204 ^N INTRODUCTION TO ZOOLOGY. 



eggs give rise to viviparous Aphides, which reproduce 

 themselves asexually for numbers of generations, from 

 unfertilized germs which are called Pseudova. These 

 viviparous Aphides have wings ; the true egg-bearing 

 females have no wings, but the males are winged. 

 Parthenogenesis sometimes occurs among other in- 

 sects, but not so markedly as ia the aphides. These 

 creatures, which are also called green-flies, suck the 

 juices of the plants they infest. They have a peculiar 

 secretion of a sweet liquid, called honey-dew, which is 

 secreted in tubes which open on the dorsal side of the 

 last segment but one. Ants are very fond of this, 

 which they suck from the aphides ; they even keep 

 them in their ant-hills for the sake of it, so that the 

 aphis has been called the ant's cow. Among the 

 higher forms classed as Hemiptera are the Cicadce 

 (Grasshoppers of classical writers), and the little green 

 insect that covers plants with spots of foam in spi-ing ; 

 it is the larva of Aphrophora (the foam-bearer), one of 

 the Gicadellidce. 



The Diptera are suctorial insects which have the 

 front wings membranous, and the back wings repre- 

 sented only by small knobs or stalks, the nature of 

 which was long a matter of dispute. The Sheep-ticks, 

 House-flies, and Blow-flies, the Gnats, and Breeze- 

 flies belong here ; also the Fleas, which have the 

 aborted wings represented by lateral plates. The 

 Crane-fly has a larva called the Wire-worm, which 

 lives on roots, and is very destructive to crops. 



The Lepidoptera, or Butterflies, are well known by 



