xxiii GALLS 525 



these lay their eggs in the galls formed by the gall-makers 

 subsequent to the growth of the galls, of which they obtain 

 the benefit: (3) Parasites; these live, like most Ichneumon-flies, 

 in the interior of the bodies of other living Insects ; they prey 

 on a considerable variety of Insects, but chiefly, it is believed, on 

 Aphididae, or on Dipterous larvae. These parasitic flies belong 

 to the sub-family Figitides. 



A great deal of discussion has occurred relative to the nature 

 and origin of galls, and many points still remain obscure. Con- 

 siderable light has been thrown on the subject by the direct 

 observations of modern naturalists. Previous to Malpighi, who 

 wrote on the subject two hundred years ago, it was supposed that 

 galls were entirely vegetable productions, and that the maggots 

 found in them were due to spontaneous generation, it having been an 

 article of belief in the Middle Ages that maggots in general arose 

 from the various organic substances in which they were found, by 

 means of the hypothetical process called, as we have said, spon- 

 taneous generation. Malpighi was aware of the unsatisfactory 

 nature of such a belief, and having found by observation that 

 galls arose from the punctures of Insects, he came to the further 

 conclusion that the growth of the gall was due to the injection 

 by the Insect into the plant of a fluid he termed Ichor, which 

 had, he considered, the effect of producing a swelling in the plant, 

 something in the same way as the sting of a bee or wasp 

 produces a swelling in an animal. Eeaumur also made observa- 

 tions on the gall-Insects, and came to the conclusion that the 

 latter part of Malpighi's views was erroneous, and that the swell- 

 ing was not due to any fluid, but simply to irritation caused by 

 the prick ; this irritation being kept up by the egg that was 

 deposited and by the subsequent development of the larva. 

 Observations since the time of Eeaumur have shown that the 

 matter is not quite so simple as he supposed, for though in the 

 case of some galls the development of the gall commences immedi- 

 ately after the introduction of the egg, yet in other cases, as in 

 the Cynipidae, it does not occur till some time thereafter, being 

 delayed even until after the hatching of the egg and the com- 

 mencement of the development of the larva. Galls are originated 



subject is no doubt destined to become a most interesting department of entomology. 

 See Riley, P. mt. Soc. WasMiigton, ii. 1893, p. 397 ; and Wasmann, Zusammengesetz- 

 ten Nester, etc., 1891. 



