THE APHIS AND THE ICHXEUMON-FLY. 547 



active Syrphida, wliicli will presently be mentioned. Several 

 Hymenoptera provision their nests with them, and others of 

 the parasitic kind, such as Chalcidicse, Proctotrupidoe, Cyni- 

 pidse, and Ichueumonidse. 



As to these last insects, the mode in which they operate on 

 the Aphis is very remarkable. As may be inferred from the 

 size of the Aphides, on which they are parasitic, they are of 

 very small dimensions, and one of these tiny creatures when 

 standing on the back of an Aphis which it is about to wound, 

 looks much like a rook perched on a sheep's back. 



When the fatal egg has been introduced under the skin of 

 the Aphis, the wounded insect, like the stricken hart, separates 

 itself from its fellows and passes to the under side of a leaf, 

 and there fixes itself. Its body soon begins to swell, and at 

 last becomes quite globular and horny, the change being 

 caused by the death of the Aphis and the rapid growth of the 

 parasite within it. The ichneumon passes rapidly through its 

 changes, and in a short time assumes the perfect form, always 

 with its head next the tail of its victim, pushes off the last two 

 or three segments of the dead Aphis and makes its escape into 

 the world. The dead and empty skins of such hapless Ajahides 

 may be found plentifully towards the end of summer, sticking 

 firmly to the leaf, and showing the round aperture through 

 which the destroying parasite has crept. 



Sometimes there is a sort of contest between the Aphis 

 and the ichneumon-fly, the former, however, having no more 

 chance against its tiny foe than has a rabbit against a weasel. 

 If the Aphis have its beak deeply plunged into the bark, the 

 ichneumon-fly has an easy task, for the Aphis can do nothing 

 but kick and struggle while anchored to the spot by its pro- 

 boscis, and all that the ichneumon-fly has to do is to make its 

 deadly lunge. But, if the Aphis be wandering about the 

 plant, the ichneumon-fly has to walk about with it, and try 

 first one side and then the other, until she can find an 

 opportunity of depositing her egg. 



We now come to a very remarkable group of Aphides which 

 have been gathered together under the generic name of 

 Eriosoma, or Wool-bodied, because their bodies are covered 

 with a substance like very fine cotton wool. These insects 



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