Nettle leaf galls, artwork. Four views of galls at the base of a stinging nettle leaf (Urtica dioica). These abnormal outgrowths are caused by the parasitic larvae of the nettle gall midge (Dasineura urticae). The larvae emit a chemical which stimulates growth and then they feed on the juices and tissue produced inside the gall cavity. A slit in the top of the gall provides an exit point. Each gall generally contains one larva. A gall may contain more than one larva (as shown in the cross-section, bottom right), indicating that the gall midge larva itself has become a victim of a endoparasitoid.