There are three groups of zoocecidia occurring on hickory trees:
1. Galls formed by species of Eriophyes (Fam. Eriophyidæ of the Acarina or mites), or an allied genus. Only two are known.
2. Galls induced by species of Phylloxera (Aphididæ of the Hemiptera). Pergande[2] has presented an excellent survey of these insects accompanied by very satisfactory descriptions of the cecidia formed by them.
3. Galls caused by species of Caryomyia (Itonididæ of the Diptera). Possibly other genera may be represented on the hickories, but according to Felt[3] “most of the hickory leaf galls are probably made by species of Caryomyia, though other midges have been reared from these deformities.”
The genus Caryomyia, which undoubtedly occupies an important place in relation to the majority of the galls described in the present paper, will be given special consideration. Felt, to whom American cecidology is heavily indebted for his extensive studies of dipterous cecidozoons, presents the following description of the genus Caryomyia in the same citation as that immediately above.
“Allied to Hormomyia, but differing by the thorax not being greatly produced over the head and by the presence of but 14 antennal segments. The males may have the flagellate antennal segments binodose or cylindric and subsessile and invariably with three low, stout circumfili. The antennal segments of the female are cylindric and with two circumfili; palpi tri- or quadri-articulate; wings rather broad, the third vein joining the costa at or near the wing apex; claws simple, the pulvilli well developed. The ovipositor of the female is short and with minute lobes apically. The genus appears to be confined to hickory leaf galls.”
Adult insects not technically known are given the old generic name “Cecidomyia.”
These galls as well as similar ones on other kinds of plants arise as the result of some stimulus (the nature of which is still not definitely known) applied by the very young larva to the growing tissue of the immature leaf. Nothing has yet been done on the development of the itonid galls of the hickories, but from studies on very similar types we have reason to believe that the ontogeny of the itonid forms is as follows: The egg is probably deposited superficially (for the ovipositor of the female Caryomyia is short) on the under side of the leaflet; on the upper side in a few cases.
Hyperplasia or excessive cell proliferation results (probably not until after the larva has emerged from the egg) forming at first a saucer-shaped structure, then cup-shaped and finally by the ingrowth of the edges, the gall becomes a closed structure enveloping the larva in a chamber. The distal growth, seldom if ever in the hickory forms, proceeds so far as to obliterate the opening which was so prominent in the very immature cup-shape stage. Hence in practically all galls of this type a minute canal or pore can be demonstrated at the distal end. In Küster’s[4] very serviceable classification of abnormal plant parts, these fall under his “umwallungen” cecidia, a term very succinctly describing their mode of development.
Two of the following described galls have been studied histologically by Cook,[5] Caryomyia holotricha O. S. and C. tubicola O. S.