubes on the abdomen of the aphis discharge the sugary excretion. 

 The tubes emit a fluid which sets on exposure to air, and is believed in some measure 

 to protect the aphis from its many insect-enemies (lady-bird larvse, Chrysopa-larvae, 

 Syrphus-larvae, &c.). The tubes can be pointed towards the place of attack, the 

 fluid is discharged, and the face or jaws of the assailant are at times so clogged that 

 he retires to rub them clean. But neither this artifice nor the friendly aid of the ants 

 are altogether adequate, for the aphides are often devoured in countless numbers.] 



