6l8 THE STUDY OF INSECTS. 



the characteristics of Neuroterus, but have been classed in 

 another genus under the name SpatJiegastcr (Spath-e-gas'ter). 

 These in turn lay eggs which develop into gall-flies like their 

 grandparents, i.e., a species of Neuroterus. Not only, says 

 Mr. Adler, do the two generations live in galls differing in 

 form, color, and situation, and the insects exhibit among 

 themselves differences of size, proportions, and structure, but 

 what renders the contrast more striking is that the Neu- 

 roterus generation is only represented by females, whilst the 

 Spathegaster generation presents individuals of both sexes. 



Although alternation of generations occurs in many 

 species of the Cynipidae, it is believed that there are other 

 species in which the parthenogenetic form exists alone ; that 

 is, the species reproduce continuously without any males ap- 

 pearing. There can be no doubt that these parthenogenetic 

 species have descended from species consisting of both sexes. 

 Still, it is said that no case is now known of a bisexual form 

 existing alone ; each bisexual species is merely a link in a 

 cycle containing a parthenogenetic generation. 



The members of this family infest many kinds of plants, 

 but their galls occur most abundantly on oaks. Among the 

 more conspicuous species are the following: 



The Fibrous Oak apple, Amphibolips coccinccz (Am-phib'- 

 o-lips coc-cin'e-ae). There are several large, spherical galls, 

 common on oaks, which have received the name of oak-apples. 

 These galls resemble each other quite closely in their ex- 

 ternal appearance, but differ much in their internal struc- 

 ture. The one which we name the Fibrous Oak-apple is 

 represented by Figure 746. In the centre of the gall there is 

 a small, hollow kernel, in the cavity of which the gall-fly is 

 developed. The space between this kernel and the dense 

 outer layer of the gall is filled with many fibres, which radi- 

 ate from the kernel. This gall is found on the scarlet oak, 

 and varies in size from three fourths inch to two inches in 

 diameter. 



The Spongy Oak-apple, Amphibolips spongifica (A. spon- 



