No. XV.B.] APPENDIX. 231 



nearly all parts of England. After migrating to a new locality, they will 

 settle down perpendicularly in a mass upon the fields to recommence their 

 work of destruction. Last summer I was informed that travellers in the 

 neighbourhood of Norwich were very much inconvenienced by the clouds 

 of aphides flying about and almost blinding them. From these manifes- 

 tations of the insect, then, we find that the injury to plants hastens the 

 transformation of the aphides. After the attacks of aphides and damage 

 to the tissue of the plant, we find upon examining the decaying tubers, 

 &c., that they become covered with parasitic fungi, of which there are 

 thirty or forty varieties. These grow and nourish in the decaying vege- 

 table matter, and are, in fact, the scavengers provided by nature to remove 

 the decomposing substance and to prevent contamination of the atmo- 

 sphere by putrid and poisonous exhalations. These fungi are in most cases 

 observed on the diseased potato. As they grow they eat up, as it were, 

 the soft and decaying parts as fast as they rot ; and thus is inorganic 

 matter converted into organic thus is death converted into life. These 

 fungi have been considered to be the source and cause of the disease ; and 

 Martius, who was the first investigator of this subject, traces many fungi, 

 and attributes the disease to them; but, in fact, they never make their 

 appearance until the potato-plant has been previously damaged, and until 

 some portion of it is already dead. I have tried many experiments to 

 produce the disease by inoculating sound potatoes, but the result has 

 been a failure. I conclude, then, that it is a necessary law that the 

 attacks of aphides are almost invariably followed by the growth of 

 fungi. 



We have now spoken of the effects of the attacks of aphides on plants, 

 and the conditions or laws of those effects. We have also noticed the 

 excessive numbers that can be produced from a single aphis ; and that, 

 assembled in vast swarms, they cause immense and serious mischief. 

 Aphides, which we have shown to be so destructive by their excessive 

 numbers, are themselves the natural prey of numerous creatures ; and 

 their increase, therefore, may lead us to believe that the natural balance of 

 creation has been disturbed, and that their natural destroyers have been 

 diminished. Ladybirds are enemies to and destroyers of aphides, and 

 being more conspicuous often lead us to the discovery of the latter. Last 

 year it is believed that ladybirds were scarce, though in many seasons, 

 particularly that of 1805, these creatures have been noticed in great abun- 

 dance on the cliffs at Dover, and other places on the coast. The hop aphis 

 often produces great havoc in the crop, and ladybirds are always much 

 welcomed in the hop-grounds. The gauzewing, too, feeds on aphides with 

 equal voracity, as also do various dypterous insects of the genus sylphidse. 

 It would be well if we could breed these creatures by millions. Various 

 hymenopterous insects are great destroyers of aphides, one genus of which, 

 called ichneumons, deposit their egg in the body of the aphis : the egg 

 becomes a maggot that feeds upon the aphis, which swells, assumes a 

 globular form, and at length dies, remaining adherent to the leaf. After 

 the death of the aphis the enclosed creature eats a hole through the case 

 which contains it, and comes out a winged insect. I have watched this 

 process, and seen the ichneumon escape from aphides in my own posses- 

 sion. Nature, amid all her wonders, goes a step further than this, for 

 another genus of hymenoptera, the chalcididse, deposit their egg within the 



