Local feeder (Diptera: Cecidomyiidae) in stems of Galium

I harvested several Galium aparine whole plants (not including roots) from my yard in the final days of June, 2023, in an effort to rear an agromyzid that feeds internally in the stems of this plant (record 0246). At the time of harvest, I stripped all leaves, flowers, and fruits from the plants, so that stems were the only plant material going into the rearing containers. The plants had mostly finished their growth and were undergoing senescence when I harvested them. I divided up the stems between three rearing bags.

On 08 July I removed some of the harvested stems from one of the rearing bags and searched them for agromyzid larvae. One of these stems had no agromyzid larvae inside but instead possessed a set of two externally visible, transverse dark brown or black marks, separated by a distance of approximately 10mm. Inside the stem between these two marks were two pale yellow-orange cecidomyiid larvae. It was unclear if the marks were related to the presence of the larvae. I transferred the larvae to another container, and left undisturbed the stems remaining in the original three rearing bags.

On 31 July a single adult cecidomyiid emerged in one of the rearing bags. After I collected the adult from the bag, I carefully searched all the stems from this bag for protruding exuviae. Eventually, with much effort, I found one set of exuviae belonging to a single cecidomyiid individual. The pupal exuviae were protruding from a small, whitish, elongate-oval, slightly raised, blisterlike chamber just under the epidermis of the stem near a node. The exuviae clearly belonged to a cecidomyiid, and my initial assumption was that they belonged to the adult that had appeared in the bag, but I could not immediately confirm this.

I photographed the larvae, pupal exuviae, and adult. Because the plant material had already begun to senesce when I collected it, it's conceivable that the stems could have been favorable habitat for a cecidomyiid feeding on fungal rather than plant material. However, the blisterlike chamber from which the exuviae protruded was suggestive of a gall formed by a plant feeder. Eventual identification of the specimens involved may shed light on the situation.