Chirosia betuleti

Newly emerged fly larvae feed on glandular hairs causing the pinnules to curl around the midrib, and partially or completely loop around the feeding larvae ... 

Eggs of Chirosia betuleti were laid in early May on the inside of leaffets, on the unfurling portion of the coiled vegetative leaf. They adhered to the sticky hairs ... on the midrib. Emerging larvae fed on .... trichomes in the upper quarter of the groove of the frond rachis. [Their] feeding [on] the expanding frond [caused] young leaflets in the immediate vicinity [to become] stunted, and curled over the midrib. 

In most fronds...[a]long that portion of the midrib on which the larva(e) fed, there [formed a] partial or, less commonly, a complete loop of the rachis. Often, only a three-quarter twist was completed such that the resulting gall was a sack-like or U-shaped alteration of the normal course of the midrib. In all galls, the larvae were covered by the pinnules. The portion of the leaf anterior to the feeding larvae continued to develop normally. At maturity the leaf was normal in overall shape and size, except for the affected portion which appeared as a looped whorl of pinnules ...  

Only one gall was formed per leaf and only rarely was the gall occupied by more than a single larva of Chirosia betiileti. Larvae that had eaten most of the trichomes of the rachis continued along the covering pinnules, as well as feeding on epidermal and ... hypodermal tissues along the adaxial side of the rachis. There was no expansion or other deformation of the midrib. ... [F]eeding ... was restricted to the adaxial surface of the rachis. With continued feeding these areas became black from the accumulation of frass and the ... breakdown of the epidermal and hypodermal tissues. 

Chirosia betuleti (Ringdahl) (Diptera: Anthomyiidae) a gall-former on the ostrich fern, Matteuccia struthiopteris, with notes on other insect-fern associates