26 THE FUNCTIONS OF A LOCAL NATURAL HISTORY SOCIETY. 
on the substance of the gall, the parasite is feeding on its tissues, 
the vital organs not being attacked until the parasite is ready to 
undergo its own transformation. Some of these parasites are so 
minute that several eggs of one species are deposited in the single 
egg of a butterfly, but it is not yet known whether this is so in 
the case of the eggs of any of the gall insects. From this it 
follows that when insects are bred from galls it is by no means 
certain that the insect emerging is the progeny of the original gall- 
maker, and especially the Cynipide and Cecidomyide, nearly allied 
insects, take advantage of the formation of the gall to provide 
pabulum for their own larvz ; and, to make the confusion more 
complete, even insects belonging to groups which are generally 
parasitic in their habits sometimes make galls themselves, as in 
the instance of Emytorica among the Chalcidide. 
The chief biological interest, however, of the gall-makers 
centres in two groups, one belonging to the Hymenoptera, viz., 
the Cynipide, and the other to the Hemiptera, viz., the Aphidze. 
The life histories of these groups are, in many cases, involved in 
great obscurity, and require the most patient investigation to work 
out, since they apparently offer examples of two of the most 
curious methods known in the reproduction of species, viz., 
parthenogenesis and the alternation of generations. I will try, in 
as few words as possible, to explain what is meant by these two 
phrases, as they may not be familiar to everyone. 
The term parthenogenesis, or the production of young by 
virgin, or rather, imperfect insects, is applied to those cases in which 
reproduction goes on for successive generations from female insects 
only, or at least from individuals which are capable of laying eggs 
which develop into perfect insects, a well-known example of 
which is the aphis, or insect which produces the honeydew. In 
one case, where specimens of Aphis dianthi were brought into a 
warm room and kept there, this process went on through many 
broods in a year for four years, and there seemed no reason then 
why it should not go on indefinitely; male insects being only 
produced out of doors at the approach of cold weather. To this 
group belong Zriosoma pyri, which produces galls, like potatoes, 
