The Gall-Flies 



gation by Kraemer. He finds that gallic acid is formed at the ex- 

 pense of the starch during the chrysalis stage of the insect. With 

 the maturing of the winged insect, the gallic acid is changed to 

 tannic acid. The transformation of gallic acid into tannin is 

 accomplished by the condensation of two molecules of the for- 

 mer with the loss of one molecule of water. 



Most of these insects are single-brooded and develop but one 

 generation in the year. Under unfavorable circumstances, how- 

 ever, this period may be greatly lengthened, and circumstances 

 are on record where the flies have emerged only after two or three 

 years. Certain moisture conditions favor the proper issuing, and 

 when these are lacking the development is retarded. 



Some very curious alternations of generations occur among 

 these insects. Of certain species, and indeed of certain genera, 

 for years only the female sex was known, and it was afterward 

 discovered, first by Riley and afterward by Adler, that what had 

 been considered two entirely distinct forms were really the same 

 species, but that one generation included both sexes while the 

 alternate generation comprised only females which reproduced 

 parthenogenetically. 



One of the most peculiar facts connected with the gall-flies 

 is that a particular part of the plant is always affected by the same 

 species, and that each species of the same generation always pro- 

 duces a deformation or gall of exactly the same character, so that 

 the gall alone identifies the species of insect, and in fact for a long 

 time generic and specific names were given to the galls before the 

 insects were named, the name subsequently being applied to the 

 insect itself. 



The full development of none of the American gall-making 

 Cynipoids has been studied with the care which this subject should 

 have and doubtless there are many interesting and important facts 

 yet to be discovered. 



The guest gall-flies closely resemble the true gall-flies, but 

 lay their eggs in galls already formed by the true gall-flies, their 

 larvae living upon the plant growth produced by the true gall- 

 fly larvae. 



The parasitic gall-flies live as true internal parasites in other 

 insects, mainly plant lice and the larvae of dipterous insects. 



About fifteen hundred species of this super-family have been 

 described. 



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