HOMOPTERA 415 



Family APHIDID^ 

 The Typical Aphids 



To this family belong the far greater ntimber of the genera and 

 species of the Aphidoidea. The distinctive characters of this family 

 are given under A in the table above. For a detailed discussion of 

 the wing-venation of these insects, see Patch ('09). 



In the Aphididce there exists a remarkable type of developrnent 

 known as heterogamy or cyclic reproduction. This is characterized 

 by an alternation of parthenogenetic generations with a sexual 

 generation. And within the series of parthenogenetic generations there 

 may be an alternation of winged and wingless forms. In some cases 

 the reproductive cycle is an exceedingly complicated one, and differ- 

 ent parts of it occur on different species of food plants. 



In those cases where different parts of the reproductive cycle 

 occur on different food-plants, the plant on which the over-wintering 

 fertilized egg is normally deposited and upon which the stem-mother 

 and her immediate progeny develop is termed the primary host; a.nd 

 that plant to which the migrants fly and from which a later form in 

 the series migrates to the primary host is known as the secondary host. 



Different species of aphids differ greatly in the details of their 

 development; it is difficult, therefore, to make generalizations re- 

 garding this matter. The following account will serve to indicate the 

 sequence of the forms occurring in the reproductive cycle of a migrat- 

 ing aphid, one in which the different parts of the cycle occur on 

 different food-plants. This account refers to what occurs in the 

 North, where the winter interrupts the production of young, and eggs 

 are developed which continue the life of the species through the 

 inclement season. In hot climates also, where there is a wet and a dry 

 season, eggs are produced to carry the species over the period when 

 succulent food is lacking. And in some cases in the North, on ex- 

 hausted vegetation the non-migratory species produce eggs during 

 the summer months. 



The stem-mother.- — In the spring there hatches from an over- 

 wintering egg a parthenogenetic, viviparous female, which lives on 

 the primar}^ host. As this female is the stock from which the summer 

 generations spring, she is known as the stem-mother or fundatrix. 

 The stem-mother is winged in some species of one of the tribes 

 (Callipterini) ; but usually she is wingless. 



The wingless agamic form. — In most species the stem-mother gives 

 birth to young which do not develop wings and which are all 

 females. These reproduce parthenogetically and are known as the 

 wingless agamic form or spurice aptercc.* These reproduce their kind 

 for a variable number of generations and then produce the next form. 

 All of these generations live on the primary host. In a few species 

 the wingless agamic form rarely appears if at all. 



*Sptiricc (New Latin, fern, pi.); Lat. spurius, an iHegitimate or spurious child. 



