Urophora cardui ...induces a large spherical or irregular multichambered gall in the stems of Cirsium arvense (L.) Scop. The fly requires moist sites ...

[In] June ... [e]ggs are laid into an apical leaf bud ... The larvae tunnel down the stem until they reach the zone of vascular differentiation where they induce the gall. The tunnel, which expands as the gall grows, fills with callus ... Larvae ... overwinter in the gall, remaining dormant as long as the gall is intact, but pupating when exposed to air. In nature this occurs in the spring ... Final gall size is a function of the number of larvae in it ... The gall ... is soft and fleshy while growing, but hard when mature ... [G]all parenchyma beyond the nutritive layer becomes lignified ...

[B&W photo on pg. 15]