156 INJURIOUS AND USEFUL INSECTS 



and some other animals which propagate in two ways ; and it 

 is solely for these winter-eggs that male aphids and oviparous 

 females are indispensably requisite. 



It has been ascertained that an aphis may infest one plant 

 for several viviparous generations, then forsake it for another, 

 and afterwards return to the first for egg-laying. This happens 

 with the aphis of the maple, that of the hop, and that of 

 the apple. 



We have seen that the aphids undergo no transformation, 

 though the winged forms are at first devoid of wings, which 

 are only acquired step by step, at successive moults. There is 

 also no resting-stage. In these respects, and in the structure 

 of the moiith-parts, aphids resemble such insects as bugs, 

 pond - skaters, water - scorpions, water - boatmen, cicadas, and 

 scale-insects. All are placed together in the same order 

 (Rhynchota or Hemiptera). The Rhynchota may be sub- 

 divided into (i) Heteroptera, with fore wings stiffer than the 

 hind ones, and often stiffer at the base than at the apex, and 

 (2) Homoptera, with uniformly membranous wings. In Heter- 

 optera the wings commonly lie flat on the back, while in 

 Homoptera they are sloped on either side, hke a roof over the 

 abdomen. The aphids belong to the sub-order Homoptera. 



There has been much discussion about the origin of honey- 

 dew, which is a sugary secretion found upon the leaves of trees 

 and many other objects in dry summer weather. The ancients 

 supposed that it fell from heaven, or was raised from the earth 

 by evaporation. Reaumur observed the actual exudation of 

 viscid drops from the intestine of aphids, and proved by tasting 

 that they had the sweetness of honey-dew. He was also aware 

 that ants are fond of the secretion, and protect and caress the 

 aphids for the sake of it. In spite of R&umur's full and 

 accurate account it was long believed that there was a second 

 sort of honey-dew, of purely vegetable origin, which exuded 

 from leaves and young stems at times when hardly any aphids 

 could be found. This vegetable honey-dew is now known to 

 be mythical, though it is often described in modern books of 

 good repute. Biisgen has shown that all honey-dew is the 

 product of aphids and scale-insects, with the unimportant 

 exception of sugary fluids secreted by certain other insects and 

 by a parasitic fungus (Claviceps). The fluid is dropped or 

 squirted out, and may fall in a fine shower upon distant objects. 



