HOMOPTERA 115 



eyes are prominent, and ocelli are also usually present. The antennas are 

 from three- to seven-jointed. On the back of the sixth abdominal seg- 

 ment there is, in many species, a pair of tubes, the cornicles, through 

 which a wax-like material is excreted. In some genera these organs are 

 merely perforated tubercles, while in still other genera they are wanting. 



Many species of aphids excrete a sweet substance known as honey- 

 dew from the posterior end of the alimentary canal. It is sometimes 

 produced in such quantities that it forms a glistening coating on the 

 leaves of the branches below the plant-lice, and stone walks beneath 

 shade-trees are often densely spotted with it. This honey-dew is fed 

 upon by bees, wasps, and ants. The bees and wasps take the food 

 where they find it, paying little if any attention to its source; but the 

 ants recognize in the plant-lice useful auxiliaries, and often care for them 

 as men care for their herds. This curious relationship will be discussed 

 further in the chapter on ants. 



In addition to honey-dew, many aphids excrete a white waxy sub- 

 stance. This may be in the form of powder, scattered over the surface of 

 the body, or it may be in large flocculent or downy masses; every grada- 

 tion between these forms exists. 



The plant-lice are remarkable for their peculiar mode of development. 

 The various species differ greatly in the details of their transformation, 

 but the following generalizations illustrated by the life history of the 

 cabbage aphid, Brevicoryne brassicce, may be made. 



The stem-mother. — In the spring there hatches from each surviving 

 egg which was laid on a cabbage stump in the fall, a female aphid known 

 as the stem-mother because she gives rise to all of the succeeding genera- 

 tions during the summer. She brings forth her young alive and is there- 

 fore viviparous. She also bears young aphids without having mated 

 (there are no male aphids in the spring) and is therefore parthenogenetic. 



The wingless agamic form. — This stem-mother gives birth to young 

 which do not develop wings and which are all females. These reproduce 

 parthenogenetically and are known as the wingless agamic forms. These 

 reproduce their kind for a variable number of generations and then pro- 

 duce the next form. 



The winged agamic form. — After a variable number of generations of 

 the wingless agamic form have been developed and the food-plant has be- 

 come overstocked by them, there appears a generation which is winged. 

 These are all parthenogenetic, viviparous females. They are known as 

 the winged agamic forms. These migrate to ether cabbage plants which 

 are not overstocked with the wingless pi ant -lice. 



When the migrating winged agamic form becomes established on fresh 

 plants, it produces young which are all females of the wingless agamic 

 form. After a variable number of generations of the winged and wing- 

 less forms have been developed and fall approaches, the egg-laying (ovip- 

 arous) females and the males are produced. 



The males and oviparous females. — These are the true sexual forms. 

 They pair and each female lays one or more eggs on the cabbage stump 

 which rest over the winter. In the case of the cabbage aphid the ovip- 

 arous female is wingless and the male is winged. Other aphids differ 

 in this respect. In some species the females are winged and in a few the 

 males are wingless. 



Primary and secondary host plants. — Some aphids deposit their eggs in 



